@article {3511, title = {Beatty Sequences for a Quadratic Irrational: Decidability and Applications}, year = {2024}, month = {2/13/2024}, abstract = {

Let α and β belong to the same quadratic field. We show that the inhomogeneous Beatty sequence (\⌊nα+β\⌋)n\≥1 is synchronized, in the sense that there is a finite automaton that takes as input the Ostrowski representations of n and y in parallel, and accepts if and only if y=\⌊nα+β\⌋. Since it is already known that the addition relation is computable for Ostrowski representations based on a quadratic number, a consequence is a new and rather simple proof that the first-order logical theory of these sequences with addition is decidable. The decision procedure is easily implemented in the free software Walnut.\ \ As an application, we show that for each r\≥1 it is decidable whether the set {\⌊nα+β\⌋:n\≥1} forms an additive basis (or asymptotic additive basis) of order r. Using our techniques, we also solve some open problems of Reble and Kimberling, and give an explicit characterization of a sequence of Hildebrand et al.

}, url = {https://arxiv.org/abs/2402.08331}, author = {Luke Schaeffer and Jeffrey Shallit and Stefan Zorcic} } @article {3504, title = {Phase transition in magic with random quantum circuits}, year = {2024}, month = {4/11/2024}, abstract = {

Magic is a property of quantum states that enables universal fault-tolerant quantum computing using simple sets of gate operations. Understanding the mechanisms by which magic is created or destroyed is, therefore, a crucial step towards efficient and practical fault-tolerant computation. We observe that a random stabilizer code subject to coherent errors exhibits a phase transition in magic, which we characterize through analytic, numeric and experimental probes. Below a critical error rate, stabilizer syndrome measurements remove the accumulated magic in the circuit, effectively protecting against coherent errors; above the critical error rate syndrome measurements concentrate magic. A better understanding of such rich behavior in the resource theory of magic could shed more light on origins of quantum speedup and pave pathways for more efficient magic state generation.

}, url = {https://arxiv.org/abs/2304.10481}, author = {Pradeep Niroula and Christopher David White and Qingfeng Wang and Sonika Johri and Daiwei Zhu and Christopher Monroe and Crystal Noel and Michael Gullans} } @article {3496, title = {The Quantum Abstract Machine}, year = {2024}, month = {2/21/2024}, abstract = {

This paper develops a model of quantum behavior that is intended to support the abstract yet accurate design and functional verification of quantum communication protocols. The work is motivated by the need for conceptual tools for the development of quantum-communication systems that are usable by non-specialists in quantum physics while also correctly capturing at a useful abstraction the underlying quantum phenomena. Our approach involves defining a quantum abstract machine (QAM) whose operations correspond to well-known quantum circuits; these operations, however, are given direct abstract semantics in a style similar to that of Berry\&$\#$39;s and Boudol\&$\#$39;s Chemical Abstract Machine. This paper defines the QAM\&$\#$39;s semantics and shows via examples how it may be used to model and reason about existing quantum communication protocols.

}, url = {https://arxiv.org/abs/2402.13469}, author = {Liyi Li and Le Chang and Rance Cleaveland and Mingwei Zhu and Xiaodi Wu} } @article {3480, title = {Strong interactions between integrated microresonators and alkali atomic vapors: towards single-atom, single-photon operation}, year = {2024}, month = {4/5/2024}, abstract = {

Cavity quantum electrodynamics (cQED), the interaction of a two-level system with a high quality factor (Q) cavity, is a foundational building block in different architectures for quantum computation, communication, and metrology. The strong interaction between the atom and the cavity enables single photon operation which is required for quantum gates and sources. Cold atoms, quantum dots, and color centers in crystals are amongst the systems that have shown single photon operations, but they require significant physical infrastructure. Atomic vapors, on the other hand, require limited experimental infrastructure and are hence much easier to deploy outside a laboratory, but they produce an ensemble of moving atoms that results in short interaction times involving multiple atoms, which can hamper quantum operations. A solution to this issue can be found in nanophotonic cavities, where light-matter interaction is enhanced and the volume of operation is small, so that fast single-atom, single-photon operations are enabled. In this work, we study the interaction of an atomically-clad microring resonator (ACMRR) with different-sized ensembles of Rb atoms. We demonstrate strong coupling between an ensemble of ~50 atoms interacting with a high-quality factor (Q \> 4 x 10^5) ACMRR, yielding a many-atom cooperativity C ~ 5.5. We continue to observe signatures of atom-photon interaction for a few (\< 3) atoms, for which we observe saturation at the level of one intracavity photon. Further development of our platform, which includes integrated thermo-optic heaters to enable cavity tuning and stabilization, should enable the observation of interactions between single photons and single atoms.

}, url = {https://arxiv.org/abs/2404.04372}, author = {Roy Zektzer and Xiyuan Lu and Khoi Tuan Hoang and Rahul Shrestha and Sharoon Austin and Feng Zhou and Ashish Chanana and Glenn Holland and Daron Westly and Paul Lett and Alexey V. Gorshkov and Kartik Srinivasan} } @article {3317, title = {Colloquium: Advances in automation of quantum dot devices control}, journal = {Reviews of Modern Physics}, volume = {95}, year = {2023}, month = {2/17/2023}, abstract = {

Arrays of quantum dots (QDs) are a promising candidate system to realize scalable, coupled qubit systems and serve as a fundamental building block for quantum computers. In such semiconductor quantum systems, devices now have tens of individual electrostatic and dynamical voltages that must be carefully set to localize the system into the single-electron regime and to realize good qubit operational performance. The mapping of requisite QD locations and charges to gate voltages presents a challenging classical control problem. With an increasing number of QD qubits, the relevant parameter space grows sufficiently to make heuristic control unfeasible. In recent years, there has been considerable effort to automate device control that combines script-based algorithms with machine learning (ML) techniques. In this Colloquium, a comprehensive overview of the recent progress in the automation of QD device control is presented, with a particular emphasis on silicon- and GaAs-based QDs formed in two-dimensional electron gases. Combining physics-based modeling with modern numerical optimization and ML has proven effective in yielding efficient, scalable control. Further integration of theoretical, computational, and experimental efforts with computer science and ML holds vast potential in advancing semiconductor and other platforms for quantum computing.

}, doi = {10.1103/revmodphys.95.011006}, url = {https://arxiv.org/abs/2112.09362}, author = {Justyna P. Zwolak and Jacob M. Taylor} } @article {3311, title = {Colloquium: Quantum and Classical Discrete Time Crystals}, year = {2023}, month = {5/15/2023}, abstract = {

The spontaneous breaking of time translation symmetry has led to the discovery of a new phase of matter - the discrete time crystal. Discrete time crystals exhibit rigid subharmonic oscillations, which result from a combination of many-body interactions, collective synchronization, and ergodicity breaking. This Colloquium reviews recent theoretical and experimental advances in the study of quantum and classical discrete time crystals. We focus on the breaking of ergodicity as the key to discrete time crystals and the delaying of ergodicity as the source of numerous phenomena that share many of the properties of discrete time crystals, including the AC Josephson effect, coupled map lattices, and Faraday waves. Theoretically, there exists a diverse array of strategies to stabilize time crystalline order in both closed and open systems, ranging from localization and prethermalization to dissipation and error correction. Experimentally, many-body quantum simulators provide a natural platform for investigating signatures of time crystalline order; recent work utilizing trapped ions, solid-state spin systems, and superconducting qubits will be reviewed. Finally, this Colloquium concludes by describing outstanding challenges in the field and a vision for new directions on both the experimental and theoretical fronts.

}, url = {https://arxiv.org/abs/2305.08904}, author = {Michael P. Zaletel and Mikhail Lukin and Christopher Monroe and Chetan Nayak and Frank Wilczek and Norman Y. Yao} } @article {3396, title = {Data Needs and Challenges of Quantum Dot Devices Automation: Workshop Report}, year = {2023}, month = {12/21/2023}, abstract = {

Gate-defined quantum dots are a promising candidate system to realize scalable, coupled qubit systems and serve as a fundamental building block for quantum computers. However, present-day quantum dot devices suffer from imperfections that must be accounted for, which hinders the characterization, tuning, and operation process. Moreover, with an increasing number of quantum dot qubits, the relevant parameter space grows sufficiently to make heuristic control infeasible. Thus, it is imperative that reliable and scalable autonomous tuning approaches are developed. In this report, we outline current challenges in automating quantum dot device tuning and operation with a particular focus on datasets, benchmarking, and standardization. We also present ideas put forward by the quantum dot community on how to overcome them.

}, doi = {https://doi.org/10.48550/arXiv.2312.14322}, url = {https://arxiv.org/abs/2312.14322}, author = {Justyna P. Zwolak and Jacob M. Taylor and Reed Andrews and Jared Benson and Garnett Bryant and Donovan Buterakos and Anasua Chatterjee and Sankar Das Sarma and Mark A. Eriksson and Eli{\v s}ka Greplov{\'a} and Michael Gullans and Fabian Hader and Tyler J. Kovach and Pranav S. Mundada and Mick Ramsey and Torbjoern Rasmussen and Brandon Severin and Anthony Sigillito and Brennan Undseth and Brian Weber} } @article {3421, title = {Digital quantum simulation of NMR experiments}, journal = {Science Advances}, volume = {9}, year = {2023}, month = {11/29/2023}, abstract = {

Simulations of nuclear magnetic resonance (NMR) experiments can be an important tool for extracting information about molecular structure and optimizing experimental protocols but are often intractable on classical computers for large molecules such as proteins and for protocols such as zero-field NMR. We demonstrate the first quantum simulation of an NMR spectrum, computing the zero-field spectrum of the methyl group of acetonitrile using four qubits of a trapped-ion quantum computer. We reduce the sampling cost of the quantum simulation by an order of magnitude using compressed sensing techniques. We show how the intrinsic decoherence of NMR systems may enable the zero-field simulation of classically hard molecules on relatively near-term quantum hardware and discuss how the experimentally demonstrated quantum algorithm can be used to efficiently simulate scientifically and technologically relevant solid-state NMR experiments on more mature devices. Our work opens a practical application for quantum computation.

}, issn = {2375-2548}, doi = {10.1126/sciadv.adh2594}, url = {https://arxiv.org/abs/2109.13298}, author = {Seetharam, Kushal and Biswas, Debopriyo and Noel, Crystal and Risinger, Andrew and Zhu, Daiwei and Katz, Or and Chattopadhyay, Sambuddha and Cetina, Marko and Monroe, Christopher and Demler, Eugene and Sels, Dries} } @article {3126, title = {Fat Pointers for Temporal Memory Safety of C}, journal = {Proceedings of the ACM on Programming Languages}, volume = {7}, year = {2023}, month = {3/20/2023}, pages = {316-347}, abstract = {

Temporal memory safety bugs, especially use-after-free and double free bugs, pose a major security threat to C programs. Real-world exploits utilizing these bugs enable attackers to read and write arbitrary memory locations, causing disastrous violations of confidentiality, integrity, and availability. Many previous solutions retrofit temporal memory safety to C, but they all either incur high performance overhead and/or miss detecting certain types of temporal memory safety bugs.
In this paper, we propose a temporal memory safety solution that is both efficient and comprehensive. Specifically, we extend Checked C, a spatially-safe extension to C, with temporally-safe pointers. These are implemented by combining two techniques: fat pointers and dynamic key-lock checks. We show that the fat-pointer solution significantly improves running time and memory overhead compared to the disjoint-metadata approach that provides the same level of protection. With empirical program data and hands-on experience porting real-world applications, we also show that our solution is practical in terms of backward compatibility -- one of the major complaints about fat pointers.

}, keywords = {Cryptography and Security (cs.CR), FOS: Computer and information sciences}, doi = {10.1145/3586038}, url = {https://arxiv.org/abs/2208.12900}, author = {Zhou, Jie and Criswell, John and Hicks, Michael} } @article {3401, title = {Logical quantum processor based on reconfigurable atom arrays}, journal = {Nature}, year = {2023}, month = {12/7/2023}, issn = {1476-4687}, doi = {10.1038/s41586-023-06927-3}, url = {https://arxiv.org/abs/2312.03982}, author = {Bluvstein, Dolev and Evered, Simon J. and Geim, Alexandra A. and Li, Sophie H. and Zhou, Hengyun and Manovitz, Tom and Ebadi, Sepehr and Cain, Madelyn and Kalinowski, Marcin and Dominik Hangleiter and Ataides, J. Pablo Bonilla and Maskara, Nishad and Cong, Iris and Gao, Xun and Rodriguez, Pedro Sales and Karolyshyn, Thomas and Semeghini, Giulia and Michael Gullans and Greiner, Markus and Vuletic, Vladan and Lukin, Mikhail D.} } @article {3271, title = {Parallel self-testing of EPR pairs under computational assumptions}, year = {2023}, month = {3/29/2023}, abstract = {

Self-testing is a fundamental feature of quantum mechanics that allows a classical verifier to force untrusted quantum devices to prepare certain states and perform certain measurements on them. The standard approach assumes at least two spatially separated devices. Recently, Metger and Vidick [Quantum, 2021] showed that a single EPR pair of a single quantum device can be self-tested under computational assumptions. In this work, we generalize their results to give the first parallel self-test of N EPR pairs and measurements on them in the single-device setting under the same computational assumptions. We show that our protocol can be passed with probability negligibly close to 1 by an honest quantum device using poly(N) resources. Moreover, we show that any quantum device that fails our protocol with probability at most ϵ must be poly(N,ϵ)-close to being honest in the appropriate sense. In particular, our protocol can test any distribution over tensor products of computational or Hadamard basis measurements, making it suitable for applications such as device-independent quantum key distribution under computational assumptions. Moreover, a simplified version of our protocol is the first that can efficiently certify an arbitrary number of qubits of a single cloud quantum computer using only classical communication.

}, url = {https://arxiv.org/abs/2201.13430}, author = {Honghao Fu and Daochen Wang and Qi Zhao} } @article {3272, title = {Parallel self-testing of EPR pairs under computational assumptions}, year = {2023}, month = {3/29/2023}, abstract = {

Self-testing is a fundamental feature of quantum mechanics that allows a classical verifier to force untrusted quantum devices to prepare certain states and perform certain measurements on them. The standard approach assumes at least two spatially separated devices. Recently, Metger and Vidick [Quantum, 2021] showed that a single EPR pair of a single quantum device can be self-tested under computational assumptions. In this work, we generalize their results to give the first parallel self-test of N EPR pairs and measurements on them in the single-device setting under the same computational assumptions. We show that our protocol can be passed with probability negligibly close to 1 by an honest quantum device using poly(N) resources. Moreover, we show that any quantum device that fails our protocol with probability at most ϵ must be poly(N,ϵ)-close to being honest in the appropriate sense. In particular, our protocol can test any distribution over tensor products of computational or Hadamard basis measurements, making it suitable for applications such as device-independent quantum key distribution under computational assumptions. Moreover, a simplified version of our protocol is the first that can efficiently certify an arbitrary number of qubits of a single cloud quantum computer using only classical communication.

}, url = {https://arxiv.org/abs/2201.13430}, author = {Honghao Fu and Daochen Wang and Qi Zhao} } @article {3314, title = {Qafny: Quantum Program Verification Through Type-guided Classical Separation Logic}, year = {2023}, month = {7/12/2023}, abstract = {

Formal verification has been proven instrumental to ensure that quantum programs implement their specifications but often requires a significant investment of time and labor. To address this challenge, we present Qafny, an automated proof system designed for verifying quantum programs. At its core, Qafny uses a type-guided quantum proof system that translates quantum operations to classical array operations. By modeling these operations as proof rules within a classical separation logic framework, Qafny provides automated support for the reasoning process that would otherwise be tedious and time-consuming. We prove the soundness and completeness of our proof system and implement a prototype compiler that transforms Qafny programs both into the Dafny programming language and into executable quantum circuits. Using Qafny, we demonstrate how to efficiently verify prominent quantum algorithms, including quantum-walk algorithms, Grover\&$\#$39;s search algorithm, and Shor\&$\#$39;s factoring algorithm, with significantly reduced human efforts.

}, url = {https://arxiv.org/abs/2211.06411}, author = {Liyi Li and Mingwei Zhu and Rance Cleaveland and Yi Lee and Le Chang and Xiaodi Wu} } @article {3355, title = {Quantum Sensing with Erasure Qubits}, year = {2023}, month = {10/2/2023}, abstract = {

The dominant noise in an \"erasure qubit\" is an erasure -- a type of error whose occurrence and location can be detected. Erasure qubits have potential to reduce the overhead associated with fault tolerance. To date, research on erasure qubits has primarily focused on quantum computing and quantum networking applications. Here, we consider the applicability of erasure qubits to quantum sensing and metrology. We show theoretically that, for the same level of noise, an erasure qubit acts as a more precise sensor or clock compared to its non-erasure counterpart. We experimentally demonstrate this by artificially injecting either erasure errors (in the form of atom loss) or dephasing errors into a differential optical lattice clock comparison, and observe enhanced precision in the case of erasure errors for the same injected error rate. Similar benefits of erasure qubits to sensing can be realized in other quantum platforms like Rydberg atoms and superconducting qubits

}, url = {https://arxiv.org/abs/2310.01512}, author = {Pradeep Niroula and Jack Dolde and Xin Zheng and Jacob Bringewatt and Adam Ehrenberg and Kevin C. Cox and Jeff Thompson and Michael Gullans and Shimon Kolkowitz and Alexey V. Gorshkov} } @article {3397, title = {Quantum-centric Supercomputing for Materials Science: A Perspective on Challenges and Future Directions}, year = {2023}, month = {12/14/2023}, abstract = {

Computational models are an essential tool for the design, characterization, and discovery of novel materials. Hard computational tasks in materials science stretch the limits of existing high-performance supercomputing centers, consuming much of their simulation, analysis, and data resources. Quantum computing, on the other hand, is an emerging technology with the potential to accelerate many of the computational tasks needed for materials science. In order to do that, the quantum technology must interact with conventional high-performance computing in several ways: approximate results validation, identification of hard problems, and synergies in quantum-centric supercomputing. In this paper, we provide a perspective on how quantum-centric supercomputing can help address critical computational problems in materials science, the challenges to face in order to solve representative use cases, and new suggested directions.

}, url = {https://arxiv.org/abs/2312.09733}, author = {Yuri Alexeev and Maximilian Amsler and Paul Baity and Marco Antonio Barroca and Sanzio Bassini and Torey Battelle and Daan Camps and David Casanova and Young jai Choi and Frederic T. Chong and Charles Chung and Chris Codella and Antonio D. Corcoles and James Cruise and Alberto Di Meglio and Jonathan Dubois and Ivan Duran and Thomas Eckl and Sophia Economou and Stephan Eidenbenz and Bruce Elmegreen and Clyde Fare and Ismael Faro and Cristina Sanz Fern{\'a}ndez and Rodrigo Neumann Barros Ferreira and Keisuke Fuji and Bryce Fuller and Laura Gagliardi and Giulia Galli and Jennifer R. Glick and Isacco Gobbi and Pranav Gokhale and Salvador de la Puente Gonzalez and Johannes Greiner and Bill Gropp and Michele Grossi and Emmanuel Gull and Burns Healy and Benchen Huang and Travis S. Humble and Nobuyasu Ito and Artur F. Izmaylov and Ali Javadi-Abhari and Douglas Jennewein and Shantenu Jha and Liang Jiang and Barbara Jones and Wibe Albert de Jong and Petar Jurcevic and William Kirby and Stefan Kister and Masahiro Kitagawa and Joel Klassen and Katherine Klymko and Kwangwon Koh and Masaaki Kondo and Doga Murat Kurkcuoglu and Krzysztof Kurowski and Teodoro Laino and Ryan Landfield and Matt Leininger and Vicente Leyton-Ortega and Ang Li and Meifeng Lin and Junyu Liu and Nicolas Lorente and Andre Luckow and Simon Martiel and Francisco Martin-Fernandez and Margaret Martonosi and Claire Marvinney and Arcesio Castaneda Medina and Dirk Merten and Antonio Mezzacapo and Kristel Michielsen and Abhishek Mitra and Tushar Mittal and Kyungsun Moon and Joel Moore and Mario Motta and Young-Hye Na and Yunseong Nam and Prineha Narang and Yu-ya Ohnishi and Daniele Ottaviani and Matthew Otten and Scott Pakin and Vincent R. Pascuzzi and Ed Penault and Tomasz Piontek and Jed Pitera and Patrick Rall and Gokul Subramanian Ravi and Niall Robertson and Matteo Rossi and Piotr Rydlichowski and Hoon Ryu and Georgy Samsonidze and Mitsuhisa Sato and Nishant Saurabh and Vidushi Sharma and Kunal Sharma and Soyoung Shin and George Slessman and Mathias Steiner and Iskandar Sitdikov and In-Saeng Suh and Eric Switzer and Wei Tang and Joel Thompson and Synge Todo and Minh Tran and Dimitar Trenev and Christian Trott and Huan-Hsin Tseng and Esin Tureci and David Garc{\'\i}a Valinas and Sofia Vallecorsa and Christopher Wever and Konrad Wojciechowski and Xiaodi Wu and Shinjae Yoo and Nobuyuki Yoshioka and Victor Wen-zhe Yu and Seiji Yunoki and Sergiy Zhuk and Dmitry Zubarev} } @article {3412, title = {A quantum-classical performance separation in nonconvex optimization}, year = {2023}, month = {11/1/2023}, abstract = {

In this paper, we identify a family of nonconvex continuous optimization instances, each d-dimensional instance with 2d local minima, to demonstrate a quantum-classical performance separation. Specifically, we prove that the recently proposed Quantum Hamiltonian Descent (QHD) algorithm [Leng et al., arXiv:2303.01471] is able to solve any d-dimensional instance from this family using O\˜(d3) quantum queries to the function value and O\˜(d4) additional 1-qubit and 2-qubit elementary quantum gates. On the other side, a comprehensive empirical study suggests that representative state-of-the-art classical optimization algorithms/solvers (including Gurobi) would require a super-polynomial time to solve such optimization instances.

}, url = {https://arxiv.org/abs/2311.00811}, author = {Jiaqi Leng and Yufan Zheng and Xiaodi Wu} } @article {3270, title = {A theory of quantum differential equation solvers: limitations and fast-forwarding}, year = {2023}, month = {3/2/2023}, abstract = {

We study the limitations and fast-forwarding of quantum algorithms for linear ordinary differential equation (ODE) systems with a particular focus on non-quantum dynamics, where the coefficient matrix in the ODE is not anti-Hermitian or the ODE is inhomogeneous. On the one hand, for generic homogeneous linear ODEs, by proving worst-case lower bounds, we show that quantum algorithms suffer from computational overheads due to two types of {\textquoteleft}{\textquoteleft}non-quantumness\&$\#$39;\&$\#$39;: real part gap and non-normality of the coefficient matrix. We then show that homogeneous ODEs in the absence of both types of {\textquoteleft}{\textquoteleft}non-quantumness\&$\#$39;\&$\#$39; are equivalent to quantum dynamics, and reach the conclusion that quantum algorithms for quantum dynamics work best. We generalize our results to the inhomogeneous case and find that existing generic quantum ODE solvers cannot be substantially improved. To obtain these lower bounds, we propose a general framework for proving lower bounds on quantum algorithms that are amplifiers, meaning that they amplify the difference between a pair of input quantum states. On the other hand, we show how to fast-forward quantum algorithms for solving special classes of ODEs which leads to improved efficiency. More specifically, we obtain quadratic improvements in the evolution time T for inhomogeneous ODEs with a negative semi-definite coefficient matrix, and exponential improvements in both T and the spectral norm of the coefficient matrix for inhomogeneous ODEs with efficiently implementable eigensystems, including various spatially discretized linear evolutionary partial differential equations. We give fast-forwarding algorithms that are conceptually different from existing ones in the sense that they neither require time discretization nor solving high-dimensional linear systems.

}, url = {https://arxiv.org/abs/2211.05246}, author = {Dong An and Jin-Peng Liu and Daochen Wang and Qi Zhao} } @article {3082, title = {Closing the Locality and Detection Loopholes in Multiparticle Entanglement Self-Testing}, journal = {Physical Review Letters}, volume = {128}, year = {2022}, month = {06/23/2022}, pages = {250401}, abstract = {

First proposed by Mayers and Yao, self-testing provides a certification method to infer the underlying physics of quantum experiments in a black-box scenario. Numerous demonstrations have been reported to self-test various types of entangled states. However, all the multiparticle self-testing experiments reported so far suffer from both detection and locality loopholes. Here, we report the first experimental realization of multiparticle entanglement self-testing closing the locality loophole in a photonic system, and the detection loophole in a superconducting system, respectively. We certify three-party and four-party GHZ states with at least 0.84 (1) and 0.86 (3) fidelities in a device-independent way. These results can be viewed as a meaningful advance in multiparticle loophole-free self-testing, and also significant progress on the foundations of quantum entanglement certification.

}, doi = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.250401}, url = {https://www.researchgate.net/profile/Dian-Wu/publication/361497881_Closing_the_Locality_and_Detection_Loopholes_in_Multiparticle_Entanglement_Self-Testing/links/62b55a8c1010dc02cc57530c/Closing-the-Locality-and-Detection-Loopholes-in-Multiparticle-Entangl}, author = {Dian Wu and Qi Zhao and Can Wang and Liang Huang and Yang-Fan Jiang and Bing Bai and You Zhou and Xue-Mei Gu and Feng-Ming Liu and Ying-Qiu Mao and Qi-Chao Sun and Ming-Cheng Chen and Jun Zhang and Cheng-Zhi Peng and Xiao-Bo Zhu and Qiang Zhang and Chao-Yang Lu and Jian-Wei Pan} } @article {3144, title = {Codimension-2 defects and higher symmetries in (3+1)D topological phases}, year = {2022}, month = {8/15/2022}, abstract = {

(3+1)D topological phases of matter can host a broad class of non-trivial topological defects of codimension-1, 2, and 3, of which the well-known point charges and flux loops are special cases. The complete algebraic structure of these defects defines a higher category, and can be viewed as an emergent higher symmetry. This plays a crucial role both in the classification of phases of matter and the possible fault-tolerant logical operations in topological quantum error correcting codes. In this paper, we study several examples of such higher codimension defects from distinct perspectives. We mainly study a class of invertible codimension-2 topological defects, which we refer to as twist strings. We provide a number of general constructions for twist strings, in terms of gauging lower dimensional invertible phases, layer constructions, and condensation defects. We study some special examples in the context of Z2 gauge theory with fermionic charges, in Z2\×Z2 gauge theory with bosonic charges, and also in non-Abelian discrete gauge theories based on dihedral (Dn) and alternating (A6) groups. The intersection between twist strings and Abelian flux loops sources Abelian point charges, which defines an H4 cohomology class that characterizes part of an underlying 3-group symmetry of the topological order. The equations involving background gauge fields for the 3-group symmetry have been explicitly written down for various cases. We also study examples of twist strings interacting with non-Abelian flux loops (defining part of a non-invertible higher symmetry), examples of non-invertible codimension-2 defects, and examples of interplay of codimension-2 defects with codimension-1 defects. We also find an example of geometric, not fully topological, twist strings in (3+1)D A6 gauge theory.

}, keywords = {FOS: Mathematics, FOS: Physical sciences, High Energy Physics - Theory (hep-th), Mathematical Physics (math-ph), Quantum Algebra (math.QA), Quantum Physics (quant-ph), Strongly Correlated Electrons (cond-mat.str-el)}, doi = {10.48550/ARXIV.2208.07367}, url = {https://arxiv.org/abs/2208.07367}, author = {Barkeshli, Maissam and Chen, Yu-An and Huang, Sheng-Jie and Kobayashi, Ryohei and Tantivasadakarn, Nathanan and Zhu, Guanyu} } @article {3080, title = {Combining machine learning with physics: A framework for tracking and sorting multiple dark solitons}, journal = {Phys. Rev. Research}, volume = {4}, year = {2022}, month = {06/01/2022}, pages = {023163 }, abstract = {

In ultracold-atom experiments, data often comes in the form of images which suffer information loss inherent in the techniques used to prepare and measure the system. This is particularly problematic when the processes of interest are complicated, such as interactions among excitations in Bose-Einstein condensates (BECs). In this paper, we describe a framework combining machine learning (ML) models with physics-based traditional analyses to identify and track multiple solitonic excitations in images of BECs. We use an ML-based object detector to locate the solitonic excitations and develop a physics-informed classifier to sort solitonic excitations into physically motivated subcategories. Lastly, we introduce a quality metric quantifying the likelihood that a specific feature is a longitudinal soliton. Our trained implementation of this framework, SolDet, is publicly available as an open-source python package. SolDet is broadly applicable to feature identification in cold-atom images when trained on a suitable user-provided dataset.

}, doi = {https://doi.org/10.1103/PhysRevResearch.4.023163}, url = {https://arxiv.org/abs/2111.04881}, author = {Shangjie Guo and Sophia M. Koh and Amilson R. Fritsch and I. B. Spielman and Justyna P. Zwolak} } @article {3021, title = {Computational self-testing of multi-qubit states and measurements}, year = {2022}, month = {1/31/2022}, abstract = {

Self-testing is a fundamental technique within quantum information theory that allows a classical verifier to force (untrusted) quantum devices to prepare certain states and perform certain measurements on them. The standard approach assumes at least two spatially separated devices. Recently, Metger and Vidick [Quantum, 2021] showed that a single EPR pair of a single quantum device can be self-tested under standard computational assumptions. In this work, we generalize their techniques to give the first protocol that self-tests N EPR pairs and measurements in the single-device setting under the same computational assumptions. We show that our protocol can be passed with probability negligibly close to 1 by an honest quantum device using poly(N) resources. Moreover, we show that any quantum device that fails our protocol with probability at most ϵ must be poly(N,ϵ)-close to being honest in the appropriate sense. In particular, a simplified version of our protocol is the first that can efficiently certify an arbitrary number of qubits of a cloud quantum computer, on which we cannot enforce spatial separation, using only classical communication.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2201.13430}, url = {https://arxiv.org/abs/2201.13430}, author = {Fu, Honghao and Daochen Wang and Zhao, Qi} } @article {3079, title = {Dark Solitons in Bose-Einstein Condensates: A Dataset for Many-body Physics Research}, year = {2022}, month = {05/17/2022}, abstract = {

We establish a dataset of over 1.6\×104 experimental images of Bose-Einstein condensates containing solitonic excitations to enable machine learning (ML) for many-body physics research. About 33 \% of this dataset has manually assigned and carefully curated labels. The remainder is automatically labeled using SolDet -- an implementation of a physics-informed ML data analysis framework -- consisting of a convolutional-neural-network-based classifier and object detector as well as a statistically motivated physics-informed classifier and a quality metric. This technical note constitutes the definitive reference of the dataset, providing an opportunity for the data science community to develop more sophisticated analysis tools, to further understand nonlinear many-body physics, and even advance cold atom experiments.

}, url = {https://arxiv.org/abs/2205.09114}, author = {Amilson R. Fritsch and Shangjie Guo and Sophia M. Koh and I. B. Spielman and Justyna P. Zwolak} } @article {3013, title = {Deconfinement and Error Thresholds in Holography}, year = {2022}, month = {2/9/2022}, abstract = {

We study the error threshold properties of holographic quantum error-correcting codes. We demonstrate that holographic CFTs admit an algebraic threshold, which is related to the confinement-deconfinement phase transition. We then apply geometric intuition from holography and the Hawking-Page phase transition to motivate the CFT result, and comment on potential extensions to other confining theories.

}, keywords = {FOS: Physical sciences, High Energy Physics - Theory (hep-th), Nuclear Theory (nucl-th), Quantum Physics (quant-ph), Strongly Correlated Electrons (cond-mat.str-el)}, doi = {10.48550/ARXIV.2202.04710}, url = {https://arxiv.org/abs/2202.04710}, author = {Bao, Ning and Cao, Charles and Zhu, Guanyu} } @article {3205, title = {Efficient and practical quantum compiler towards multi-qubit systems with deep reinforcement learning}, year = {2022}, month = {4/14/2022}, abstract = {

Efficient quantum compiling tactics greatly enhance the capability of quantum computers to execute complicated quantum algorithms. Due to its fundamental importance, a plethora of quantum compilers has been designed in past years. However, there are several caveats to current protocols, which are low optimality, high inference time, limited scalability, and lack of universality. To compensate for these defects, here we devise an efficient and practical quantum compiler assisted by advanced deep reinforcement learning (RL) techniques, i.e., data generation, deep Q-learning, and AQ* search. In this way, our protocol is compatible with various quantum machines and can be used to compile multi-qubit operators. We systematically evaluate the performance of our proposal in compiling quantum operators with both inverse-closed and inverse-free universal basis sets. In the task of single-qubit operator compiling, our proposal outperforms other RL-based quantum compilers in the measure of compiling sequence length and inference time. Meanwhile, the output solution is near-optimal, guaranteed by the Solovay-Kitaev theorem. Notably, for the inverse-free universal basis set, the achieved sequence length complexity is comparable with the inverse-based setting and dramatically advances previous methods. These empirical results contribute to improving the inverse-free Solovay-Kitaev theorem. In addition, for the first time, we demonstrate how to leverage RL-based quantum compilers to accomplish two-qubit operator compiling. The achieved results open an avenue for integrating RL with quantum compiling to unify efficiency and practicality and thus facilitate the exploration of quantum advantages.

}, keywords = {FOS: Computer and information sciences, FOS: Physical sciences, Machine Learning (cs.LG), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2204.06904}, url = {https://arxiv.org/abs/2204.06904}, author = {Chen, Qiuhao and Du, Yuxuan and Zhao, Qi and Jiao, Yuling and Lu, Xiliang and Wu, Xingyao} } @article {3139, title = {Experimental Implementation of an Efficient Test of Quantumness}, year = {2022}, month = {9/28/2022}, abstract = {

A test of quantumness is a protocol where a classical user issues challenges to a quantum device to determine if it exhibits non-classical behavior, under certain cryptographic assumptions. Recent attempts to implement such tests on current quantum computers rely on either interactive challenges with efficient verification, or non-interactive challenges with inefficient (exponential time) verification. In this paper, we execute an efficient non-interactive test of quantumness on an ion-trap quantum computer. Our results significantly exceed the bound for a classical device\&$\#$39;s success.

}, keywords = {FOS: Physical sciences, Other Condensed Matter (cond-mat.other), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2209.14316}, url = {https://arxiv.org/abs/2209.14316}, author = {Lewis, Laura and Zhu, Daiwei and Gheorghiu, Alexandru and Noel, Crystal and Katz, Or and Harraz, Bahaa and Wang, Qingfeng and Risinger, Andrew and Feng, Lei and Biswas, Debopriyo and Egan, Laird and Vidick, Thomas and Cetina, Marko and Monroe, Christopher} } @article {2913, title = {Hamiltonian simulation with random inputs}, journal = {Phys. Rev. Lett. 129, 270502}, volume = {129}, year = {2022}, month = {12/30/2022}, abstract = {

The algorithmic error of digital quantum simulations is usually explored in terms of the spectral norm distance between the actual and ideal evolution operators. In practice, this worst-case error analysis may be unnecessarily pessimistic. To address this, we develop a theory of average-case performance of Hamiltonian simulation with random initial states. We relate the average-case error to the Frobenius norm of the multiplicative error and give upper bounds for the product formula (PF) and truncated Taylor series methods. As applications, we estimate average-case error for digital Hamiltonian simulation of general lattice Hamiltonians and k-local Hamiltonians. In particular, for the nearest-neighbor Heisenberg chain with n spins, the error is quadratically reduced from O(n) in the worst case to O(n\−\−\√) on average for both the PF method and the Taylor series method. Numerical evidence suggests that this theory accurately characterizes the average error for concrete models. We also apply our results to error analysis in the simulation of quantum scrambling.

}, doi = {https://doi.org/10.1103/PhysRevLett.129.270502}, url = {https://arxiv.org/abs/2111.04773}, author = {Qi Zhao and You Zhou and Alexander F. Shaw and Tongyang Li and Andrew M. Childs} } @article {3070, title = {Infinite-randomness criticality in monitored quantum dynamics with static disorder}, year = {2022}, month = {5/27/2022}, abstract = {

We consider a model of monitored quantum dynamics with quenched spatial randomness: specifically, random quantum circuits with spatially varying measurement rates. These circuits undergo a measurement-induced phase transition (MIPT) in their entanglement structure, but the nature of the critical point differs drastically from the case with constant measurement rate. In particular, at the critical measurement rate, we find that the entanglement of a subsystem of size l scales as S\∼l\√; moreover, the dynamical critical exponent z=\∞. The MIPT is flanked by Griffiths phases with continuously varying dynamical exponents. We argue for this infinite-randomness scenario on general grounds and present numerical evidence that it captures some features of the universal critical properties of MIPT using large-scale simulations of Clifford circuits. These findings demonstrate that the relevance and irrelevance of perturbations to the MIPT can naturally be interpreted using a powerful heuristic known as the Harris criterion.\ 

}, url = {https://arxiv.org/abs/2205.14002}, author = {Aidan Zabalo and Justin H. Wilson and Michael Gullans and Romain Vasseur and Sarang Gopalakrishnan and David A. Huse and J. H. Pixley} } @article {3014, title = {Machine-assisted discovery of integrable symplectic mappings}, year = {2022}, month = {3/22/2022}, abstract = {

We present a new automated method for finding integrable symplectic maps of the plane. These dynamical systems possess a hidden symmetry associated with an existence of conserved quantities, i.e. integrals of motion. The core idea of the algorithm is based on the knowledge that the evolution of an integrable system in the phase space is restricted to a lower-dimensional submanifold. Limiting ourselves to polygon invariants of motion, we analyze the shape of individual trajectories thus successfully distinguishing integrable motion from chaotic cases. For example, our method rediscovers some of the famous McMillan-Suris integrable mappings and discrete Painlev{\'e} equations. In total, over 100 new integrable families are presented and analyzed; some of them are isolated in the space of parameters, and some of them are families with one parameter (or the ratio of parameters) being continuous or discrete. At the end of the paper, we suggest how newly discovered maps are related to a general 2D symplectic map via an introduction of discrete perturbation theory and propose a method on how to construct smooth near-integrable dynamical systems based on mappings with polygon invariants.

}, keywords = {Accelerator Physics (physics.acc-ph), Adaptation and Self-Organizing Systems (nlin.AO), Exactly Solvable and Integrable Systems (nlin.SI), FOS: Physical sciences}, doi = {10.48550/ARXIV.2201.13133}, url = {https://arxiv.org/abs/2201.13133}, author = {Zolkin, Timofey and Kharkov, Yaroslav and Nagaitsev, Sergei} } @article {3006, title = {Operator Scaling Dimensions and Multifractality at Measurement-Induced Transitions}, journal = {Physical Review Letters}, volume = {128}, year = {2022}, month = {2/11/2022}, abstract = {

Repeated local measurements of quantum many body systems can induce a phase transition in their entanglement structure. These measurement-induced phase transitions (MIPTs) have been studied for various types of dynamics, yet most cases yield quantitatively similar values of the critical exponents, making it unclear if there is only one underlying universality class. Here, we directly probe the properties of the conformal field theories governing these MIPTs using a numerical transfer-matrix method, which allows us to extract the effective central charge, as well as the first few low-lying scaling dimensions of operators at these critical points. Our results provide convincing evidence that the generic and Clifford MIPTs for qubits lie in different universality classes and that both are distinct from the percolation transition for qudits in the limit of large onsite Hilbert space dimension. For the generic case, we find strong evidence of multifractal scaling of correlation functions at the critical point, reflected in a continuous spectrum of scaling dimensions.

}, doi = {10.1103/physrevlett.128.050602}, url = {https://arxiv.org/abs/2107.03393}, author = {Aidan Zabalo and Michael Gullans and Justin H. Wilson and Romain Vasseur and Andreas W. W. Ludwig and Sarang Gopalakrishnan and David A. Huse and J. H. Pixley} } @article {3150, title = {Quantum Algorithms for Sampling Log-Concave Distributions and Estimating Normalizing Constants}, journal = {Advances in Neural Information Processing Systems (NeurIPS 2022)}, volume = {35}, year = {2022}, month = {10/12/2022}, keywords = {FOS: Computer and information sciences, FOS: Mathematics, FOS: Physical sciences, Machine Learning (cs.LG), Optimization and Control (math.OC), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2210.06539}, url = {https://arxiv.org/abs/2210.06539}, author = {Andrew M. Childs and Li, Tongyang and Liu, Jin-Peng and Wang, Chunhao and Zhang, Ruizhe} } @article {3191, title = {Quantum Natural Proof: A New Perspective of Hybrid Quantum-Classical Program Verification}, year = {2022}, month = {11/11/2022}, abstract = {

Many quantum programs are assured by formal verification, but such verification is usually laborious and time-consuming. This paper proposes quantum natural proof (QNP), an automated proof system for verifying hybrid quantum-classical algorithms. Natural proofs are a subclass of proofs that are amenable to completely automated reasoning, provide sound but incomplete procedures, and capture common reasoning tactics in program verification. The core of QNP is a type-guided quantum proof system, named Qafny, which views quantum operations as some classical array operations that can be modeled as proof rules in a classical separation logic framework, suitable for automated reasoning. We proved the soundness and completeness of the Qafny proof system as well as the soundness of the proof system compilation from Qafny to Dafny. By using the QNP implementation in Dafny, automated verification can be efficiently perform for many hybrid quantum-classical algorithms, including GHZ, Shor\&$\#$39;s, Grover\&$\#$39;s, and quantum walk algorithms, which saves a great amount of human efforts. In addition, quantum programs written in Qafny can be compiled to quantum circuits so that every verified quantum program can be run on a quantum machine.

}, keywords = {FOS: Computer and information sciences, FOS: Physical sciences, Programming Languages (cs.PL), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2211.06411}, url = {https://arxiv.org/abs/2211.06411}, author = {Li, Liyi and Zhu, Mingwei and Lee, Yi and Chang, Le and Wu, Xiaodi} } @article {2997, title = {Quantum Simulation for High Energy Physics}, year = {2022}, month = {4/7/2022}, abstract = {

It is for the first time that Quantum Simulation for High Energy Physics (HEP) is studied in the U.S. decadal particle-physics community planning, and in fact until recently, this was not considered a mainstream topic in the community. This fact speaks of a remarkable rate of growth of this subfield over the past few years, stimulated by the impressive advancements in Quantum Information Sciences (QIS) and associated technologies over the past decade, and the significant investment in this area by the government and private sectors in the U.S. and other countries. High-energy physicists have quickly identified problems of importance to our understanding of nature at the most fundamental level, from tiniest distances to cosmological extents, that are intractable with classical computers but may benefit from quantum advantage. They have initiated, and continue to carry out, a vigorous program in theory, algorithm, and hardware co-design for simulations of relevance to the HEP mission. This community whitepaper is an attempt to bring this exciting and yet challenging area of research to the spotlight, and to elaborate on what the promises, requirements, challenges, and potential solutions are over the next decade and beyond.

}, keywords = {FOS: Physical sciences, High Energy Physics - Lattice (hep-lat), High Energy Physics - Phenomenology (hep-ph), High Energy Physics - Theory (hep-th), Nuclear Theory (nucl-th), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2204.03381}, url = {https://arxiv.org/abs/2204.03381}, author = {Bauer, Christian W. and Zohreh Davoudi and Balantekin, A. Baha and Bhattacharya, Tanmoy and Carena, Marcela and de Jong, Wibe A. and Draper, Patrick and El-Khadra, Aida and Gemelke, Nate and Hanada, Masanori and Kharzeev, Dmitri and Lamm, Henry and Li, Ying-Ying and Liu, Junyu and Lukin, Mikhail and Meurice, Yannick and Monroe, Christopher and Nachman, Benjamin and Pagano, Guido and Preskill, John and Rinaldi, Enrico and Roggero, Alessandro and Santiago, David I. and Savage, Martin J. and Siddiqi, Irfan and Siopsis, George and Van Zanten, David and Wiebe, Nathan and Yamauchi, Yukari and Yeter-Aydeniz, K{\"u}bra and Zorzetti, Silvia} } @article {3187, title = {Quantum simulation of real-space dynamics}, journal = {Quantum}, volume = {6}, year = {2022}, month = {11/8/2022}, pages = {860}, abstract = {

Quantum simulation is a prominent application of quantum computers. While there is extensive previous work on simulating finite-dimensional systems, less is known about quantum algorithms for real-space dynamics. We conduct a systematic study of such algorithms. In particular, we show that the dynamics of a d-dimensional Schr{\"o}dinger equation with η particles can be simulated with gate complexity O~(ηdFpoly(log(g\′/ϵ))), where ϵ is the discretization error, g\′ controls the higher-order derivatives of the wave function, and F measures the time-integrated strength of the potential. Compared to the best previous results, this exponentially improves the dependence on ϵ and g\′ from poly(g\′/ϵ) to poly(log(g\′/ϵ)) and polynomially improves the dependence on T and d, while maintaining best known performance with respect to η. For the case of Coulomb interactions, we give an algorithm using η3(d+η)Tpoly(log(ηdTg\′/(Δϵ)))/Δ one- and two-qubit gates, and another using η3(4d)d/2Tpoly(log(ηdTg\′/(Δϵ)))/Δ one- and two-qubit gates and QRAM operations, where T is the evolution time and the parameter Δ regulates the unbounded Coulomb interaction. We give applications to several computational problems, including faster real-space simulation of quantum chemistry, rigorous analysis of discretization error for simulation of a uniform electron gas, and a quadratic improvement to a quantum algorithm for escaping saddle points in nonconvex optimization.

}, doi = {https://doi.org/10.22331/q-2022-11-17-860}, url = {https://doi.org/10.22331\%2Fq-2022-11-17-860}, author = {Andrew M. Childs and Jiaqi Leng and Tongyang Li and Jin-Peng Liu and Chenyi Zhang} } @article {3204, title = {A scheme to create and verify scalable entanglement in optical lattice}, journal = {npj Quantum Information}, volume = {8}, year = {2022}, month = {9/4/2022}, abstract = {

To achieve scalable quantum information processing, great efforts have been devoted to the creation of large-scale entangled states in various physical systems. Ultracold atom in optical lattice is considered as one of the promising platforms due to its feasible initialization and parallel manipulation. In this work, we propose an efficient scheme to generate and characterize global entanglement in the optical lattice. With only two-layer quantum circuits, the generation utilizes two-qubit entangling gates based on the superexchange interaction in double wells. The parallelism of these operations enables the generation to be fast and scalable. To verify the entanglement of this non-stabilizer state, we mainly design three complementary detection protocols which are less resource-consuming compared to the full tomography. In particular, one just needs two homogenous local measurement settings to identify the entanglement property. Our entanglement generation and verification protocols provide the foundation for the further quantum information processing in optical lattice.

}, doi = {10.1038/s41534-022-00609-0}, url = {https://arxiv.org/abs/2209.01531}, author = {You Zhou and Bo Xiao and Meng-Da Li and Qi Zhao and Zhen-Sheng Yuan and Xiongfeng Ma and Jian-Wei Pan} } @article {3009, title = {Self-Testing of a Single Quantum System: Theory and Experiment}, year = {2022}, month = {3/17/2022}, abstract = {

Certifying individual quantum devices with minimal assumptions is crucial for the development of quantum technologies. Here, we investigate how to leverage single-system contextuality to realize self-testing. We develop a robust self-testing protocol based on the simplest contextuality witness for the simplest contextual quantum system, the Klyachko-Can-Binicio{\u g}lu-Shumovsky (KCBS) inequality for the qutrit. We establish a lower bound on the fidelity of the state and the measurements (to an ideal configuration) as a function of the value of the witness under a pragmatic assumption on the measurements we call the KCBS orthogonality condition. We apply the method in an experiment with randomly chosen measurements on a single trapped 40Ca+ and near-perfect detection efficiency. The observed statistics allow us to self-test the system and provide the first experimental demonstration of quantum self-testing of a single system. Further, we quantify and report that deviations from our assumptions are minimal, an aspect previously overlooked by contextuality experiments.

}, keywords = {Atomic Physics (physics.atom-ph), FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {https://doi.org/10.48550/arXiv.2203.09003}, url = {https://arxiv.org/abs/2203.09003}, author = {Hu, Xiao-Min and Xie, Yi and Arora, Atul Singh and Ai, Ming-Zhong and Bharti, Kishor and Zhang, Jie and Wu, Wei and Chen, Ping-Xing and Cui, Jin-Ming and Liu, Bi-Heng and Huang, Yun-Feng and Li, Chuan-Feng and Guo, Guang-Can and Roland, J{\'e}r{\'e}mie and Cabello, Ad{\'a}n and Kwek, Leong-Chuan} } @article {3017, title = {Shadow Distillation: Quantum Error Mitigation with Classical Shadows for Near-Term Quantum Processors}, year = {2022}, month = {3/14/2022}, abstract = {

Mitigating errors in quantum information processing devices is especially important in the absence of fault tolerance. An effective method in suppressing state-preparation errors is using multiple copies to distill the ideal component from a noisy quantum state. Here, we use classical shadows and randomized measurements to circumvent the need for coherent access to multiple copies at an exponential cost. We study the scaling of resources using numerical simulations and find that the overhead is still favorable compared to full state tomography. We optimize measurement resources under realistic experimental constraints and apply our method to an experiment preparing Greenberger-Horne-Zeilinger (GHZ) state with trapped ions. In addition to improving stabilizer measurements, the analysis of the improved results reveals the nature of errors affecting the experiment. Hence, our results provide a directly applicable method for mitigating errors in near-term quantum computers.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2203.07309}, url = {https://arxiv.org/abs/2203.07309}, author = {Seif, Alireza and Cian, Ze-Pei and Zhou, Sisi and Chen, Senrui and Jiang, Liang} } @article {2765, title = {Theoretical bounds on data requirements for the ray-based classification}, journal = {SN Comput. Sci.}, volume = {3}, year = {2022}, month = {02/26/2022}, abstract = {

The problem of classifying high-dimensional shapes in real-world data grows in complexity as the dimension of the space increases. For the case of identifying convex shapes of different geometries, a new classification framework has recently been proposed in which the intersections of a set of one-dimensional representations, called rays, with the boundaries of the shape are used to identify the specific geometry. This ray-based classification (RBC) has been empirically verified using a synthetic dataset of two- and three-dimensional shapes [1] and, more recently, has also been validated experimentally [2]. Here, we establish a bound on the number of rays necessary for shape classification, defined by key angular metrics, for arbitrary convex shapes. For two dimensions, we derive a lower bound on the number of rays in terms of the shape\&$\#$39;s length, diameter, and exterior angles. For convex polytopes in R^N, we generalize this result to a similar bound given as a function of the dihedral angle and the geometrical parameters of polygonal faces. This result enables a different approach for estimating high-dimensional shapes using substantially fewer data elements than volumetric or surface-based approaches.

}, doi = {https://doi.org/10.1007/s42979-021-00921-0}, url = {https://arxiv.org/abs/2103.09577}, author = {Brian J. Weber and Sandesh S. Kalantre and Thomas McJunkin and J. M. Taylor and Justyna P. Zwolak} } @article {3192, title = {A theory of quantum differential equation solvers: limitations and fast-forwarding}, year = {2022}, month = {11/9/2022}, abstract = {

We study the limitations and fast-forwarding of quantum algorithms for solving linear ordinary differential equation (ODE) systems with particular focus on non-quantum dynamics, where the coefficient matrix in the ODE is not anti-Hermitian or the ODE is inhomogeneous. On the one hand, for generic homogeneous linear ODEs, by proving worst-case lower bounds, we show that quantum algorithms suffer from computational overheads due to two types of {\textquoteleft}{\textquoteleft}non-quantumness\&$\#$39;\&$\#$39;: real part gap and non-normality of the coefficient matrix. We then show that ODEs in the absence of both types of {\textquoteleft}{\textquoteleft}non-quantumness\&$\#$39;\&$\#$39; are equivalent to quantum dynamics, and reach the conclusion that quantum algorithms for quantum dynamics work best. We generalize our results to the inhomogeneous case and find that existing generic quantum ODE solvers cannot be substantially improved. To obtain these lower bounds, we propose a general framework for proving lower bounds on quantum algorithms that are amplifiers, meaning that they amplify the difference between a pair of input quantum states. On the other hand, we show how to fast-forward quantum algorithms for solving special classes of ODEs which leads to improved efficiency. More specifically, we obtain quadratic to exponential improvements in terms of the evolution time T and the spectral norm of the coefficient matrix for the following classes of ODEs: inhomogeneous ODEs with a negative definite coefficient matrix, inhomogeneous ODEs with a coefficient matrix having an eigenbasis that can be efficiently prepared on a quantum computer and eigenvalues that can be efficiently computed classically, and the spatially discretized inhomogeneous heat equation and advection-diffusion equation. We give fast-forwarding algorithms that are conceptually different from existing ones in the sense that they neither require time discretization nor solving high-dimensional linear systems.

}, keywords = {FOS: Mathematics, FOS: Physical sciences, Numerical Analysis (math.NA), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2211.05246}, url = {https://arxiv.org/abs/2211.05246}, author = {Dong An and Liu, Jin-Peng and Wang, Daochen and Zhao, Qi} } @article {3005, title = {Toward Robust Autotuning of Noisy Quantum dot Devices}, journal = {Physical Review Applied}, volume = {17}, year = {2022}, month = {02/26/2022}, abstract = {

The current autotuning approaches for quantum dot (QD) devices, while showing some success, lack an assessment of data reliability. This leads to unexpected failures when noisy or otherwise low-quality data is processed by an autonomous system. In this work, we propose a framework for robust autotuning of QD devices that combines a machine learning (ML) state classifier with a data quality control module. The data quality control module acts as a \"gatekeeper\" system, ensuring that only reliable data are processed by the state classifier. Lower data quality results in either device recalibration or termination. To train both ML systems, we enhance the QD simulation by incorporating synthetic noise typical of QD experiments. We confirm that the inclusion of synthetic noise in the training of the state classifier significantly improves the performance, resulting in an accuracy of 95.0(9) \% when tested on experimental data. We then validate the functionality of the data quality control module by showing that the state classifier performance deteriorates with decreasing data quality, as expected. Our results establish a robust and flexible ML framework for autonomous tuning of noisy QD devices.

}, doi = {https://doi.org/10.1103/PhysRevApplied.17.024069}, url = {https://arxiv.org/abs/2108.00043}, author = {Joshua Ziegler and Thomas McJunkin and E.S. Joseph and Sandesh S. Kalantre and Benjamin Harpt and D.E. Savage and M.G. Lagally and M.A. Eriksson and Jacob M. Taylor and Justyna P. Zwolak} } @article {2919, title = {Cross-Platform Comparison of Arbitrary Quantum Computations}, year = {2021}, month = {7/27/2021}, abstract = {

As we approach the era of quantum advantage, when quantum computers (QCs) can outperform any classical computer on particular tasks, there remains the difficult challenge of how to validate their performance. While algorithmic success can be easily verified in some instances such as number factoring or oracular algorithms, these approaches only provide pass/fail information for a single QC. On the other hand, a comparison between different QCs on the same arbitrary circuit provides a lower-bound for generic validation: a quantum computation is only as valid as the agreement between the results produced on different QCs. Such an approach is also at the heart of evaluating metrological standards such as disparate atomic clocks. In this paper, we report a cross-platform QC comparison using randomized and correlated measurements that results in a wealth of information on the QC systems. We execute several quantum circuits on widely different physical QC platforms and analyze the cross-platform fidelities.

}, url = {https://arxiv.org/abs/2107.11387}, author = {Daiwei Zhu and Ze-Pei Cian and Crystal Noel and Andrew Risinger and Debopriyo Biswas and Laird Egan and Yingyue Zhu and Alaina M. Green and Cinthia Huerta Alderete and Nhung H. Nguyen and Qingfeng Wang and Andrii Maksymov and Yunseong Nam and Marko Cetina and Norbert M. Linke and Mohammad Hafezi and Christopher Monroe} } @article {2631, title = {Device-independent Randomness Expansion with Entangled Photons}, journal = {Nat. Phys. }, year = {2021}, month = {01/28/2021}, abstract = {

With the growing availability of experimental loophole-free Bell tests, it has become possible to implement a new class of device-independent random number generators whose output can be certified to be uniformly random without requiring a detailed model of the quantum devices used. However, all of these experiments require many input bits in order to certify a small number of output bits, and it is an outstanding challenge to develop a system that generates more randomness than is used. Here, we devise a device-independent spot-checking protocol which uses only uniform bits as input. Implemented with a photonic loophole-free Bell test, we can produce 24\% more certified output bits (1,181,264,237) than consumed input bits (953,301,640), which is 5 orders of magnitude more efficient than our previous work [arXiv:1812.07786]. The experiment ran for 91.0 hours, creating randomness at an average rate of 3606 bits/s with a soundness error bounded by 5.7\×10\−7 in the presence of classical side information. Our system will allow for greater trust in public sources of randomness, such as randomness beacons, and the protocols may one day enable high-quality sources of private randomness as the device footprint shrinks.

}, doi = {https://doi.org/10.1038/s41567-020-01153-4}, url = {https://arxiv.org/abs/1912.11158}, author = {Lynden K. Shalm and Yanbao Zhang and Joshua C. Bienfang and Collin Schlager and Martin J. Stevens and Michael D. Mazurek and Carlos Abell{\'a}n and Waldimar Amaya and Morgan W. Mitchell and Mohammad A. Alhejji and Honghao Fu and Joel Ornstein and Richard P. Mirin and Sae Woo Nam and Emanuel Knill} } @article {2881, title = {EasyPQC: Verifying Post-Quantum Cryptography}, journal = {ACM CCS 2021}, year = {2021}, month = {9/20/2021}, abstract = {

EasyCrypt is a formal verification tool used extensively for formalizing concrete security proofs of cryptographic constructions. However, the EasyCrypt formal logics consider only classical attackers, which means that post-quantum security proofs cannot be formalized and machine-checked with this tool. In this paper we prove that a natural extension of the EasyCrypt core logics permits capturing a wide class of post-quantum cryptography proofs, settling a question raised by (Unruh, POPL 2019). Leveraging our positive result, we implement EasyPQC, an extension of EasyCrypt for post-quantum security proofs, and use EasyPQC to verify post-quantum security of three classic constructions: PRF-based MAC, Full Domain Hash and GPV08 identity-based encryption.

}, doi = {https://dx.doi.org/10.1145/3460120.3484567}, author = {Manuel Barbosa and Gilles Barthe and Xiong Fan and Benjamin Gr{\'e}goire and Shih-Han Hung and Jonathan Katz and Pierre-Yves Strub and Xiaodi Wu and Li Zhou} } @article {2756, title = {Exploiting anticommutation in Hamiltonian simulation}, year = {2021}, month = {3/14/2021}, abstract = {

Quantum computing can efficiently simulate Hamiltonian dynamics of many-body quantum physics, a task that is generally intractable with classical computers. The hardness lies at the ubiquitous anti-commutative relations of quantum operators, in corresponding with the notorious negative sign problem in classical simulation. Intuitively, Hamiltonians with more commutative terms are also easier to simulate on a quantum computer, and anti-commutative relations generally cause more errors, such as in the product formula method. Here, we theoretically explore the role of anti-commutative relation in Hamiltonian simulation. We find that, contrary to our intuition, anti-commutative relations could also reduce the hardness of Hamiltonian simulation. Specifically, Hamiltonians with mutually anti-commutative terms are easy to simulate, as what happens with ones consisting of mutually commutative terms. Such a property is further utilized to reduce the algorithmic error or the gate complexity in the truncated Taylor series quantum algorithm for general problems. Moreover, we propose two modified linear combinations of unitaries methods tailored for Hamiltonians with different degrees of anti-commutation. We numerically verify that the proposed methods exploiting anti-commutative relations could significantly improve the simulation accuracy of electronic Hamiltonians. Our work sheds light on the roles of commutative and anti-commutative relations in simulating quantum systems.

}, url = {https://arxiv.org/abs/2103.07988}, author = {Qi Zhao and Xiao Yuan} } @article {2908, title = {Interactive Protocols for Classically-Verifiable Quantum Advantage}, year = {2021}, month = {12/9/2021}, abstract = {

Achieving quantum computational advantage requires solving a classically intractable problem on a quantum device. Natural proposals rely upon the intrinsic hardness of classically simulating quantum mechanics; however, verifying the output is itself classically intractable. On the other hand, certain quantum algorithms (e.g. prime factorization via Shor\&$\#$39;s algorithm) are efficiently verifiable, but require more resources than what is available on near-term devices. One way to bridge the gap between verifiability and implementation is to use \"interactions\" between a prover and a verifier. By leveraging cryptographic functions, such protocols enable the classical verifier to enforce consistency in a quantum prover\&$\#$39;s responses across multiple rounds of interaction. In this work, we demonstrate the first implementation of an interactive quantum advantage protocol, using an ion trap quantum computer. We execute two complementary protocols -- one based upon the learning with errors problem and another where the cryptographic construction implements a computational Bell test. To perform multiple rounds of interaction, we implement mid-circuit measurements on a subset of trapped ion qubits, with subsequent coherent evolution. For both protocols, the performance exceeds the asymptotic bound for classical behavior; maintaining this fidelity at scale would conclusively demonstrate verifiable quantum advantage.

}, url = {https://arxiv.org/abs/2112.05156}, author = {Daiwei Zhu and Gregory D. Kahanamoku-Meyer and Laura Lewis and Crystal Noel and Or Katz and Bahaa Harraz and Qingfeng Wang and Andrew Risinger and Lei Feng and Debopriyo Biswas and Laird Egan and Alexandru Gheorghiu and Yunseong Nam and Thomas Vidick and Umesh Vazirani and Norman Y. Yao and Marko Cetina and Christopher Monroe} } @article {2923, title = {Machine learning outperforms thermodynamics in measuring how well a many-body system learns a drive}, journal = {Scientific Reports}, volume = {11}, year = {2021}, month = {10/22/2021}, abstract = {

Diverse many-body systems, from soap bubbles to suspensions to polymers, learn and remember patterns in the drives that push them far from equilibrium. This learning may be leveraged for computation, memory, and engineering. Until now, many-body learning has been detected with thermodynamic properties, such as work absorption and strain. We progress beyond these macroscopic properties first defined for equilibrium contexts: We quantify statistical mechanical learning using representation learning, a machine-learning model in which information squeezes through a bottleneck. By calculating properties of the bottleneck, we measure four facets of many-body systems\&$\#$39; learning: classification ability, memory capacity, discrimination ability, and novelty detection. Numerical simulations of a classical spin glass illustrate our technique. This toolkit exposes self-organization that eludes detection by thermodynamic measures: Our toolkit more reliably and more precisely detects and quantifies learning by matter while providing a unifying framework for many-body learning.\ 

}, issn = {2045-2322}, doi = {https://doi.org/10.1038/s41598-021-88311-7}, url = {https://arxiv.org/abs/2004.03604}, author = {Zhong, Weishun and Gold, Jacob M. and Marzen, Sarah and England, Jeremy L. and Nicole Yunger Halpern} } @article {2730, title = {Machine-learning enhanced dark soliton detection in Bose-Einstein condensates}, journal = {Mach. Learn.: Sci. Technol. }, volume = {2}, year = {2021}, month = {6/17/2021}, pages = {035020}, abstract = {

Most data in cold-atom experiments comes from images, the analysis of which is limited by our preconceptions of the patterns that could be present in the data. We focus on the well-defined case of detecting dark solitons -- appearing as local density depletions in a BEC -- using a methodology that is extensible to the general task of pattern recognition in images of cold atoms. Studying soliton dynamics over a wide range of parameters requires the analysis of large datasets, making the existing human-inspection-based methodology a significant bottleneck. Here we describe an automated classification and positioning system for identifying localized excitations in atomic Bose-Einstein condensates (BECs) utilizing deep convolutional neural networks to eliminate the need for human image examination. Furthermore, we openly publish our labeled dataset of dark solitons, the first of its kind, for further machine learning research.

}, doi = {https://doi.org/10.1088/2632-2153/abed1e}, url = {https://arxiv.org/abs/2101.05404}, author = {Shangjie Guo and Amilson R. Fritsch and Craig Greenberg and I. B. Spielman and Justyna P. Zwolak} } @article {2805, title = {Observation of measurement-induced quantum phases in a trapped-ion quantum computer}, year = {2021}, month = {6/10/2021}, abstract = {

Many-body open quantum systems balance internal dynamics against decoherence from interactions with an environment. Here, we explore this balance via random quantum circuits implemented on a trapped ion quantum computer, where the system evolution is represented by unitary gates with interspersed projective measurements. As the measurement rate is varied, a purification phase transition is predicted to emerge at a critical point akin to a fault-tolerent threshold. We probe the \"pure\" phase, where the system is rapidly projected to a deterministic state conditioned on the measurement outcomes, and the \"mixed\" or \"coding\" phase, where the initial state becomes partially encoded into a quantum error correcting codespace. We find convincing evidence of the two phases and show numerically that, with modest system scaling, critical properties of the transition clearly emerge.

}, url = {https://arxiv.org/abs/2106.05881}, author = {Crystal Noel and Pradeep Niroula and Daiwei Zhu and Andrew Risinger and Laird Egan and Debopriyo Biswas and Marko Cetina and Alexey V. Gorshkov and Michael Gullans and David A. Huse and Christopher Monroe} } @article {2806, title = {Phase-engineered bosonic quantum codes}, journal = {Physical Review A}, volume = {103}, year = {2021}, month = {6/29/2021}, pages = {062427}, abstract = {

Continuous-variable systems protected by bosonic quantum codes have emerged as a promising platform for quantum information. To date, the design of code words has centered on optimizing the state occupation in the relevant basis to generate the distance needed for error correction. Here, we show tuning the phase degree of freedom in the design of code words can affect, and potentially enhance, the protection against Markovian errors that involve excitation exchange with the environment. As illustrations, we first consider phase engineering bosonic codes with uniform spacing in the Fock basis that correct excitation loss with a Kerr unitary and show that these modified codes feature destructive interference between error code words and, with an adapted \“two-level\” recovery, the error protection is significantly enhanced. We then study protection against energy decay with the presence of mode nonlinearities \…

}, url = {https://authors.library.caltech.edu/109764/2/1901.05358.pdf}, author = {Linshu Li and Dylan J Young and Victor V. Albert and Kyungjoo Noh and Chang-Ling Zou and Liang Jiang} } @article {2645, title = {Quantum Algorithms for Escaping from Saddle Points}, journal = {Quantum}, volume = {5}, year = {2021}, month = {8/19/2021}, abstract = {

We initiate the study of quantum algorithms for escaping from saddle points with provable guarantee. Given a function f:Rn\→R, our quantum algorithm outputs an ϵ-approximate second-order stationary point using O~(log2n/ϵ1.75) queries to the quantum evaluation oracle (i.e., the zeroth-order oracle). Compared to the classical state-of-the-art algorithm by Jin et al. with O~(log6n/ϵ1.75) queries to the gradient oracle (i.e., the first-order oracle), our quantum algorithm is polynomially better in terms of n and matches its complexity in terms of 1/ϵ. Our quantum algorithm is built upon two techniques: First, we replace the classical perturbations in gradient descent methods by simulating quantum wave equations, which constitutes the polynomial speedup in n for escaping from saddle points. Second, we show how to use a quantum gradient computation algorithm due to Jordan to replace the classical gradient queries by quantum evaluation queries with the same complexity. Finally, we also perform numerical experiments that support our quantum speedup.

}, doi = {https://doi.org/10.22331/q-2021-08-20-529}, url = {https://arxiv.org/abs/2007.10253}, author = {Chenyi Zhang and Jiaqi Leng and Tongyang Li} } @article {2826, title = {Quantum Meets the Minimum Circuit Size Problem}, year = {2021}, month = {8/6/2021}, abstract = {

In this work, we initiate the study of the Minimum Circuit Size Problem (MCSP) in the quantum setting. MCSP is a problem to compute the circuit complexity of Boolean functions. It is a fascinating problem in complexity theory---its hardness is mysterious, and a better understanding of its hardness can have surprising implications to many fields in computer science.
We first define and investigate the basic complexity-theoretic properties of minimum quantum circuit size problems for three natural objects: Boolean functions, unitaries, and quantum states. We show that these problems are not trivially in NP but in QCMA (or have QCMA protocols). Next, we explore the relations between the three quantum MCSPs and their variants. We discover that some reductions that are not known for classical MCSP exist for quantum MCSPs for unitaries and states, e.g., search-to-decision reduction and self-reduction. Finally, we systematically generalize results known for classical MCSP to the quantum setting (including quantum cryptography, quantum learning theory, quantum circuit lower bounds, and quantum fine-grained complexity) and also find new connections to tomography and quantum gravity. Due to the fundamental differences between classical and quantum circuits, most of our results require extra care and reveal properties and phenomena unique to the quantum setting. Our findings could be of interest for future studies, and we post several open problems for further exploration along this direction.

}, url = {https://arxiv.org/abs/2108.03171}, author = {Nai-Hui Chia and Chi-Ning Chou and Jiayu Zhang and Ruizhe Zhang} } @article {2874, title = {Quantum Simulation with Hybrid Tensor Networks}, journal = {Physical Review Letters}, volume = {127}, year = {2021}, month = {8/31/2021}, abstract = {

Tensor network theory and quantum simulation are respectively the key classical and quantum computing methods in understanding quantum many-body physics. Here, we introduce the framework of hybrid tensor networks with building blocks consisting of measurable quantum states and classically contractable tensors, inheriting both their distinct features in efficient representation of many-body wave functions. With the example of hybrid tree tensor networks, we demonstrate efficient quantum simulation using a quantum computer whose size is significantly smaller than the one of the target system. We numerically benchmark our method for finding the ground state of 1D and 2D spin systems of up to 8\×8 and 9\×8 qubits with operations only acting on 8+1 and 9+1 qubits,~respectively. Our approach sheds light on simulation of large practical problems with intermediate-scale quantum computers, with potential applications in chemistry, quantum many-body physics, quantum field theory, and quantum gravity thought experiments.

}, issn = {1079-7114}, doi = {10.1103/physrevlett.127.040501}, url = {http://dx.doi.org/10.1103/PhysRevLett.127.040501}, author = {Yuan, Xiao and Sun, Jinzhao and Liu, Junyu and Zhao, Qi and Zhou, You} } @article {2766, title = {Ray-based framework for state identification in quantum dot devices}, journal = {PRX Quantum}, volume = {2}, year = {2021}, month = {06/17/2021}, abstract = {

Quantum dots (QDs) defined with electrostatic gates are a leading platform for a scalable quantum computing implementation. However, with increasing numbers of qubits, the complexity of the control parameter space also grows. Traditional measurement techniques, relying on complete or near-complete exploration via two-parameter scans (images) of the device response, quickly become impractical with increasing numbers of gates. Here, we propose to circumvent this challenge by introducing a measurement technique relying on one-dimensional projections of the device response in the multi-dimensional parameter space. Dubbed as the ray-based classification (RBC) framework, we use this machine learning (ML) approach to implement a classifier for QD states, enabling automated recognition of qubit-relevant parameter regimes. We show that RBC surpasses the 82 \% accuracy benchmark from the experimental implementation of image-based classification techniques from prior work while cutting down the number of measurement points needed by up to 70 \%. The reduction in measurement cost is a significant gain for time-intensive QD measurements and is a step forward towards the scalability of these devices. We also discuss how the RBC-based optimizer, which tunes the device to a multi-qubit regime, performs when tuning in the two- and three-dimensional parameter spaces defined by plunger and barrier gates that control the dots. This work provides experimental validation of both efficient state identification and optimization with ML techniques for non-traditional measurements in quantum systems with high-dimensional parameter spaces and time-intensive measurements.

}, doi = {https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.2.020335}, url = {https://arxiv.org/abs/2102.11784}, author = {Justyna P. Zwolak and Thomas McJunkin and Sandesh S. Kalantre and Samuel F. Neyens and E. R. MacQuarrie and Mark A. Eriksson and J. M. Taylor} } @article {2818, title = {Robust Self-Testing of Multiparticle Entanglement}, journal = {Phys. Rev. Lett. }, volume = {127}, year = {2021}, month = {12/7/2021}, pages = {230503}, abstract = {

Quantum self-testing is a device-independent way to certify quantum states and measurements using only the input-output statistics, with minimal assumptions about the quantum devices. Due to the high demand on tolerable noise, however, experimental self-testing was limited to two-photon systems. Here, we demonstrate the first robust self-testing for multi-particle quantum entanglement. We prepare two examples of four-photon graph states, the Greenberger-Horne-Zeilinger (GHZ) states with a fidelity of 0.957(2) and the linear cluster states with a fidelity of 0.945(2). Based on the observed input-output statistics, we certify the genuine four-photon entanglement and further estimate their qualities with respect to realistic noise in a device-independent manner.

}, doi = {https://doi.org/10.1103/PhysRevLett.127.230503}, url = {https://arxiv.org/abs/2105.10298}, author = {Dian Wu and Qi Zhao and Xue-Mei Gu and Han-Sen Zhong and You Zhou and Li-Chao Peng and Jian Qin and Yi-Han Luo and Kai Chen and Li Li and Nai-Le Liu and Chao-Yang Lu and Jian-Wei Pan} } @article {2767, title = {RPPLNS: Pay-per-last-N-shares with a Randomised Twist}, year = {2021}, month = {2/15/2021}, abstract = {

\"Pay-per-last-N-shares\" (PPLNS) is one of the most common payout strategies used by mining pools in Proof-of-Work (PoW) cryptocurrencies. As with any payment scheme, it is imperative to study issues of incentive compatibility of miners within the pool. For PPLNS this question has only been partially answered; we know that reasonably-sized miners within a PPLNS pool prefer following the pool protocol over employing specific deviations. In this paper, we present a novel modification to PPLNS where we randomise the protocol in a natural way. We call our protocol \"Randomised pay-per-last-N-shares\" (RPPLNS), and note that the randomised structure of the protocol greatly simplifies the study of its incentive compatibility. We show that RPPLNS maintains the strengths of PPLNS (i.e., fairness, variance reduction, and resistance to pool hopping), while also being robust against a richer class of strategic mining than what has been shown for PPLNS.

}, url = {https://arxiv.org/abs/2102.07681}, author = {Philip Lazos and Francisco J. Marmolejo-Coss{\'\i}o and Xinyu Zhou and Jonathan Katz} } @article {2506, title = {Theory of Trotter Error with Commutator Scaling}, journal = {Phys. Rev. X}, volume = {11}, year = {2021}, month = {2/1/2021}, pages = {49}, chapter = {011020}, abstract = {

The Lie-Trotter formula, together with its higher-order generalizations, provides a direct approach to decomposing the exponential of a sum of operators. Despite significant effort, the error scaling of such product formulas remains poorly understood. We develop a theory of Trotter error that overcomes the limitations of prior approaches based on truncating the Baker-Campbell-Hausdorff expansion. Our analysis directly exploits the commutativity of operator summands, producing tighter error bounds for both real- and imaginary-time evolutions. Whereas previous work achieves similar goals for systems with geometric locality or Lie-algebraic structure, our approach holds in general. We give a host of improved algorithms for digital quantum simulation and quantum Monte Carlo methods, including simulations of second-quantized plane-wave electronic structure, k-local Hamiltonians, rapidly decaying power-law interactions, clustered Hamiltonians, the transverse field Ising model, and quantum ferromagnets, nearly matching or even outperforming the best previous results. We obtain further speedups using the fact that product formulas can preserve the locality of the simulated system. Specifically, we show that local observables can be simulated with complexity independent of the system size for power-law interacting systems, which implies a Lieb-Robinson bound as a byproduct. Our analysis reproduces known tight bounds for first- and second-order formulas. Our higher-order bound overestimates the complexity of simulating a one-dimensional Heisenberg model with an even-odd ordering of terms by only a factor of 5, and is close to tight for power-law interactions and other orderings of terms. This suggests that our theory can accurately characterize Trotter error in terms of both asymptotic scaling and constant prefactor.

}, doi = {https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.011020}, url = {https://arxiv.org/abs/1912.08854}, author = {Andrew M. Childs and Yuan Su and Minh C. Tran and Nathan Wiebe and Shuchen Zhu} } @article {2731, title = {A Threshold for Quantum Advantage in Derivative Pricing}, journal = {Quantum}, volume = {5}, year = {2021}, pages = {463}, abstract = {

We give an upper bound on the resources required for valuable quantum advantage in pricing derivatives. To do so, we give the first complete resource estimates for useful quantum derivative pricing, using autocallable and Target Accrual Redemption Forward (TARF) derivatives as benchmark use cases. We uncover blocking challenges in known approaches and introduce a new method for quantum derivative pricing - the re-parameterization method - that avoids them. This method combines pre-trained variational circuits with fault-tolerant quantum computing to dramatically reduce resource requirements. We find that the benchmark use cases we examine require 7.5k logical qubits and a T-depth of 46 million and thus estimate that quantum advantage would require a logical clock speed of 10Mhz. While the resource requirements given here are out of reach of current systems, we hope they will provide a roadmap for further improvements in algorithms, implementations, and planned hardware architectures.\ 

}, doi = {https://doi.org/10.22331/q-2021-06-01-463}, url = {https://arxiv.org/abs/2012.03819}, author = {Shouvanik Chakrabarti and Rajiv Krishnakumar and Guglielmo Mazzola and Nikitas Stamatopoulos and Stefan Woerner and William J. Zeng} } @article {2450, title = {Ultralight dark matter detection with mechanical quantum sensors}, journal = {New Journal of Physics}, volume = {23}, year = {2021}, month = {3/10/2021}, pages = {023041}, abstract = {

We consider the use of quantum-limited mechanical force sensors to detect ultralight (sub-meV) dark matter candidates which are weakly coupled to the standard model. We show that mechanical sensors with masses around or below the milligram scale, operating around the standard quantum limit, would enable novel searches for dark matter with natural frequencies around the kHz scale. This would complement existing strategies based on torsion balances, atom interferometers, and atomic clock systems

}, issn = {1367-2630}, doi = {https://doi.org/10.1088/1367-2630/abd9e7}, url = {https://arxiv.org/abs/1908.04797}, author = {Daniel Carney and Anson Hook and Zhen Liu and J. M. Taylor and Yue Zhao} } @article {2449, title = {Auto-tuning of double dot devices in situ with machine learning}, journal = {Phys. Rev. Applied }, volume = {13}, year = {2020}, month = {4/1/2020}, abstract = {

There are myriad quantum computing approaches, each having its own set of challenges to understand and effectively control their operation. Electrons confined in arrays of semiconductor nanostructures, called quantum dots (QDs), is one such approach. The easy access to control parameters, fast measurements, long qubit lifetimes, and the potential for scalability make QDs especially attractive. However, as the size of the QD array grows, so does the number of parameters needed for control and thus the tuning complexity. The current practice of manually tuning the qubits is a relatively time-consuming procedure and is inherently impractical for scaling up and applications. In this work, we report on the in situ implementation of an auto-tuning protocol proposed by Kalantre et al. [arXiv:1712.04914]. In particular, we discuss how to establish a seamless communication protocol between a machine learning (ML)-based auto-tuner and the experimental apparatus. We then show that a ML algorithm trained exclusively on synthetic data coming from a physical model to quantitatively classify the state of the QD device, combined with an optimization routine, can be used to replace manual tuning of gate voltages in devices. A success rate of over 85 \% is determined for tuning to a double quantum dot regime when at least one of the plunger gates is initiated sufficiently close to the desired state. Modifications to the training network, fitness function, and optimizer are discussed as a path towards further improvement in the success rate when starting both near and far detuned from the target double dot range.

}, doi = {https://doi.org/10.1103/PhysRevApplied.13.034075}, url = {https://arxiv.org/abs/1909.08030}, author = {Justyna P. Zwolak and Thomas McJunkin and Sandesh S. Kalantre and J. P. Dodson and E. R. MacQuarrie and D. E. Savage and M. G. Lagally and S. N. Coppersmith and Mark A. Eriksson and J. M. Taylor} } @article {2572, title = {Constructing Multipartite Bell inequalities from stabilizers}, year = {2020}, month = {2/5/2020}, abstract = {

Bell inequality with self-testing property has played an important role in quantum information field with both fundamental and practical applications. However, it is generally challenging to find Bell inequalities with self-testing property for multipartite states and actually there are not many known candidates. In this work, we propose a systematical framework to construct Bell inequalities from stabilizers which are maximally violated by general stabilizer states, with two observables for each local party. We show that the constructed Bell inequalities can self-test any stabilizer state which is essentially device-independent, if and only if these stabilizers can uniquely determine the state in a device-dependent manner. This bridges the gap between device-independent and device-dependent verification methods. Our framework can provide plenty of Bell inequalities for self-testing stabilizer states. Among them, we give two families of Bell inequalities with different advantages: (1) a family of Bell inequalities with a constant ratio of quantum and classical bounds using 2N correlations, (2) Single pair inequalities improving on all previous robustness self-testing bounds using N+1 correlations, which are both efficient and suitable for realizations in multipartite systems. Our framework can not only inspire more fruitful multipartite Bell inequalities from conventional verification methods, but also pave the way for their practical applications.

}, url = {https://arxiv.org/abs/2002.01843}, author = {Qi Zhao and You Zhou} } @article {2540, title = {Efficient randomness certification by quantum probability estimation}, journal = {Phys. Rev. Research }, volume = {2}, year = {2020}, month = {1/7/2020}, abstract = {

For practical applications of quantum randomness generation, it is important to certify and further produce a fixed block of fresh random bits with as few trials as possible. Consequently, protocols with high finite-data efficiency are preferred. To yield such protocols with respect to quantum side information, we develop quantum probability estimation. Our approach is applicable to device-independent as well as device-dependent scenarios, and it generalizes techniques from previous works [Miller and Shi, SIAM J. Comput. 46, 1304 (2017); Arnon-Friedman et al., Nat. Commun. 9, 459 (2018)]. Quantum probability estimation can adapt to changing experimental conditions, allows stopping the experiment as soon as the prespecified randomness goal is achieved, and can tolerate imperfect knowledge of the input distribution. Moreover, the randomness rate achieved at constant error is asymptotically optimal. For the device-independent scenario, our approach certifies the amount of randomness available in experimental results without first searching for relations between randomness and violations of fixed Bell inequalities. We implement quantum probability estimation for device-independent randomness generation in the CHSH Bell-test configuration, and we show significant improvements in finite-data efficiency, particularly at small Bell violations which are typical in current photonic loophole-free Bell tests.

}, doi = {https://doi.org/10.1103/PhysRevResearch.2.013016}, author = {Yanbao Zhang and Honghao Fu and Emanuel Knill} } @article {2457, title = {Entanglement Bounds on the Performance of Quantum Computing Architectures}, journal = {Phys. Rev. Research}, volume = {2}, year = {2020}, month = {9/22/2020}, abstract = {

There are many possible architectures for future quantum computers that designers will need to choose between. However, the process of evaluating a particular connectivity graph\&$\#$39;s performance as a quantum architecture can be difficult. In this paper, we establish a connection between a quantity known as the isoperimetric number and a lower bound on the time required to create highly entangled states. The metric we propose counts resources based on the use of two-qubit unitary operations, while allowing for arbitrarily fast measurements and classical feedback. We describe how these results can be applied to the evaluation of the hierarchical architecture proposed in Phys. Rev. A 98, 062328 (2018). We also show that the time-complexity bound we place on the creation of highly-entangled states can be saturated up to a multiplicative factor logarithmic in the number of qubits.

}, doi = {https://doi.org/10.1103/PhysRevResearch.2.033316}, url = {https://arxiv.org/abs/1908.04802}, author = {Zachary Eldredge and Leo Zhou and Aniruddha Bapat and James R. Garrison and Abhinav Deshpande and Frederic T. Chong and Alexey V. Gorshkov} } @article {2329, title = {Experimental Low-Latency Device-Independent Quantum Randomness}, journal = {Phys. Rev. Lett. }, volume = {124}, year = {2020}, month = {12/24/2019}, abstract = {

Applications of randomness such as private key generation and public randomness beacons require small blocks of certified random bits on demand. Device-independent quantum random number generators can produce such random bits, but existing quantum-proof protocols and loophole-free implementations suffer from high latency, requiring many hours to produce any random bits. We demonstrate device-independent quantum randomness generation from a loophole-free Bell test with a more efficient quantum-proof protocol, obtaining multiple blocks of 512 bits with an average experiment time of less than 5 min per block and with certified error bounded by 2\−64\≈5.42\×10\−20.

}, doi = {https://doi.org/10.1103/PhysRevLett.124.010505}, url = {https://arxiv.org/abs/1812.07786}, author = {Yanbao Zhang and Lynden K. Shalm and Joshua C. Bienfang and Martin J. Stevens and Michael D. Mazurek and Sae Woo Nam and Carlos Abell{\'a}n and Waldimar Amaya and Morgan W. Mitchell and Honghao Fu and Carl Miller and Alan Mink and Emanuel Knill} } @article {2686, title = {Fault-Tolerant Operation of a Quantum Error-Correction Code}, year = {2020}, month = {9/24/2020}, abstract = {

Quantum error correction protects fragile quantum information by encoding it in a larger quantum system whose extra degrees of freedom enable the detection and correction of errors. An encoded logical qubit thus carries increased complexity compared to a bare physical qubit. Fault-tolerant protocols contain the spread of errors and are essential for realizing error suppression with an error-corrected logical qubit. Here we experimentally demonstrate fault-tolerant preparation, rotation, error syndrome extraction, and measurement on a logical qubit encoded in the 9-qubit Bacon-Shor code. For the logical qubit, we measure an average fault-tolerant preparation and measurement error of 0.6\% and a transversal Clifford gate with an error of 0.3\% after error correction. The result is an encoded logical qubit whose logical fidelity exceeds the fidelity of the entangling operations used to create it. We compare these operations with non-fault-tolerant protocols capable of generating arbitrary logical states, and observe the expected increase in error. We directly measure the four Bacon-Shor stabilizer generators and are able to detect single qubit Pauli errors. These results show that fault-tolerant quantum systems are currently capable of logical primitives with error rates lower than their constituent parts. With the future addition of intermediate measurements, the full power of scalable quantum error-correction can be achieved.\ 

}, url = {https://arxiv.org/abs/2009.11482}, author = {Laird Egan and Dripto M. Debroy and Crystal Noel and Andrew Risinger and Daiwei Zhu and Debopriyo Biswas and Michael Newman and Muyuan Li and Kenneth R. Brown and Marko Cetina and Christopher Monroe} } @article {2687, title = {Mechanical Quantum Sensing in the Search for Dark Matter}, year = {2020}, month = {8/13/2020}, type = {FERMILAB-PUB-20-378-QIS-T}, abstract = {

Numerous astrophysical and cosmological observations are best explained by the existence of dark matter, a mass density which interacts only very weakly with visible, baryonic matter. Searching for the extremely weak signals produced by this dark matter strongly motivate the development of new, ultra-sensitive detector technologies. Paradigmatic advances in the control and readout of massive mechanical systems, in both the classical and quantum regimes, have enabled unprecedented levels of sensitivity. In this white paper, we outline recent ideas in the potential use of a range of solid-state mechanical sensing technologies to aid in the search for dark matter in a number of energy scales and with a variety of coupling mechanisms.

}, url = {https://arxiv.org/abs/2008.06074}, author = {D. Carney and G. Krnjaic and D. C. Moore and C. A. Regal and G. Afek and S. Bhave and B. Brubaker and T. Corbitt and J. Cripe and N. Crisosto and A.Geraci and S. Ghosh and J. G. E. Harris and A. Hook and E. W. Kolb and J. Kunjummen and R. F. Lang and T. Li and T. Lin and Z. Liu and J. Lykken and L. Magrini and J. Manley and N. Matsumoto and A. Monte and F. Monteiro and T. Purdy and C. J. Riedel and R. Singh and S. Singh and K. Sinha and J. M. Taylor and J. Qin and D. J. Wilson and Y. Zhao} } @article {2692, title = {More of the Bulk from Extremal Area Variations}, journal = {Classical and Quantum Gravity}, volume = {38}, year = {2020}, month = {12/24/2020}, pages = {047001}, abstract = {

It was shown recently, building on work of Alexakis, Balehowksy, and Nachman that the geometry of (some portion of) a manifold with boundary is uniquely fixed by the areas of a foliation of two-dimensional disk-shaped surfaces anchored to the boundary. In the context of AdS/CFT, this implies that (a portion of) a four-dimensional bulk geometry can be fixed uniquely from the entanglement entropies of disk-shaped boundary regions, subject to several constraints. In this Note, we loosen some of these constraints, in particular allowing for the bulk foliation of extremal surfaces to be local and removing the constraint of disk topology; these generalizations ensure uniqueness of more of the deep bulk geometry by allowing for e.g. surfaces anchored on disconnected asymptotic boundaries, or HRT surfaces past a phase transition. We also explore in more depth the generality of the local foliation requirement, showing that even in a highly dynamical geometry like AdS-Vaidya it is satisfied.

}, doi = {https://iopscience.iop.org/article/10.1088/1361-6382/abcfd0/pdf}, url = {https://arxiv.org/abs/2009.07850}, author = {Ning Bao and ChunJun Cao and Sebastian Fischetti and Jason Pollack and Yibo Zhong} } @article {2571, title = {A note on blind contact tracing at scale with applications to the COVID-19 pandemic}, year = {2020}, month = {4/10/2020}, abstract = {

The current COVID-19 pandemic highlights the utility of contact tracing, when combined with case isolation and social distancing, as an important tool for mitigating the spread of a disease [1]. Contact tracing provides a mechanism of identifying individuals with a high likelihood of previous exposure to a contagious disease, allowing additional precautions to be put in place to prevent continued transmission. Here we consider a cryptographic approach to contact tracing based on secure two-party computation (2PC). We begin by considering the problem of comparing a set of location histories held by two parties to determine whether they have come within some threshold distance while at the same time maintaining the privacy of the location histories. We propose a solution to this problem using pre-shared keys, adapted from an equality testing protocol due to Ishai et al [2]. We discuss how this protocol can be used to maintain privacy within practical contact tracing scenarios, including both app-based approaches and approaches which leverage location history held by telecoms and internet service providers. We examine the efficiency of this approach and show that existing infrastructure is sufficient to support anonymised contact tracing at a national level.

}, url = {https://arxiv.org/abs/2004.05116}, author = {Jack K. Fitzsimons and Atul Mantri and Robert Pisarczyk and Tom Rainforth and Zhikuan Zhao} } @article {2732, title = {One-shot dynamical resource theory}, year = {2020}, month = {12/4/2020}, abstract = {

A fundamental problem in resource theory is to study the manipulation of the resource. Focusing on a general dynamical resource theory of quantum channels, here we consider tasks of one-shot resource distillation and dilution with a single copy of the resource. For any target of unitary channel or pure state preparation channel, we establish a universal strategy to determine upper and lower bounds on rates that convert between any given resource and the target. We show that the rates are related to resource measures based on the channel robustness and the channel hypothesis testing entropy, with regularization factors of the target resource measures. The strategy becomes optimal with converged bounds when the channel robustness is finite and measures of the target resource collapse to the same value. The single-shot result also applies to asymptotic parallel manipulation of channels to obtain asymptotic resource conversion rates. We give several examples of dynamical resources, including the purity, classical capacity, quantum capacity, non-uniformity, coherence, and entanglement of quantum channels. Our results are applicable to general dynamical resource theories with potential applications in quantum communication, fault-tolerant quantum computing, and quantum thermodynamics.

}, url = {https://arxiv.org/abs/2012.02781}, author = {Xiao Yuan and Pei Zeng and Minbo Gao and Qi Zhao} } @article {2456, title = {The operator L{\'e}vy flight: light cones in chaotic long-range interacting systems}, journal = {Phys. Rev. Lett. }, volume = {124}, year = {2020}, month = {7/6/2020}, abstract = {

We propose a generic light cone phase diagram for chaotic long-range r\−α interacting systems, where a linear light cone appears for α\≥d+1/2 in d dimension. Utilizing the dephasing nature of quantum chaos, we argue that the universal behavior of the squared commutator is described by a stochastic model, for which the exact phase diagram is known. We provide an interpretation in terms of the L{\'e}vy flights and show that this suffices to capture the scaling of the squared commutator. We verify these phenomena in numerical computation of a long-range spin chain with up to 200 sites.\ 

}, doi = {https://doi.org/10.1103/PhysRevLett.124.180601}, url = {https://arxiv.org/abs/1909.08646}, author = {Tianci Zhou and Shenglong Xu and Xiao Chen and Andrew Guo and Brian Swingle} } @article {2558, title = {On the Principles of Differentiable Quantum Programming Languages}, year = {2020}, month = {4/2/2020}, abstract = {

Variational Quantum Circuits (VQCs), or the so-called quantum neural-networks, are predicted to be one of the most important near-term quantum applications, not only because of their similar promises as classical neural-networks, but also because of their feasibility on near-term noisy intermediate-size quantum (NISQ) machines. The need for gradient information in the training procedure of VQC applications has stimulated the development of auto-differentiation techniques for quantum circuits. We propose the first formalization of this technique, not only in the context of quantum circuits but also for imperative quantum programs (e.g., with controls), inspired by the success of differentiable programming languages in classical machine learning. In particular, we overcome a few unique difficulties caused by exotic quantum features (such as quantum no-cloning) and provide a rigorous formulation of differentiation applied to bounded-loop imperative quantum programs, its code-transformation rules, as well as a sound logic to reason about their correctness. Moreover, we have implemented our code transformation in OCaml and demonstrated the resource-efficiency of our scheme both analytically and empirically. We also conduct a case study of training a VQC instance with controls, which shows the advantage of our scheme over existing auto-differentiation for quantum circuits without controls.

}, doi = {https://doi.org/10.1145/3385412.3386011}, url = {https://arxiv.org/abs/2004.01122}, author = {Shaopeng Zhu and Shih-Han Hung and Shouvanik Chakrabarti and Xiaodi Wu} } @article {2604, title = {Probing many-body localization on a noisy quantum computer}, year = {2020}, month = {6/22/2020}, abstract = {

A disordered system of interacting particles exhibits localized behavior when the disorder is large compared to the interaction strength. Studying this phenomenon on a quantum computer without error correction is challenging because even weak coupling to a thermal environment destroys most signatures of localization. Fortunately, spectral functions of local operators are known to contain features that can survive the presence of noise. In these spectra, discrete peaks and a soft gap at low frequencies compared to the thermal phase indicate localization. Here, we present the computation of spectral functions on a trapped-ion quantum computer for a one-dimensional Heisenberg model with disorder. Further, we design an error-mitigation technique which is effective at removing the noise from the measurement allowing clear signatures of localization to emerge as the disorder increases. Thus, we show that spectral functions can serve as a robust and scalable diagnostic of many-body localization on the current generation of quantum computers.\ 

}, url = {https://arxiv.org/abs/2006.12355}, author = {D. Zhu and S. Johri and N. H. Nguyen and C. Huerta Alderete and K. A. Landsman and N. M. Linke and C. Monroe and A. Y. Matsuura} } @article {2646, title = {Quantum simulation with hybrid tensor networks}, year = {2020}, month = {7/2/2020}, abstract = {

Tensor network theory and quantum simulation are respectively the key classical and quantum methods in understanding many-body quantum physics. Here we show hybridization of these two seemingly independent methods, inheriting both their distinct advantageous features of efficient representations of many-body wave functions. We introduce the framework of hybrid tensor networks with building blocks consisting of measurable quantum states and classically contractable tensors. As an example, we demonstrate efficient quantum simulation with hybrid tree tensor networks that use quantum hardware whose size is significantly smaller than the one of the target system. We numerically test our method for finding the ground state of 1D and 2D spin systems of up to 8\×8 and 4\×3 qubits with operations only acting on 8+1 and 4+1 qubits, respectively. Our approach paves the way to the near-term quantum simulation of large practical problems with intermediate size quantum hardware, with potential applications in quantum chemistry, quantum many-body physics, quantum field theory, and quantum gravity thought experiments.

}, url = {https://arxiv.org/abs/2007.00958}, author = {Xiao Yuan and Jinzhao Sun and Junyu Liu and Qi Zhao and You Zhou} } @article {2570, title = {Quantum walks and Dirac cellular automata on a programmable trapped-ion quantum computer}, year = {2020}, month = {2/6/2020}, abstract = {

The quantum walk formalism is a widely used and highly successful framework for modeling quantum systems, such as simulations of the Dirac equation, different dynamics in both the low and high energy regime, and for developing a wide range of quantum algorithms. Here we present the circuit-based implementation of a discrete-time quantum walk in position space on a five-qubit trapped-ion quantum processor. We encode the space of walker positions in particular multi-qubit states and program the system to operate with different quantum walk parameters, experimentally realizing a Dirac cellular automaton with tunable mass parameter. The quantum walk circuits and position state mapping scale favorably to a larger model and physical systems, allowing the implementation of any algorithm based on discrete-time quantum walks algorithm and the dynamics associated with the discretized version of the Dirac equation.

}, url = {https://arxiv.org/abs/2002.02537}, author = {C. Huerta Alderete and Shivani Singh and Nhung H. Nguyen and Daiwei Zhu and Radhakrishnan Balu and Christopher Monroe and C. M. Chandrashekar and Norbert M. Linke} } @article {2685, title = {Ray-based classification framework for high-dimensional data}, journal = {Proceedings of the Machine Learning and the Physical Sciences Workshop at NeurIPS 2020, Vancouver, Canada}, year = {2020}, month = {10/1/2020}, abstract = {

While classification of arbitrary structures in high dimensions may require complete quantitative information, for simple geometrical structures, low-dimensional qualitative information about the boundaries defining the structures can suffice. Rather than using dense, multi-dimensional data, we propose a deep neural network (DNN) classification framework that utilizes a minimal collection of one-dimensional representations, called \emph{rays}, to construct the \"fingerprint\" of the structure(s) based on substantially reduced information. We empirically study this framework using a synthetic dataset of double and triple quantum dot devices and apply it to the classification problem of identifying the device state. We show that the performance of the ray-based classifier is already on par with traditional 2D images for low dimensional systems, while significantly cutting down the data acquisition cost.

}, url = {https://arxiv.org/abs/2010.00500}, author = {Justyna P. Zwolak and Sandesh S. Kalantre and Thomas McJunkin and Brian J. Weber and J. M. Taylor} } @article {2264, title = {Confined Dynamics in Long-Range Interacting Quantum Spin Chains}, journal = {Phys. Rev. Lett.}, volume = {122 }, year = {2019}, month = {04/17/2019}, abstract = {

We study the quasiparticle excitation and quench dynamics of the one-dimensional transverse-field Ising model with power-law (1/rα) interactions. We find that long-range interactions give rise to a confining potential, which couples pairs of domain walls (kinks) into bound quasiparticles, analogous to mesonic bound states in high-energy physics. We show that these bound states have dramatic consequences for the non-equilibrium dynamics following a global quantum quench, such as suppressed spreading of quantum information and oscillations of order parameters. The masses of these bound states can be read out from the Fourier spectrum of these oscillating order parameters. We then use a two-kink model to qualitatively explain the phenomenon of long-range-interaction-induced confinement. The masses of the bound states predicted by this model are in good quantitative agreement with exact diagonalization results. Moreover, we illustrate that these bound states lead to weak thermalization of local observables for initial states with energy near the bottom of the many-body energy spectrum. Our work is readily applicable to current trapped-ion experiments.

}, doi = {https://doi.org/10.1103/PhysRevLett.122.150601}, url = {https://arxiv.org/abs/1810.02365}, author = {Fangli Liu and Rex Lundgren and Paraj Titum and Guido Pagano and Jiehang Zhang and Christopher Monroe and Alexey V. Gorshkov} } @article {2143, title = {Interacting Qubit-Photon Bound States with Superconducting Circuits}, journal = {Phys. Rev. }, volume = {X 9}, year = {2019}, month = {2018/01/30}, abstract = {

Qubits strongly coupled to a photonic crystal give rise to many exotic physical scenarios, beginning with single and multi-excitation qubit-photon dressed bound states comprising induced spatially localized photonic modes, centered around the qubits, and the qubits themselves. The localization of these states changes with qubit detuning from the band-edge, offering an avenue of in situ control of bound state interaction. Here, we present experimental results from a device with two qubits coupled to a superconducting microwave photonic crystal and realize tunable on-site and inter-bound state interactions. We observe a fourth-order two photon virtual process between bound states indicating strong coupling between the photonic crystal and qubits. Due to their localization-dependent interaction, these states offer the ability to create one-dimensional chains of bound states with tunable and potentially long-range interactions that preserve the qubits\&$\#$39; spatial organization, a key criterion for realization of certain quantum many-body models. The widely tunable, strong and robust interactions demonstrated with this system are promising benchmarks towards realizing larger, more complex systems of bound states.

}, doi = {https://doi.org/10.1103/PhysRevX.9.011021}, url = {http://arxiv.org/abs/1801.10167}, author = {Neereja M. Sundaresan and Rex Lundgren and Guanyu Zhu and Alexey V. Gorshkov and Andrew A. Houck} } @article {2460, title = {Nondestructive cooling of an atomic quantum register via state-insensitive Rydberg interactions}, year = {2019}, month = {7/28/2019}, abstract = {

We propose a protocol for sympathetically cooling neutral atoms without destroying the quantum information stored in their internal states. This is achieved by designing state-insensitive Rydberg interactions between the data-carrying atoms and cold auxiliary atoms. The resulting interactions give rise to an effective phonon coupling, which leads to the transfer of heat from the data atoms to the auxiliary atoms, where the latter can be cooled by conventional methods. This can be used to extend the lifetime of quantum storage based on neutral atoms and can have applications for long quantum computations. The protocol can also be modified to realize state-insensitive interactions between the data and the auxiliary atoms but tunable and non-trivial interactions among the data atoms, allowing one to simultaneously cool and simulate a quantum spin-model.\ 

}, url = {https://arxiv.org/abs/1907.11156}, author = {Ron Belyansky and Jeremy T. Young and Przemyslaw Bienias and Zachary Eldredge and Adam M. Kaufman and Peter Zoller and Alexey V. Gorshkov} } @article {2362, title = {Opportunities for Nuclear Physics \& Quantum Information Science}, year = {2019}, month = {03/13/2019}, abstract = {

his whitepaper is an outcome of the workshop Intersections between Nuclear Physics and Quantum Information held at Argonne National Laboratory on 28-30 March 2018 [www.phy.anl.gov/npqi2018/]. The workshop brought together 116 national and international experts in nuclear physics and quantum information science to explore opportunities for the two fields to collaborate on topics of interest to the U.S. Department of Energy (DOE) Office of Science, Office of Nuclear Physics, and more broadly to U.S. society and industry. The workshop consisted of 22 invited and 10 contributed talks, as well as three panel discussion sessions. Topics discussed included quantum computation, quantum simulation, quantum sensing, nuclear physics detectors, nuclear many-body problem, entanglement at collider energies, and lattice gauge theories.

}, url = {https://arxiv.org/abs/1903.05453}, author = {I. C. Clo{\"e}t and Matthew R. Dietrich and John Arrington and Alexei Bazavov and Michael Bishof and Adam Freese and Alexey V. Gorshkov and Anna Grassellino and Kawtar Hafidi and Zubin Jacob and Michael McGuigan and Yannick Meurice and Zein-Eddine Meziani and Peter Mueller and Christine Muschik and James Osborn and Matthew Otten and Peter Petreczky and Tomas Polakovic and Alan Poon and Raphael Pooser and Alessandro Roggero and Mark Saffman and Brent VanDevender and Jiehang Zhang and Erez Zohar} } @article {2390, title = {Photon pair condensation by engineered dissipation}, journal = {Phys. Rev. Lett. }, volume = {123}, year = {2019}, month = {04/02/2019}, abstract = {

Dissipation can usually induce detrimental decoherence in a quantum system. However, engineered dissipation can be used to prepare and stabilize coherent quantum many-body states. Here, we show that by engineering dissipators containing photon pair operators, one can stabilize an exotic dark state, which is a condensate of photon pairs with a phase-nematic order. In this system, the usual superfluid order parameter, i.e. single-photon correlation, is absent, while the photon pair correlation exhibits long-range order. Although the dark state is not unique due to multiple parity sectors, we devise an additional type of dissipators to stabilize the dark state in a particular parity sector via a diffusive annihilation process which obeys Glauber dynamics in an Ising model. Furthermore, we propose an implementation of these photon-pair dissipators in circuit-QED architecture.\ 

}, doi = {10.1103/PhysRevLett.123.063602}, url = {https://arxiv.org/abs/1904.00016}, author = {Ze-Pei Cian and Guanyu Zhu and Su-Kuan Chu and Alireza Seif and Wade DeGottardi and Liang Jiang and Mohammad Hafezi} } @article {2385, title = {ReQWIRE: Reasoning about Reversible Quantum Circuits}, journal = {EPTCS }, volume = {287}, year = {2019}, type = {In Proceedings QPL 2018, arXiv:1901.09476}, chapter = {299-312}, abstract = {

Common quantum algorithms make heavy use of ancillae: scratch qubits that are initialized at some state and later returned to that state and discarded. Existing quantum circuit languages let programmers assert that a qubit has been returned to the |0\> state before it is discarded, allowing for a range of optimizations. However, existing languages do not provide the tools to verify these assertions, introducing a potential source of errors. In this paper we present methods for verifying that ancillae are discarded in the desired state, and use these methods to implement a verified compiler from classical functions to quantum oracles.

}, doi = {https://doi.org/10.4204/EPTCS.287.17}, url = {https://arxiv.org/abs/1901.10118}, author = {Robert Rand and Jennifer Paykin and Dong-Ho Lee and Steve Zdancewic} } @article {2217, title = {Scale-Invariant Continuous Entanglement Renormalization of a Chern Insulator}, journal = {Phys. Rev. Lett}, volume = {122}, year = {2019}, month = {03/27/2019}, abstract = {

The multi-scale entanglement renormalization ansatz (MERA) postulates the existence of quantum circuits that renormalize entanglement in real space at different length scales. Chern insulators, however, cannot have scale-invariant discrete MERA circuits with finite bond dimension. In this Letter, we show that the continuous MERA (cMERA), a modified version of MERA adapted for field theories, possesses a fixed point wavefunction with nonzero Chern number. Additionally, it is well known that reversed MERA circuits can be used to prepare quantum states efficiently in time that scales logarithmically with the size of the system. However, state preparation via MERA typically requires the advent of a full-fledged universal quantum computer. In this Letter, we demonstrate that our cMERA circuit can potentially be realized in existing analog quantum computers, i.e., an ultracold atomic Fermi gas in an optical lattice with light-induced spin-orbit coupling.\ 

}, doi = {https://doi.org/10.1103/PhysRevLett.122.120502}, url = {https://arxiv.org/abs/1807.11486}, author = {Su-Kuan Chu and Guanyu Zhu and James R. Garrison and Zachary Eldredge and Ana Vald{\'e}s Curiel and Przemyslaw Bienias and I. B. Spielman and Alexey V. Gorshkov} } @article {2392, title = {Toward convergence of effective field theory simulations on digital quantum computers}, year = {2019}, month = {04/18/2019}, abstract = {

We report results for simulating an effective field theory to compute the binding energy of the deuteron nucleus using a hybrid algorithm on a trapped-ion quantum computer. Two increasingly complex unitary coupled-cluster ansaetze have been used to compute the binding energy to within a few percent for successively more complex Hamiltonians. By increasing the complexity of the Hamiltonian, allowing more terms in the effective field theory expansion and calculating their expectation values, we present a benchmark for quantum computers based on their ability to scalably calculate the effective field theory with increasing accuracy. Our result of E4=\−2.220\±0.179MeV may be compared with the exact Deuteron ground-state energy \−2.224MeV. We also demonstrate an error mitigation technique using Richardson extrapolation on ion traps for the first time. The error mitigation circuit represents a record for deepest quantum circuit on a trapped-ion quantum computer.\ 

}, url = {https://arxiv.org/abs/1904.04338}, author = {Omar Shehab and Kevin A. Landsman and Yunseong Nam and Daiwei Zhu and Norbert M. Linke and Matthew J. Keesan and Raphael C. Pooser and Christopher R. Monroe} } @article {2412, title = {Two-qubit entangling gates within arbitrarily long chains of trapped ions}, year = {2019}, month = {05/28/2019}, abstract = {

Ion trap systems are a leading platform for large scale quantum computers. Trapped ion qubit crystals are fully-connected and reconfigurable, owing to their long range Coulomb interaction that can be modulated with external optical forces. However, the spectral crowding of collective motional modes could pose a challenge to the control of such interactions for large numbers of qubits. Here, we show that high-fidelity quantum gate operations are still possible with very large trapped ion crystals, simplifying the scaling of ion trap quantum computers. To this end, we present analytical work that determines how parallel entangling gates produce a crosstalk error that falls off as the inverse cube of the distance between the pairs. We also show experimental work demonstrating entangling gates on a fully-connected chain of seventeen 171Yb+ ions with fidelities as high as 97(1)\%.

}, url = {https://arxiv.org/abs/1905.10421}, author = {Kevin A. Landsman and Yukai Wu and Pak Hong Leung and Daiwei Zhu and Norbert M. Linke and Kenneth R. Brown and Luming Duan and Christopher R. Monroe} } @article {2283, title = {A Coherent Spin-Photon Interface in Silicon}, journal = {Nature }, volume = {555}, year = {2018}, month = {2018/03/29}, pages = {599-603}, abstract = {

Electron spins in silicon quantum dots are attractive systems for quantum computing due to their long coherence times and the promise of rapid scaling using semiconductor fabrication techniques. While nearest neighbor exchange coupling of two spins has been demonstrated, the interaction of spins via microwave frequency photons could enable long distance spin-spin coupling and \"all-to-all\" qubit connectivity. Here we demonstrate strong-coupling between a single spin in silicon and a microwave frequency photon with spin-photon coupling rates g_s/(2π) \> 10 MHz. The mechanism enabling coherent spin-photon interactions is based on spin-charge hybridization in the presence of a magnetic field gradient. In addition to spin-photon coupling, we demonstrate coherent control of a single spin in the device and quantum non-demolition spin state readout using cavity photons. These results open a direct path toward entangling single spins using microwave frequency photons.

}, doi = {https://doi.org/10.1038/nature25769}, url = {https://arxiv.org/abs/1710.03265}, author = {X. Mi and M. Benito and S. Putz and D. M. Zajac and J. M. Taylor and Guido Burkard and J. R. Petta} } @article {2148, title = {A coherent spin{\textendash}photon interface in silicon}, journal = {Nature}, year = {2018}, month = {2018/02/14}, abstract = {

Electron spins in silicon quantum dots are attractive systems for quantum computing owing to their long coherence times and the promise of rapid scaling of the number of dots in a system using semiconductor fabrication techniques. Although nearest-neighbour exchange coupling of two spins has been demonstrated, the interaction of spins via microwave-frequency photons could enable long-distance spin\–spin coupling and connections between arbitrary pairs of qubits (\‘all-to-all\’ connectivity) in a spin-based quantum processor. Realizing coherent spin\–photon coupling is challenging because of the small magnetic-dipole moment of a single spin, which limits magnetic-dipole coupling rates to less than 1 kilohertz. Here we demonstrate strong coupling between a single spin in silicon and a single microwave-frequency photon, with spin\–photon coupling rates of more than 10 megahertz. The mechanism that enables the coherent spin\–photon interactions is based on spin\–charge hybridization in the presence of a magnetic-field gradient. In addition to spin\–photon coupling, we demonstrate coherent control and dispersive readout of a single spin. These results open up a direct path to entangling single spins using microwave-frequency photons.

}, doi = {10.1038/nature25769}, url = {https://www.nature.com/articles/nature25769$\#$author-information}, author = {X. Mi and M. Benito and S. Putz and D. M. Zajac and J. M. Taylor and Guido Burkard and J. R. Petta} } @article {2289, title = {Cryogenic Trapped-Ion System for Large Scale Quantum Simulation}, year = {2018}, abstract = {

We present a cryogenic ion trapping system designed for large scale quantum simulation of spin models. Our apparatus is based on a segmented-blade ion trap enclosed in a 4 K cryostat, which enables us to routinely trap over 100 171Yb+ ions in a linear configuration for hours due to a low background gas pressure from differential cryo-pumping. We characterize the cryogenic vacuum by using trapped ion crystals as a pressure gauge, measuring both inelastic and elastic collision rates with the molecular background gas. We demonstrate nearly equidistant ion spacing for chains of up to 44 ions using anharmonic axial potentials. This reliable production and lifetime enhancement of large linear ion chains will enable quantum simulation of spin models that are intractable with classical computer modelling.

}, url = {https://arxiv.org/abs/1802.03118}, author = {G. Pagano and P. W. Hess and H. B. Kaplan and W. L. Tan and P. Richerme and P. Becker and A. Kyprianidis and J. Zhang and E. Birckelbaw and M. R. Hernandez and Y. Wu and C. Monroe} } @article {2141, title = {Dark state optical lattice with sub-wavelength spatial structure}, journal = {Phys. Rev. Lett.}, volume = {120}, year = {2018}, month = {2018/02/20}, pages = {083601}, abstract = {

We report on the experimental realization of a conservative optical lattice for cold atoms with a subwavelength spatial structure. The potential is based on the nonlinear optical response of three-level atoms in laser-dressed dark states, which is not constrained by the diffraction limit of the light generating the potential. The lattice consists of a one-dimensional array of ultranarrow barriers with widths less than 10\ nm, well below the wavelength of the lattice light, physically realizing a Kronig-Penney potential. We study the band structure and dissipation of this lattice and find good agreement with theoretical predictions. Even on resonance, the observed lifetimes of atoms trapped in the lattice are as long as 44\ ms, nearly\ 105times the excited state lifetime, and could be further improved with more laser intensity. The potential is readily generalizable to higher dimensions and different geometries, allowing, for example, nearly perfect box traps, narrow tunnel junctions for atomtronics applications, and dynamically generated lattices with subwavelength spacings.

}, doi = {10.1103/PhysRevLett.120.083601}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.120.083601}, author = {Yang Wang and Sarthak Subhankar and Przemyslaw Bienias and Mateusz Lacki and Tsz-Chun Tsui and Mikhail A. Baranov and Alexey V. Gorshkov and Peter Zoller and James V. Porto and Steven L. Rolston} } @article {2104, title = {Electro-optomechanical equivalent circuits for quantum transduction}, year = {2018}, month = {2018/10/15}, abstract = {

Using the techniques of optomechanics, a high-Q mechanical oscillator may serve as a link between electromagnetic modes of vastly different frequencies. This approach has successfully been exploited for the frequency conversion of classical signals and has the potential of performing quantum state transfer between superconducting circuitry and a traveling optical signal. Such transducers are often operated in a linear regime, where the hybrid system can be described using linear response theory based on the Heisenberg-Langevin equations. While mathematically straightforward to solve, this approach yields little intuition about the dynamics of the hybrid system to aid the optimization of the transducer. As an analysis and design tool for such electro-optomechanical transducers, we introduce an equivalent circuit formalism, where the entire transducer is represented by an electrical circuit. Thereby we integrate the transduction functionality of optomechanical (OM) systems into the toolbox of electrical engineering allowing the use of its well-established design techniques. This unifying impedance description can be applied both for static (DC) and harmonically varying (AC) drive fields, accommodates arbitrary linear circuits, and is not restricted to the resolved-sideband regime. Furthermore, by establishing the quantized input/output formalism for the equivalent circuit, we obtain the scattering matrix for linear transducers using circuit analysis, and thereby have a complete quantum mechanical characterization of the transducer. Hence, this mapping of the entire transducer to the language of electrical engineering both sheds light on how the transducer performs and can at the same time be used to optimize its performance by aiding the design of a suitable electrical circuit.

}, doi = {https://doi.org/10.1103/PhysRevApplied.10.044036}, url = {https://arxiv.org/abs/1710.10136}, author = {Emil Zeuthen and Albert Schliesser and J. M. Taylor and Anders S. S{\o}rensen} } @article {2054, title = {Entanglement of purification: from spin chains to holography}, journal = {Journal of High Energy Physics}, year = {2018}, month = {2018/01/22}, pages = {98}, abstract = {

Purification is a powerful technique in quantum physics whereby a mixed quantum state is extended to a pure state on a larger system. This process is not unique, and in systems composed of many degrees of freedom, one natural purification is the one with minimal entanglement. Here we study the entropy of the minimally entangled purification, called the entanglement of purification, in three model systems: an Ising spin chain, conformal field theories holographically dual to Einstein gravity, and random stabilizer tensor networks. We conjecture values for the entanglement of purification in all these models, and we support our conjectures with a variety of numerical and analytical results. We find that such minimally entangled purifications have a number of applications, from enhancing entanglement-based tensor network methods for describing mixed states to elucidating novel aspects of the emergence of geometry from entanglement in the AdS/CFT correspondence.

}, doi = {10.1007/JHEP01(2018)098}, url = {https://link.springer.com/article/10.1007\%2FJHEP01\%282018\%29098$\#$citeas}, author = {Phuc Nguyen and Trithep Devakul and Matthew G. Halbasch and Michael P. Zaletel and Brian Swingle} } @article {2282, title = {Experimentally Generated Randomness Certified by the Impossibility of Superluminal Signals}, journal = {Nature}, volume = {556}, year = {2018}, month = {2018/04/11}, pages = {223-226}, abstract = {

From dice to modern complex circuits, there have been many attempts to build increasingly better devices to generate random numbers. Today, randomness is fundamental to security and cryptographic systems, as well as safeguarding privacy. A key challenge with random number generators is that it is hard to ensure that their outputs are unpredictable. For a random number generator based on a physical process, such as a noisy classical system or an elementary quantum measurement, a detailed model describing the underlying physics is required to assert unpredictability. Such a model must make a number of assumptions that may not be valid, thereby compromising the integrity of the device. However, it is possible to exploit the phenomenon of quantum nonlocality with a loophole-free Bell test to build a random number generator that can produce output that is unpredictable to any adversary limited only by general physical principles. With recent technological developments, it is now possible to carry out such a loophole-free Bell test. Here we present certified randomness obtained from a photonic Bell experiment and extract 1024 random bits uniform to within 10\−12. These random bits could not have been predicted within any physical theory that prohibits superluminal signaling and allows one to make independent measurement choices. To certify and quantify the randomness, we describe a new protocol that is optimized for apparatuses characterized by a low per-trial violation of Bell inequalities. We thus enlisted an experimental result that fundamentally challenges the notion of determinism to build a system that can increase trust in random sources. In the future, random number generators based on loophole-free Bell tests may play a role in increasing the security and trust of our cryptographic systems and infrastructure.

}, doi = {https://doi.org/10.1038/s41586-018-0019-0}, url = {https://arxiv.org/abs/1803.06219}, author = {Peter Bierhorst and Emanuel Knill and Scott Glancy and Yanbao Zhang and Alan Mink and Stephen Jordan and Andrea Rommal and Yi-Kai Liu and Bradley Christensen and Sae Woo Nam and Martin J. Stevens and Lynden K. Shalm} } @article {2147, title = {High-fidelity quantum gates in Si/SiGe double quantum dots}, journal = {Physical Review B}, volume = {97}, year = {2018}, month = {2018/02/15}, pages = {085421}, abstract = {

Motivated by recent experiments of Zajac\ et\ al.\ [Science359, 439 (2018)], we theoretically describe high-fidelity two-qubit gates using the exchange interaction between the spins in neighboring quantum dots subject to a magnetic field gradient. We use a combination of analytical calculations and numerical simulations to provide the optimal pulse sequences and parameter settings for the gate operation. We present a synchronization method which avoids detrimental spin flips during the gate operation and provide details about phase mismatches accumulated during the two-qubit gates which occur due to residual exchange interaction, nonadiabatic pulses, and off-resonant driving. By adjusting the gate times, synchronizing the resonant and off-resonant transitions, and compensating these phase mismatches by phase control, the overall gate fidelity can be increased significantly.

}, doi = {10.1103/PhysRevB.97.085421}, url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.085421}, author = {Maximilian Russ and D. M. Zajac and A. J. Sigillito and F. Borjans and J. M. Taylor and J. R. Petta and Guido Burkard} } @article {2286, title = {Parallel Entangling Operations on a Universal Ion Trap Quantum Computer}, year = {2018}, abstract = {

The circuit model of a quantum computer consists of sequences of gate operations between quantum bits (qubits), drawn from a universal family of discrete operations. The ability to execute parallel entangling quantum gates offers clear efficiency gains in numerous quantum circuits as well as for entire algorithms such as Shor\&$\#$39;s factoring algorithm and quantum simulations. In cases such as full adders and multiple-control Toffoli gates, parallelism can provide an exponential improvement in overall execution time. More importantly, quantum gate parallelism is essential for the practical fault-tolerant error correction of qubits that suffer from idle errors. The implementation of parallel quantum gates is complicated by potential crosstalk, especially between qubits fully connected by a common-mode bus, such as in Coulomb-coupled trapped atomic ions or cavity-coupled superconducting transmons. Here, we present the first experimental results for parallel 2-qubit entangling gates in an array of fully-connected trapped ion qubits. We demonstrate an application of this capability by performing a 1-bit full addition operation on a quantum computer using a depth-4 quantum circuit. These results exploit the power of highly connected qubit systems through classical control techniques, and provide an advance toward speeding up quantum circuits and achieving fault tolerance with trapped ion quantum computers.

}, url = {https://arxiv.org/abs/1810.11948}, author = {C. Figgatt and A. Ostrander and N. M. Linke and K. A. Landsman and D. Zhu and D. Maslov and C. Monroe} } @article {2218, title = {Photon propagation through dissipative Rydberg media at large input rates}, year = {2018}, abstract = {

We study the dissipative propagation of quantized light in interacting Rydberg media under the conditions of electromagnetically induced transparency (EIT). Rydberg blockade physics in optically dense atomic media leads to strong dissipative interactions between single photons. The regime of high incoming photon flux constitutes a challenging many-body dissipative problem. We experimentally study in detail for the first time the pulse shapes and the second-order correlation function of the outgoing field and compare our data with simulations based on two novel theoretical approaches well-suited to treat this many-photon limit. At low incoming flux, we report good agreement between both theories and the experiment. For higher input flux, the intensity of the outgoing light is lower than that obtained from theoretical predictions. We explain this discrepancy using a simple phenomenological model taking into account pollutants, which are nearly-stationary Rydberg excitations coming from the reabsorption of scattered probe photons. At high incoming photon rates, the blockade physics results in unconventional shapes of measured correlation functions.\ 

}, url = {https://arxiv.org/abs/1807.07586}, author = {Przemyslaw Bienias and James Douglas and Asaf Paris-Mandoki and Paraj Titum and Ivan Mirgorodskiy and Christoph Tresp and Emil Zeuthen and Michael Gullans and Marco Manzoni and Sebastian Hofferberth and Darrick Chang and Alexey V. Gorshkov} } @article {2319, title = {Practitioner{\textquoteright}s guide to social network analysis: Examining physics anxiety in an active-learning setting}, year = {2018}, abstract = {

The application of social network analysis (SNA) has recently grown prevalent in science, technology, engineering, and mathematics education research. Research on classroom networks has led to greater understandings of student persistence in physics majors, changes in their career-related beliefs (e.g., physics interest), and their academic success. In this paper, we aim to provide a practitioner\&$\#$39;s guide to carrying out research using SNA, including how to develop data collection instruments, set up protocols for gathering data, as well as identify network methodologies relevant to a wide range of research questions beyond what one might find in a typical primer. We illustrate these techniques using student anxiety data from active-learning physics classrooms. We explore the relationship between students\&$\#$39; physics anxiety and the social networks they participate in throughout the course of a semester. We find that students\&$\#$39; with greater numbers of outgoing interactions are more likely to experience negative anxiety shifts even while we control for {\it pre} anxiety, gender, and final course grade. We also explore the evolution of student networks and find that the second half of the semester is a critical period for participating in interactions associated with decreased physics anxiety. Our study further supports the benefits of dynamic group formation strategies that give students an opportunity to interact with as many peers as possible throughout a semester. To complement our guide to SNA in education research, we also provide a set of tools for letting other researchers use this approach in their work -- the {\it SNA toolbox} -- that can be accessed on GitHub.\ 

}, url = {https://arxiv.org/abs/1809.00337}, author = {Remy Dou and Justyna P. Zwolak} } @article {2268, title = {QFlow lite dataset: A machine-learning approach to the charge states in quantum dot experiments}, journal = {PLOS ONE}, volume = {13}, year = {2018}, month = {2018}, pages = {e0205844}, type = {2018/10/17}, abstract = {

Over the past decade, machine learning techniques have revolutionized how research is done, from designing new materials and predicting their properties to assisting drug discovery to advancing cybersecurity. Recently, we added to this list by showing how a machine learning algorithm (a so-called learner) combined with an optimization routine can assist experimental efforts in the realm of tuning semiconductor quantum dot (QD) devices. Among other applications, semiconductor QDs are a candidate system for building quantum computers. The present-day tuning techniques for bringing the QD devices into a desirable configuration suitable for quantum computing that rely on heuristics do not scale with the increasing size of the quantum dot arrays required for even near-term quantum computing demonstrations. Establishing a reliable protocol for tuning that does not rely on the gross-scale heuristics developed by experimentalists is thus of great importance. To implement the machine learning-based approach, we constructed a dataset of simulated QD device characteristics, such as the conductance and the charge sensor response versus the applied electrostatic gate voltages. Here, we describe the methodology for generating the dataset, as well as its validation in training convolutional neural networks. We show that the learner\&$\#$39;s accuracy in recognizing the state of a device is ~96.5 \% in both current- and charge-sensor-based training. We also introduce a tool that enables other researchers to use this approach for further research: QFlow lite - a Python-based mini-software suite that uses the dataset to train neural networks to recognize the state of a device and differentiate between states in experimental data. This work gives the definitive reference for the new dataset that will help enable researchers to use it in their experiments or to develop new machine learning approaches and concepts

}, doi = {https://doi.org/10.1371/journal.pone.0205844}, url = {https://arxiv.org/abs/1809.10018}, author = {Justyna P. Zwolak and Sandesh S. Kalantre and Xingyao Wu and Stephen Ragole and J. M. Taylor} } @article {2306, title = {Quantitative Robustness Analysis of Quantum Programs (Extended Version)}, journal = {Proc. ACM Program. Lang.}, volume = {3}, year = {2018}, month = {2018/12/1}, pages = {Article 31}, abstract = {

Quantum computation is a topic of significant recent interest, with practical advances coming from both research and industry. A major challenge in quantum programming is dealing with errors (quantum noise) during execution. Because quantum resources (e.g., qubits) are scarce, classical error correction techniques applied at the level of the architecture are currently cost-prohibitive. But while this reality means that quantum programs are almost certain to have errors, there as yet exists no principled means to reason about erroneous behavior. This paper attempts to fill this gap by developing a semantics for erroneous quantum while-programs, as well as a logic for reasoning about them. This logic permits proving a property we have identified, called ε-robustness, which characterizes possible \"distance\" between an ideal program and an erroneous one. We have proved the logic sound, and showed its utility on several case studies, notably: (1) analyzing the robustness of noisy versions of the quantum Bernoulli factory (QBF) and quantum walk (QW); (2) demonstrating the (in)effectiveness of different error correction schemes on single-qubit errors; and (3) analyzing the robustness of a fault-tolerant version of QBF.

}, doi = {https://doi.org/10.1145/3290344}, url = {https://arxiv.org/abs/1811.03585}, author = {Shih-Han Hung and Kesha Hietala and Shaopeng Zhu and Mingsheng Ying and Michael Hicks and Xiaodi Wu} } @article {2150, title = {Resonantly driven CNOT gate for electron spins}, journal = {Science}, volume = {359}, year = {2018}, month = {2018/01/26}, pages = {439-442}, abstract = {

Single-qubit rotations and two-qubit CNOT operations are crucial ingredients for universal quantum computing. Although high-fidelity single-qubit operations have been achieved using the electron spin degree of freedom, realizing a robust CNOT gate has been challenging because of rapid nuclear spin dephasing and charge noise. We demonstrate an efficient resonantly driven CNOT gate for electron spins in silicon. Our platform achieves single-qubit rotations with fidelities greater than 99\%, as verified by randomized benchmarking. Gate control of the exchange coupling allows a quantum CNOT gate to be implemented with resonant driving in ~200 nanoseconds. We used the CNOT gate to generate a Bell state with 78\% fidelity (corrected for errors in state preparation and measurement). Our quantum dot device architecture enables multi-qubit algorithms in silicon.

}, doi = {10.1126/science.aao5965}, url = {http://science.sciencemag.org/content/359/6374/439}, author = {D. M. Zajac and A. J. Sigillito and M. Russ and F. Borjans and J. M. Taylor and Guido Burkard and J. R. Petta} } @article {2320, title = {Studying community development: a network analytical approach}, year = {2018}, abstract = {

Research shows that community plays a central role in learning, and strong community engages students and aids in student persistence. Thus, understanding the function and structure of communities in learning environments is essential to education. We use social network analysis to explore the community dynamics of students in a pre-matriculation, two-week summer program. Unlike previous network analysis studies in PER, we build our networks from classroom video that has been coded for student interactions using labeled, directed ties. We define 3 types of interaction: on task interactions (regarding the assigned task), on topic interactions (having to do with science, technology, engineering, and mathematics (STEM)), and off topic interactions (unrelated to the assignment or STEM). To study the development of community in this program, we analyze the shift in conversation topicality over the course of the program. Conversations are more on-task toward the end of the program and we propose that this conversational shift represents a change in student membership within their forming community.\ 

}, url = {https://arxiv.org/abs/1808.08193}, author = {C. A. Hass and Florian Genz and Mary Bridget Kustusch and Pierre-P. A. Ouime and Katarzyna Pomian and Eleanor C. Sayre and Justyna P. Zwolak} } @article {1814, title = {Correlated Photon Dynamics in Dissipative Rydberg Media}, journal = {Physical Review Letters}, volume = {119}, year = {2017}, month = {2017/07/26}, pages = {043602}, abstract = {

Rydberg blockade physics in optically dense atomic media under the conditions of electromagnetically induced transparency (EIT) leads to strong dissipative interactions between single photons. We introduce a new approach to analyzing this challenging many-body problem in the limit of large optical depth per blockade radius. In our approach, we separate the single-polariton EIT physics from Rydberg-Rydberg interactions in a serialized manner while using a hard-sphere model for the latter, thus capturing the dualistic particle-wave nature of light as it manifests itself in dissipative Rydberg-EIT media. Using this approach, we analyze the saturation behavior of the transmission through one-dimensional Rydberg-EIT media in the regime of non-perturbative dissipative interactions relevant to current experiments. Our model is in good agreement with experimental data. We also analyze a scheme for generating regular trains of single photons from continuous-wave input and derive its scaling behavior in the presence of imperfect single-photon EIT.

}, doi = {10.1103/PhysRevLett.119.043602}, url = {https://arxiv.org/abs/1608.06068}, author = {Emil Zeuthen and Michael Gullans and Mohammad F. Maghrebi and Alexey V. Gorshkov} } @article {2009, title = {Extreme learning machines for regression based on V-matrix method}, journal = {Cognitive Neurodynamics}, year = {2017}, month = {2017/06/10}, abstract = {

This paper studies the joint effect of V-matrix, a recently proposed framework for statistical inferences, and extreme learning machine (ELM) on regression problems. First of all, a novel algorithm is proposed to efficiently evaluate the V-matrix. Secondly, a novel weighted ELM algorithm called V-ELM is proposed based on the explicit kernel mapping of ELM and the V-matrix method. Though V-matrix method could capture the geometrical structure of training data, it tends to assign a higher weight to instance with smaller input value. In order to avoid this bias, a novel method called VI-ELM is proposed by minimizing both the regression error and the V-matrix weighted error simultaneously. Finally, experiment results on 12 real world benchmark datasets show the effectiveness of our proposed methods.

}, issn = {1871-4099}, doi = {10.1007/s11571-017-9444-2}, url = {http://dx.doi.org/10.1007/s11571-017-9444-2}, author = {Yang, Zhiyong and Zhang, Taohong and Lu, Jingcheng and Yuan Su and Zhang, Dezheng and Duan, Yaowu} } @article {1990, title = {Genuine N -partite entanglement without N -partite correlation functions}, journal = {Physical Review A}, volume = {95}, year = {2017}, month = {2017/06/26}, pages = {062331}, abstract = {

A genuinely\ N-partite entangled state may display vanishing\ N-partite correlations measured for arbitrary local observables. In such states the genuine entanglement is noticeable solely in correlations between subsets of particles. A straightforward way to obtain such states for odd\ N\ is to design an \“antistate\” in which all correlations between an odd number of observers are exactly opposite. Evenly mixing a state with its antistate then produces a mixed state with no\ N-partite correlations, with many of them genuinely multiparty entangled. Intriguingly, all known examples of \“entanglement without correlations\” involve an\ odd\ number of particles. Here we further develop the idea of antistates, thereby shedding light on the different properties of even and odd particle systems. We conjecture that there is no antistate to any pure even-N-party entangled state making the simple construction scheme unfeasible. However, as we prove by construction, higher-rank examples of entanglement without correlations for arbitrary even\ N\ indeed exist. These classes of states exhibit genuine entanglement and even violate an\ N-partite Bell inequality, clearly demonstrating the nonclassical features of these states as well as showing their applicability for quantum information processing.

}, doi = {doi.org/10.1103/PhysRevA.95.062331}, url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.95.062331}, author = {Minh C. Tran and Margherita Zuppardo and Anna de Rosier and Lukas Knips and Wieslaw Laskowski and Tomasz Paterek and Harald Weinfurter} } @article {2103, title = {Machine Learning techniques for state recognition and auto-tuning in quantum dots}, year = {2017}, month = {2017/12/13}, abstract = {

Recent progress in building large-scale quantum devices for exploring quantum computing and simulation paradigms has relied upon effective tools for achieving and maintaining good experimental parameters, i.e. tuning up devices. In many cases, including in quantum-dot based architectures, the parameter space grows substantially with the number of qubits, and may become a limit to scalability. Fortunately, machine learning techniques for pattern recognition and image classification using so-called deep neural networks have shown surprising successes for computer-aided understanding of complex systems. In this work, we use deep and convolutional neural networks to characterize states and charge configurations of semiconductor quantum dot arrays when one can only measure a current-voltage characteristic of transport (here conductance) through such a device. For simplicity, we model a semiconductor nanowire connected to leads and capacitively coupled to depletion gates using the Thomas-Fermi approximation and Coulomb blockade physics. We then generate labeled training data for the neural networks, and find at least 90 \% accuracy for charge and state identification for single and double dots purely from the dependence of the nanowire\’s conductance upon gate voltages. Using these characterization networks, we can then optimize the parameter space to achieve a desired configuration of the array, a technique we call \‘auto-tuning\’. Finally, we show how such techniques can be implemented in an experimental setting by applying our approach to an experimental data set, and outline further problems in this domain, from using charge sensing data to extensions to full one and two-dimensional arrays, that can be tackled with machine learning.

}, url = {https://arxiv.org/abs/1712.04914}, author = {Sandesh S. Kalantre and Justyna P. Zwolak and Stephen Ragole and Xingyao Wu and Neil M. Zimmerman and M. D. Stewart and J. M. Taylor} } @article {2053, title = {Observation of a Many-Body Dynamical Phase Transition with a 53-Qubit Quantum Simulator}, journal = {Nature}, volume = {551}, year = {2017}, month = {2017/11/29}, pages = {601-604}, abstract = {

A quantum simulator is a restricted class of quantum computer that controls the interactions between quantum bits in a way that can be mapped to certain difficult quantum many-body problems. As more control is exerted over larger numbers of qubits, the simulator can tackle a wider range of problems, with the ultimate limit being a universal quantum computer that can solve general classes of hard problems. We use a quantum simulator composed of up to 53 qubits to study a non-equilibrium phase transition in the transverse field Ising model of magnetism, in a regime where conventional statistical mechanics does not apply. The qubits are represented by trapped ion spins that can be prepared in a variety of initial pure states. We apply a global long-range Ising interaction with controllable strength and range, and measure each individual qubit with near 99\% efficiency. This allows the single-shot measurement of arbitrary many-body correlations for the direct probing of the dynamical phase transition and the uncovering of computationally intractable features that rely on the long-range interactions and high connectivity between the qubits.

}, doi = {10.1038/nature24654}, url = {https://www.nature.com/articles/nature24654}, author = {J. Zhang and G. Pagano and P. W. Hess and A. Kyprianidis and P. Becker and H. Kaplan and Alexey V. Gorshkov and Z. -X. Gong and C. Monroe} } @article {1787, title = {Quantum state tomography via reduced density matrices}, journal = {Physical Review Letters}, volume = {118}, year = {2017}, month = {2017/01/09}, pages = {020401}, abstract = {

Quantum state tomography via local measurements is an efficient tool for characterizing quantum states. However it requires that the original global state be uniquely determined (UD) by its local reduced density matrices (RDMs). In this work we demonstrate for the first time a class of states that are UD by their RDMs under the assumption that the global state is pure, but fail to be UD in the absence of that assumption. This discovery allows us to classify quantum states according to their UD properties, with the requirement that each class be treated distinctly in the practice of simplifying quantum state tomography. Additionally we experimentally test the feasibility and stability of performing quantum state tomography via the measurement of local RDMs for each class. These theoretical and experimental results advance the project of performing efficient and accurate quantum state tomography in practice.

}, doi = {10.1103/PhysRevLett.118.020401}, url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.020401}, author = {Tao Xin and Dawei Lu and Joel Klassen and Nengkun Yu and Zhengfeng Ji and Jianxin Chen and Xian Ma and Guilu Long and Bei Zeng and Raymond Laflamme} } @article {2048, title = {On the readiness of quantum optimization machines for industrial applications}, year = {2017}, month = {2017/08/31}, abstract = {

There have been multiple attempts to demonstrate that quantum annealing and, in particular, quantum annealing on quantum annealing machines, has the potential to outperform current classical optimization algorithms implemented on CMOS technologies. The benchmarking of these devices has been controversial. Initially, random spin-glass problems were used, however, these were quickly shown to be not well suited to detect any quantum speedup. Subsequently, benchmarking shifted to carefully crafted synthetic problems designed to highlight the quantum nature of the hardware while (often) ensuring that classical optimization techniques do not perform well on them. Even worse, to date a true sign of improved scaling with the number problem variables remains elusive when compared to classical optimization techniques. Here, we analyze the readiness of quantum annealing machines for real-world application problems. These are typically not random and have an underlying structure that is hard to capture in synthetic benchmarks, thus posing unexpected challenges for optimization techniques, both classical and quantum alike. We present a comprehensive computational scaling analysis of fault diagnosis in digital circuits, considering architectures beyond D-wave quantum annealers. We find that the instances generated from real data in multiplier circuits are harder than other representative random spin-glass benchmarks with a comparable number of variables. Although our results show that transverse-field quantum annealing is outperformed by state-of-the-art classical optimization algorithms, these benchmark instances are hard and small in the size of the input, therefore representing the first industrial application ideally suited for near-term quantum annealers.

}, url = {https://arxiv.org/abs/1708.09780}, author = {Alejandro Perdomo-Ortiz and Alexander Feldman and Asier Ozaeta and Sergei V. Isakov and Zheng Zhu and Bryan O{\textquoteright}Gorman and Helmut G. Katzgraber and Alexander Diedrich and Hartmut Neven and Johan de Kleer and Brad Lackey and Rupak Biswas} } @article {1816, title = {Valley Blockade in a Silicon Double Quantum Dot}, journal = {Physical Review B}, volume = {96}, year = {2017}, month = {2017/11/13}, pages = {205302}, abstract = {

Electrical transport in double quantum dots (DQDs) illuminates many interesting features of the dots\&$\#$39; carrier states. Recent advances in silicon quantum information technologies have renewed interest in the valley states of electrons confined in silicon. Here we show measurements of DC transport through a mesa-etched silicon double quantum dot. Comparing bias triangles (i.e., regions of allowed current in DQDs) at positive and negative bias voltages we find a systematic asymmetry in the size of the bias triangles at the two bias polarities. Asymmetries of this nature are associated with blocking of tunneling events due to the occupation of a metastable state. Several features of our data lead us to conclude that the states involved are not simple spin states. Rather, we develop a model based on selective filling of valley states in the DQD that is consistent with all of the qualitative features of our data.

}, doi = {10.1103/PhysRevB.96.205302}, url = {https://arxiv.org/abs/1607.06107}, author = {Justin K. Perron and Michael Gullans and J. M. Taylor and M. D. Stewart, Jr. and Neil M. Zimmerman} } @article {1452, title = {Detecting Consistency of Overlapping Quantum Marginals by Separability}, journal = {Physical Review A}, volume = {93}, year = {2016}, month = {2016/03/03}, pages = {032105}, abstract = { The quantum marginal problem asks whether a set of given density matrices are consistent, i.e., whether they can be the reduced density matrices of a global quantum state. Not many non-trivial analytic necessary (or sufficient) conditions are known for the problem in general. We propose a method to detect consistency of overlapping quantum marginals by considering the separability of some derived states. Our method works well for the $k$-symmetric extension problem in general, and for the general overlapping marginal problems in some cases. Our work is, in some sense, the converse to the well-known $k$-symmetric extension criterion for separability. }, doi = {10.1103/PhysRevA.93.032105}, url = {http://arxiv.org/abs/1509.06591}, author = {Jianxin Chen and Zhengfeng Ji and Nengkun Yu and Bei Zeng} } @article {1912, title = {Figures of merit for quantum transducers}, year = {2016}, month = {2016/10/04}, abstract = {

Recent technical advances have sparked renewed interest in physical systems that couple simultaneously to different parts of the electromagnetic spectrum, thus enabling transduction of signals between vastly different frequencies at the level of single photons. Such hybrid systems have demonstrated frequency conversion of classical signals and have the potential of enabling quantum state transfer, e.g., between superconducting circuits and traveling optical signals. This Letter describes a simple approach for the theoretical characterization of performance for quantum transducers. Given that, in practice, one cannot attain ideal one-to-one quantum conversion, we will explore how well the transducer performs in various scenarios ranging from classical signal detection to applications for quantum information processing. While the performance of the transducer depends on the particular application in which it enters, we show that the performance can be characterized by defining two simple parameters: the signal transfer efficiency\ η\ and the added noise\ N.

}, url = {https://arxiv.org/abs/1610.01099}, author = {Emil Zeuthen and Albert Schliesser and Anders S. S{\o}rensen and J. M. Taylor} } @article {1783, title = {Joint product numerical range and geometry of reduced density matrices}, year = {2016}, month = {2016/06/23}, abstract = {The reduced density matrices of a many-body quantum system form a convex set, whose three-dimensional projection Θ is convex in R3. The boundary ∂Θ of Θ may exhibit nontrivial geometry, in particular ruled surfaces. Two physical mechanisms are known for the origins of ruled surfaces: symmetry breaking and gapless. In this work, we study the emergence of ruled surfaces for systems with local Hamiltonians in infinite spatial dimension, where the reduced density matrices are known to be separable as a consequence of the quantum de Finetti{\textquoteright}s theorem. This allows us to identify the reduced density matrix geometry with joint product numerical range Π of the Hamiltonian interaction terms. We focus on the case where the interaction terms have certain structures, such that ruled surface emerge naturally when taking a convex hull of Π. We show that, a ruled surface on ∂Θ sitting in Π has a gapless origin, otherwise it has a symmetry breaking origin. As an example, we demonstrate that a famous ruled surface, known as the oloid, is a possible shape of Θ, with two boundary pieces of symmetry breaking origin separated by two gapless lines.}, url = {http://arxiv.org/abs/1606.07422}, author = {Jianxin Chen and Cheng Guo and Zhengfeng Ji and Yiu-Tung Poon and Nengkun Yu and Bei Zeng and Jie Zhou} } @article {2508, title = {Measurement Protocol for the Entanglement Spectrum of Cold Atoms}, journal = {Phys. Rev. X }, volume = {6(4)}, year = {2016}, month = {2016/11/22}, abstract = {

Entanglement, and, in particular the entanglement spectrum, plays a major role in characterizing many-body quantum systems. While there has been a surge of theoretical works on the subject, no experimental measurement has been performed to date because of the lack of an implementable measurement scheme. Here, we propose a measurement protocol to access the entanglement spectrum of many-body states in experiments with cold atoms in optical lattices. Our scheme effectively performs a Ramsey spectroscopy of the entanglement Hamiltonian and is based on the ability to produce several copies of the state under investigation together with the possibility to perform a global swap gate between two copies conditioned on the state of an auxiliary qubit. We show how the required conditional swap gate can be implemented with cold atoms, either by using Rydberg interactions or coupling the atoms to a cavity mode. We illustrate these ideas on a simple (extended) Bose-Hubbard model where such a measurement protocol reveals topological features of the Haldane phase.\ 

}, doi = {https://doi.org/10.1103/PhysRevX.6.041033}, url = {https://arxiv.org/abs/1605.08624}, author = {Hannes Pichler and Guanyu Zhu and Alireza Seif and Peter Zoller and Mohammad Hafezi} } @article {2005, title = {{O}bservation of {P}rethermalization in {L}ong-{R}ange {I}nteracting {S}pin {C}hains}, year = {2016}, month = {2016/08/02}, abstract = {

Statistical mechanics can predict thermal equilibrium states for most classical systems, but for an isolated quantum system there is no general understanding on how equilibrium states dynamically emerge from the microscopic Hamiltonian. For instance, quantum systems that are near-integrable usually fail to thermalize in an experimentally realistic time scale and, instead, relax to quasi-stationary prethermal states that can be described by statistical mechanics when approximately conserved quantities are appropriately included in a generalized Gibbs ensemble (GGE). Here we experimentally study the relaxation dynamics of a chain of up to 22 spins evolving under a long-range transverse field Ising Hamiltonian following a sudden quench. For sufficiently long-ranged interactions the system relaxes to a new type of prethermal state that retains a strong memory of the initial conditions. In this case, the prethermal state cannot be described by a GGE, but rather arises from an emergent double-well potential felt by the spin excitations. This result shows that prethermalization occurs in a significantly broader context than previously thought, and reveals new challenges for a generic understanding of the thermalization of quantum systems, particularly in the presence of long-range interactions.

}, url = {https://arxiv.org/abs/1608.00681}, author = {B. Neyenhuis and J. Smith and A. C. Lee and J. Zhang and P. Richerme and P. W. Hess and Z. -X. Gong and Alexey V. Gorshkov and C. Monroe} } @article {1705, title = {Performance of QAOA on Typical Instances of Constraint Satisfaction Problems with Bounded Degree}, year = {2016}, month = {2016/01/08}, abstract = {We consider constraint satisfaction problems of bounded degree, with a good notion of "typicality", e.g. the negation of the variables in each constraint is taken independently at random. Using the quantum approximate optimization algorithm (QAOA), we show that μ+Ω(1/D--√) fraction of the constraints can be satisfied for typical instances, with the assignment efficiently produced by QAOA. We do so by showing that the averaged fraction of constraints being satisfied is μ+Ω(1/D--√), with small variance. Here μ is the fraction that would be satisfied by a uniformly random assignment, and D is the number of constraints that each variable can appear. CSPs with typicality include Max-kXOR and Max-kSAT. We point out how it can be applied to determine the typical ground-state energy of some local Hamiltonians. We also give a similar result for instances with "no overlapping constraints", using the quantum algorithm. We sketch how the classical algorithm might achieve some partial result.}, url = {http://arxiv.org/abs/1601.01744}, author = {Cedric Yen-Yu Lin and Yechao Zhu} } @article {1706, title = {Pure-state tomography with the expectation value of Pauli operators}, journal = {Physical Review A}, volume = {93}, year = {2016}, month = {2016/03/31}, pages = {032140}, abstract = {

We examine the problem of finding the minimum number of Pauli measurements needed to uniquely determine an arbitrary n-qubit pure state among all quantum states. We show that only 11 Pauli measurements are needed to determine an arbitrary two-qubit pure state compared to the full quantum state tomography with 16 measurements, and only 31 Pauli measurements are needed to determine an arbitrary three-qubit pure state compared to the full quantum state tomography with 64 measurements. We demonstrate that our protocol is robust under depolarizing error with simulated random pure states. We experimentally test the protocol on two- and three-qubit systems with nuclear magnetic resonance techniques. We show that the pure state tomography protocol saves us a number of measurements without considerable loss of fidelity. We compare our protocol with same-size sets of randomly selected Pauli operators and find that our selected set of Pauli measurements significantly outperforms those random sampling sets. As a direct application, our scheme can also be used to reduce the number of settings needed for pure-state tomography in quantum optical systems.

}, doi = {http://dx.doi.org/10.1103/PhysRevA.93.032140}, url = {http://arxiv.org/abs/1601.05379}, author = {Xian Ma and Tyler Jackson and Hui Zhou and Jianxin Chen and Dawei Lu and Michael D. Mazurek and Kent A.G. Fisher and Xinhua Peng and David Kribs and Kevin J. Resch and Zhengfeng Ji and Bei Zeng and Raymond Laflamme} } @article {1689, title = {Tomography is necessary for universal entanglement detection with single-copy observables}, journal = {Physical Review Letters}, volume = {116}, year = {2016}, month = {2016/06/07}, pages = {230501}, abstract = {Entanglement, one of the central mysteries of quantum mechanics, plays an essential role in numerous applications of quantum information theory. A natural question of both theoretical and experimental importance is whether universal entanglement detection is possible without full state tomography. In this work, we prove a no-go theorem that rules out this possibility for any non-adaptive schemes that employ single-copy measurements only. We also examine in detail a previously implemented experiment, which claimed to detect entanglement of two-qubit states via adaptive single-copy measurements without full state tomography. By performing the experiment and analyzing the data, we demonstrate that the information gathered is indeed sufficient to reconstruct the state. These results reveal a fundamental limit for single-copy measurements in entanglement detection, and provides a general framework to study the detection of other interesting properties of quantum states, such as the positivity of partial transpose and the k-symmetric extendibility.}, doi = {10.1103/PhysRevLett.116.230501}, url = {http://arxiv.org/abs/1511.00581}, author = {Dawei Lu and Tao Xin and Nengkun Yu and Zhengfeng Ji and Jianxin Chen and Guilu Long and Jonathan Baugh and Xinhua Peng and Bei Zeng and Raymond Laflamme} } @article {1602, title = {Whose Information? Information About What?}, journal = {Quantum [Un]Speakables II: 50 Years of Bell{\textquoteright}s Theorem}, year = {2016}, month = {2016/01/01}, author = {Jeffrey Bub and Anton Zeilinger and Reinhold Bertlmann} } @article {1462, title = {Discontinuity of Maximum Entropy Inference and Quantum Phase Transitions}, journal = {New Journal of Physics}, volume = {17}, year = {2015}, month = {2015/08/10}, pages = {083019}, abstract = { In this paper, we discuss the connection between two genuinely quantum phenomena --- the discontinuity of quantum maximum entropy inference and quantum phase transitions at zero temperature. It is shown that the discontinuity of the maximum entropy inference of local observable measurements signals the non-local type of transitions, where local density matrices of the ground state change smoothly at the transition point. We then propose to use the quantum conditional mutual information of the ground state as an indicator to detect the discontinuity and the non-local type of quantum phase transitions in the thermodynamic limit. }, doi = {10.1088/1367-2630/17/8/083019}, url = {http://arxiv.org/abs/1406.5046v2}, author = {Jianxin Chen and Zhengfeng Ji and Chi-Kwong Li and Yiu-Tung Poon and Yi Shen and Nengkun Yu and Bei Zeng and Duanlu Zhou} } @article {1601, title = {Quantum Entanglement and Information}, journal = {The Stanford Encyclopedia of Philosophy}, year = {2015}, month = {02/07/2015}, abstract = {Quantum entanglement is a physical resource, like energy, associated with the peculiar nonclassical correlations that are possible between separated quantum systems. Entanglement can be measured, transformed, and purified. A pair of quantum systems in an entangled state can be used as a quantum information channel to perform computational and cryptographic tasks that are impossible for classical systems. The general study of the information-processing capabilities of quantum systems is the subject of quantum information theory.}, url = {http://plato.stanford.edu/archives/sum2015/entries/qt-entangle/}, author = {Jeffrey Bub and Edward N. Zalta} } @article {1450, title = {Universal Subspaces for Local Unitary Groups of Fermionic Systems}, journal = {Communications in Mathematical Physics}, volume = {333}, year = {2015}, month = {2014/10/10}, pages = {541 - 563}, abstract = { Let $\mathcal{V}=\wedge^N V$ be the $N$-fermion Hilbert space with $M$-dimensional single particle space $V$ and $2N\le M$. We refer to the unitary group $G$ of $V$ as the local unitary (LU) group. We fix an orthonormal (o.n.) basis $\ket{v_1},...,\ket{v_M}$ of $V$. Then the Slater determinants $e_{i_1,...,i_N}:= \ket{v_{i_1}\we v_{i_2}\we...\we v_{i_N}}$ with $i_1<...3. If $M$ is even, the well known BCS states are not LU-equivalent to any single occupancy state. Our main result is that for N=3 and $M$ even there is a universal subspace $\cW\subseteq\cS$ spanned by $M(M-1)(M-5)/6$ states $e_{i_1,...,i_N}$. Moreover the number $M(M-1)(M-5)/6$ is minimal. }, doi = {10.1007/s00220-014-2187-6}, url = {http://arxiv.org/abs/1301.3421v2}, author = {Lin Chen and Jianxin Chen and Dragomir Z. Djokovic and Bei Zeng} } @article {1342, title = {Optical detection of radio waves through a nanomechanical transducer}, journal = {Nature}, volume = {507}, year = {2014}, month = {2014/3/5}, pages = {81 - 85}, abstract = {Low-loss transmission and sensitive recovery of weak radio-frequency (rf) and microwave signals is an ubiquitous technological challenge, crucial in fields as diverse as radio astronomy, medical imaging, navigation and communication, including those of quantum states. Efficient upconversion of rf-signals to an optical carrier would allow transmitting them via optical fibers dramatically reducing losses, and give access to the mature toolbox of quantum optical techniques, routinely enabling quantum-limited signal detection. Research in the field of cavity optomechanics has shown that nanomechanical oscillators can couple very strongly to either microwave or optical fields. An oscillator accommodating both functionalities would bear great promise as the intermediate platform in a radio-to-optical transduction cascade. Here, we demonstrate such an opto-electro-mechanical transducer utilizing a high-Q nanomembrane. A moderate voltage bias (<10V) is sufficient to induce strong coupling between the voltage fluctuations in a rf resonance circuit and the membrane{\textquoteright}s displacement, which is simultaneously coupled to light reflected off its metallized surface. The circuit acts as an antenna; the voltage signals it induces are detected as an optical phase shift with quantum-limited sensitivity. The half-wave voltage is in the microvolt range, orders of magnitude below that of standard optical modulators. The noise added by the membrane is suppressed by the electro-mechanical cooperativity C~6800 and has a temperature of 40mK, far below 300K where the entire device is operated. This corresponds to a sensitivity limit as low as 5 pV/Hz^1/2, or -210dBm/Hz in a narrow band around 1 MHz. Our work introduces an entirely new approach to all-optical, ultralow-noise detection of classical electronic signals, and sets the stage for coherent upconversion of low-frequency quantum signals to the optical domain. }, doi = {10.1038/nature13029}, url = {http://arxiv.org/abs/1307.3467v2}, author = {T. Bagci and A. Simonsen and S. Schmid and L. G. Villanueva and E. Zeuthen and J. Appel and J. M. Taylor and A. S{\o}rensen and K. Usami and A. Schliesser and E. S. Polzik} } @article {1840, title = {Probing many-body interactions in an optical lattice clock}, journal = {Ann. Phys.}, volume = {340}, year = {2014}, pages = {311}, url = {http://www.sciencedirect.com/science/article/pii/S0003491613002546}, author = {Rey, A M and Alexey V. Gorshkov and Kraus, C V and Martin, M J and Bishof, M and Swallows, M D and Zhang, X and Benko, C and Ye, J and Lemke, N D and Ludlow, A D} } @article {1479, title = {Suppressing the loss of ultracold molecules via the continuous quantum Zeno effect }, journal = {Physical Review Letters}, volume = {112}, year = {2014}, month = {2014/2/20}, abstract = { We investigate theoretically the suppression of two-body losses when the on-site loss rate is larger than all other energy scales in a lattice. This work quantitatively explains the recently observed suppression of chemical reactions between two rotational states of fermionic KRb molecules confined in one-dimensional tubes with a weak lattice along the tubes [Yan et al., Nature 501, 521-525 (2013)]. New loss rate measurements performed for different lattice parameters but under controlled initial conditions allow us to show that the loss suppression is a consequence of the combined effects of lattice confinement and the continuous quantum Zeno effect. A key finding, relevant for generic strongly reactive systems, is that while a single-band theory can qualitatively describe the data, a quantitative analysis must include multiband effects. Accounting for these effects reduces the inferred molecule filling fraction by a factor of five. A rate equation can describe much of the data, but to properly reproduce the loss dynamics with a fixed filling fraction for all lattice parameters we develop a mean-field model and benchmark it with numerically exact time-dependent density matrix renormalization group calculations. }, doi = {10.1103/PhysRevLett.112.070404}, url = {http://arxiv.org/abs/1310.2221v2}, author = {Bihui Zhu and Bryce Gadway and Michael Foss-Feig and Johannes Schachenmayer and Michael Wall and Kaden R. A. Hazzard and Bo Yan and Steven A. Moses and Jacob P. Covey and Deborah S. Jin and Jun Ye and Murray Holland and Ana Maria Rey} } @article {1451, title = {Symmetric Extension of Two-Qubit States}, journal = {Physical Review A}, volume = {90}, year = {2014}, month = {2014/9/17}, abstract = { Quantum key distribution uses public discussion protocols to establish shared secret keys. In the exploration of ultimate limits to such protocols, the property of symmetric extendibility of underlying bipartite states $\rho_{AB}$ plays an important role. A bipartite state $\rho_{AB}$ is symmetric extendible if there exits a tripartite state $\rho_{ABB{\textquoteright}}$, such that the $AB$ marginal state is identical to the $AB{\textquoteright}$ marginal state, i.e. $\rho_{AB{\textquoteright}}=\rho_{AB}$. For a symmetric extendible state $\rho_{AB}$, the first task of the public discussion protocol is to break this symmetric extendibility. Therefore to characterize all bi-partite quantum states that possess symmetric extensions is of vital importance. We prove a simple analytical formula that a two-qubit state $\rho_{AB}$ admits a symmetric extension if and only if $\tr(\rho_B^2)\geq \tr(\rho_{AB}^2)-4\sqrt{\det{\rho_{AB}}}$. Given the intimate relationship between the symmetric extension problem and the quantum marginal problem, our result also provides the first analytical necessary and sufficient condition for the quantum marginal problem with overlapping marginals. }, doi = {10.1103/PhysRevA.90.032318}, url = {http://arxiv.org/abs/1310.3530v2}, author = {Jianxin Chen and Zhengfeng Ji and David Kribs and Norbert L{\"u}tkenhaus and Bei Zeng} } @article {1445, title = {Unextendible Product Basis for Fermionic Systems}, journal = {Journal of Mathematical Physics}, volume = {55}, year = {2014}, month = {2014/01/01}, pages = {082207}, abstract = { We discuss the concept of unextendible product basis (UPB) and generalized UPB for fermionic systems, using Slater determinants as an analogue of product states, in the antisymmetric subspace $\wedge^ N \bC^M$. We construct an explicit example of generalized fermionic unextendible product basis (FUPB) of minimum cardinality $N(M-N)+1$ for any $N\ge2,M\ge4$. We also show that any bipartite antisymmetric space $\wedge^ 2 \bC^M$ of codimension two is spanned by Slater determinants, and the spaces of higher codimension may not be spanned by Slater determinants. Furthermore, we construct an example of complex FUPB of $N=2,M=4$ with minimum cardinality $5$. In contrast, we show that a real FUPB does not exist for $N=2,M=4$ . Finally we provide a systematic construction for FUPBs of higher dimensions using FUPBs and UPBs of lower dimensions. }, doi = {10.1063/1.4893358}, url = {http://arxiv.org/abs/1312.4218v1}, author = {Jianxin Chen and Lin Chen and Bei Zeng} } @article {1842, title = {A quantum many-body spin system in an optical lattice clock}, journal = {Science}, volume = {341}, year = {2013}, pages = {632}, url = {http://www.sciencemag.org/content/341/6146/632.abstract}, author = {M J Martin and Bishof, M and Swallows, M D and X Zhang and C Benko and J von-Stecher and Alexey V. Gorshkov and Rey, A M and Jun Ye} } @article {1460, title = {Symmetries of Codeword Stabilized Quantum Codes}, journal = {8th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2013)}, volume = {22}, year = {2013}, month = {2013/03/28}, pages = {192-206}, abstract = { Symmetry is at the heart of coding theory. Codes with symmetry, especially cyclic codes, play an essential role in both theory and practical applications of classical error-correcting codes. Here we examine symmetry properties for codeword stabilized (CWS) quantum codes, which is the most general framework for constructing quantum error-correcting codes known to date. A CWS code Q can be represented by a self-dual additive code S and a classical code C, i.,e., Q=(S,C), however this representation is in general not unique. We show that for any CWS code Q with certain permutation symmetry, one can always find a self-dual additive code S with the same permutation symmetry as Q such that Q=(S,C). As many good CWS codes have been found by starting from a chosen S, this ensures that when trying to find CWS codes with certain permutation symmetry, the choice of S with the same symmetry will suffice. A key step for this result is a new canonical representation for CWS codes, which is given in terms of a unique decomposition as union stabilizer codes. For CWS codes, so far mainly the standard form (G,C) has been considered, where G is a graph state. We analyze the symmetry of the corresponding graph of G, which in general cannot possess the same permutation symmetry as Q. We show that it is indeed the case for the toric code on a square lattice with translational symmetry, even if its encoding graph can be chosen to be translational invariant. }, doi = {10.4230/LIPIcs.TQC.2013.192}, url = {http://arxiv.org/abs/1303.7020v2}, author = {Salman Beigi and Jianxin Chen and Markus Grassl and Zhengfeng Ji and Qiang Wang and Bei Zeng} } @article {1197, title = {Topologically Protected Quantum State Transfer in a Chiral Spin Liquid}, journal = {Nature Communications}, volume = {4}, year = {2013}, month = {2013/3/12}, pages = {1585}, abstract = { Topology plays a central role in ensuring the robustness of a wide variety of physical phenomena. Notable examples range from the robust current carrying edge states associated with the quantum Hall and the quantum spin Hall effects to proposals involving topologically protected quantum memory and quantum logic operations. Here, we propose and analyze a topologically protected channel for the transfer of quantum states between remote quantum nodes. In our approach, state transfer is mediated by the edge mode of a chiral spin liquid. We demonstrate that the proposed method is intrinsically robust to realistic imperfections associated with disorder and decoherence. Possible experimental implementations and applications to the detection and characterization of spin liquid phases are discussed. }, doi = {10.1038/ncomms2531}, url = {http://arxiv.org/abs/1110.3788v1}, author = {Norman Y. Yao and Chris R. Laumann and Alexey V. Gorshkov and Hendrik Weimer and Liang Jiang and J. Ignacio Cirac and Peter Zoller and Mikhail D. Lukin} } @article {1459, title = {Uniqueness of Quantum States Compatible with Given Measurement Results}, journal = {Physical Review A}, volume = {88}, year = {2013}, month = {2013/7/11}, abstract = { We discuss the uniqueness of quantum states compatible with given results for measuring a set of observables. For a given pure state, we consider two different types of uniqueness: (1) no other pure state is compatible with the same measurement results and (2) no other state, pure or mixed, is compatible with the same measurement results. For case (1), it is known that for a d-dimensional Hilbert space, there exists a set of 4d-5 observables that uniquely determines any pure state. We show that for case (2), 5d-7 observables suffice to uniquely determine any pure state. Thus there is a gap between the results for (1) and (2), and we give some examples to illustrate this. The case of observables corresponding to reduced density matrices (RDMs) of a multipartite system is also discussed, where we improve known bounds on local dimensions for case (2) in which almost all pure states are uniquely determined by their RDMs. We further discuss circumstances where (1) can imply (2). We use convexity of the numerical range of operators to show that when only two observables are measured, (1) always implies (2). More generally, if there is a compact group of symmetries of the state space which has the span of the observables measured as the set of fixed points, then (1) implies (2). We analyze the possible dimensions for the span of such observables. Our results extend naturally to the case of low rank quantum states. }, doi = {10.1103/PhysRevA.88.012109}, url = {http://arxiv.org/abs/1212.3503v2}, author = {Jianxin Chen and Hillary Dawkins and Zhengfeng Ji and Nathaniel Johnston and David Kribs and Frederic Shultz and Bei Zeng} } @article {1444, title = {Universal Entanglers for Bosonic and Fermionic Systems}, year = {2013}, month = {2013/05/31}, abstract = { A universal entangler (UE) is a unitary operation which maps all pure product states to entangled states. It is known that for a bipartite system of particles $1,2$ with a Hilbert space $\mathbb{C}^{d_1}\otimes\mathbb{C}^{d_2}$, a UE exists when $\min{(d_1,d_2)}\geq 3$ and $(d_1,d_2)\neq (3,3)$. It is also known that whenever a UE exists, almost all unitaries are UEs; however to verify whether a given unitary is a UE is very difficult since solving a quadratic system of equations is NP-hard in general. This work examines the existence and construction of UEs of bipartite bosonic/fermionic systems whose wave functions sit in the symmetric/antisymmetric subspace of $\mathbb{C}^{d}\otimes\mathbb{C}^{d}$. The development of a theory of UEs for these types of systems needs considerably different approaches from that used for UEs of distinguishable systems. This is because the general entanglement of identical particle systems cannot be discussed in the usual way due to the effect of (anti)-symmetrization which introduces "pseudo entanglement" that is inaccessible in practice. We show that, unlike the distinguishable particle case, UEs exist for bosonic/fermionic systems with Hilbert spaces which are symmetric (resp. antisymmetric) subspaces of $\mathbb{C}^{d}\otimes\mathbb{C}^{d}$ if and only if $d\geq 3$ (resp. $d\geq 8$). To prove this we employ algebraic geometry to reason about the different algebraic structures of the bosonic/fermionic systems. Additionally, due to the relatively simple coherent state form of unentangled bosonic states, we are able to give the explicit constructions of two bosonic UEs. Our investigation provides insight into the entanglement properties of systems of indisitinguishable particles, and in particular underscores the difference between the entanglement structures of bosonic, fermionic and distinguishable particle systems. }, url = {http://arxiv.org/abs/1305.7489v1}, author = {Joel Klassen and Jianxin Chen and Bei Zeng} } @article {1458, title = {Comment on some results of Erdahl and the convex structure of reduced density matrices}, journal = {Journal of Mathematical Physics}, volume = {53}, year = {2012}, month = {2012/05/16}, pages = {072203}, abstract = { In J. Math. Phys. 13, 1608-1621 (1972), Erdahl considered the convex structure of the set of $N$-representable 2-body reduced density matrices in the case of fermions. Some of these results have a straightforward extension to the $m$-body setting and to the more general quantum marginal problem. We describe these extensions, but can not resolve a problem in the proof of Erdahl{\textquoteright}s claim that every extreme point is exposed in finite dimensions. Nevertheless, we can show that when $2m \geq N$ every extreme point of the set of $N$-representable $m$-body reduced density matrices has a unique pre-image in both the symmetric and anti-symmetric setting. Moreover, this extends to the quantum marginal setting for a pair of complementary $m$-body and $(N-m)$-body reduced density matrices. }, doi = {10.1063/1.4736842}, url = {http://arxiv.org/abs/1205.3682v1}, author = {Jianxin Chen and Zhengfeng Ji and Mary Beth Ruskai and Bei Zeng and Duan-Lu Zhou} } @article {1443, title = {Correlations in excited states of local Hamiltonians}, journal = {Physical Review A}, volume = {85}, year = {2012}, month = {2012/4/9}, abstract = { Physical properties of the ground and excited states of a $k$-local Hamiltonian are largely determined by the $k$-particle reduced density matrices ($k$-RDMs), or simply the $k$-matrix for fermionic systems---they are at least enough for the calculation of the ground state and excited state energies. Moreover, for a non-degenerate ground state of a $k$-local Hamiltonian, even the state itself is completely determined by its $k$-RDMs, and therefore contains no genuine ${>}k$-particle correlations, as they can be inferred from $k$-particle correlation functions. It is natural to ask whether a similar result holds for non-degenerate excited states. In fact, for fermionic systems, it has been conjectured that any non-degenerate excited state of a 2-local Hamiltonian is simultaneously a unique ground state of another 2-local Hamiltonian, hence is uniquely determined by its 2-matrix. And a weaker version of this conjecture states that any non-degenerate excited state of a 2-local Hamiltonian is uniquely determined by its 2-matrix among all the pure $n$-particle states. We construct explicit counterexamples to show that both conjectures are false. It means that correlations in excited states of local Hamiltonians could be dramatically different from those in ground states. We further show that any non-degenerate excited state of a $k$-local Hamiltonian is a unique ground state of another $2k$-local Hamiltonian, hence is uniquely determined by its $2k$-RDMs (or $2k$-matrix). }, doi = {10.1103/PhysRevA.85.040303}, url = {http://arxiv.org/abs/1106.1373v2}, author = {Jianxin Chen and Zhengfeng Ji and Zhaohui Wei and Bei Zeng} } @article {1448, title = {From Ground States to Local Hamiltonians}, journal = {Physical Review A}, volume = {86}, year = {2012}, month = {2012/8/30}, abstract = { Traditional quantum physics solves ground states for a given Hamiltonian, while quantum information science asks for the existence and construction of certain Hamiltonians for given ground states. In practical situations, one would be mainly interested in local Hamiltonians with certain interaction patterns, such as nearest neighbour interactions on some type of lattices. A necessary condition for a space $V$ to be the ground-state space of some local Hamiltonian with a given interaction pattern, is that the maximally mixed state supported on $V$ is uniquely determined by its reduced density matrices associated with the given pattern, based on the principle of maximum entropy. However, it is unclear whether this condition is in general also sufficient. We examine the situations for the existence of such a local Hamiltonian to have $V$ satisfying the necessary condition mentioned above as its ground-state space, by linking to faces of the convex body of the local reduced states. We further discuss some methods for constructing the corresponding local Hamiltonians with given interaction patterns, mainly from physical points of view, including constructions related to perturbation methods, local frustration-free Hamiltonians, as well as thermodynamical ensembles. }, doi = {10.1103/PhysRevA.86.022339}, url = {http://arxiv.org/abs/1110.6583v4}, author = {Jianxin Chen and Zhengfeng Ji and Bei Zeng and D. L. Zhou} } @article {1454, title = {Ground-State Spaces of Frustration-Free Hamiltonians}, journal = {Journal of Mathematical Physics}, volume = {53}, year = {2012}, month = {2012/01/01}, pages = {102201}, abstract = { We study the ground-state space properties for frustration-free Hamiltonians. We introduce a concept of {\textquoteleft}reduced spaces{\textquoteright} to characterize local structures of ground-state spaces. For a many-body system, we characterize mathematical structures for the set $\Theta_k$ of all the $k$-particle reduced spaces, which with a binary operation called join forms a semilattice that can be interpreted as an abstract convex structure. The smallest nonzero elements in $\Theta_k$, called atoms, are analogs of extreme points. We study the properties of atoms in $\Theta_k$ and discuss its relationship with ground states of $k$-local frustration-free Hamiltonians. For spin-1/2 systems, we show that all the atoms in $\Theta_2$ are unique ground states of some 2-local frustration-free Hamiltonians. Moreover, we show that the elements in $\Theta_k$ may not be the join of atoms, indicating a richer structure for $\Theta_k$ beyond the convex structure. Our study of $\Theta_k$ deepens the understanding of ground-state space properties for frustration-free Hamiltonians, from a new angle of reduced spaces. }, doi = {10.1063/1.4748527}, url = {http://arxiv.org/abs/1112.0762v1}, author = {Jianxin Chen and Zhengfeng Ji and David Kribs and Zhaohui Wei and Bei Zeng} } @article {1449, title = {Minimum Entangling Power is Close to Its Maximum}, year = {2012}, month = {2012/10/04}, abstract = { Given a quantum gate $U$ acting on a bipartite quantum system, its maximum (average, minimum) entangling power is the maximum (average, minimum) entanglement generation with respect to certain entanglement measure when the inputs are restricted to be product states. In this paper, we mainly focus on the {\textquoteright}weakest{\textquoteright} one, i.e., the minimum entangling power, among all these entangling powers. We show that, by choosing von Neumann entropy of reduced density operator or Schmidt rank as entanglement measure, even the {\textquoteright}weakest{\textquoteright} entangling power is generically very close to its maximal possible entanglement generation. In other words, maximum, average and minimum entangling powers are generically close. We then study minimum entangling power with respect to other Lipschitiz-continuous entanglement measures and generalize our results to multipartite quantum systems. As a straightforward application, a random quantum gate will almost surely be an intrinsically fault-tolerant entangling device that will always transform every low-entangled state to near-maximally entangled state. }, url = {http://arxiv.org/abs/1210.1296v1}, author = {Jianxin Chen and Zhengfeng Ji and David W Kribs and Bei Zeng} } @article {1503, title = {Nanoplasmonic Lattices for Ultracold atoms}, journal = {Physical Review Letters}, volume = {109}, year = {2012}, month = {2012/12/6}, abstract = { We propose to use sub-wavelength confinement of light associated with the near field of plasmonic systems to create nanoscale optical lattices for ultracold atoms. Our approach combines the unique coherence properties of isolated atoms with the sub-wavelength manipulation and strong light-matter interaction associated with nano-plasmonic systems. It allows one to considerably increase the energy scales in the realization of Hubbard models and to engineer effective long-range interactions in coherent and dissipative many-body dynamics. Realistic imperfections and potential applications are discussed. }, doi = {10.1103/PhysRevLett.109.235309}, url = {http://arxiv.org/abs/1208.6293v3}, author = {Michael Gullans and T. Tiecke and D. E. Chang and J. Feist and J. D. Thompson and J. I. Cirac and P. Zoller and M. D. Lukin} } @article {1493, title = {Photonic quantum simulation of ground state configurations of Heisenberg square and checkerboard lattice spin systems }, year = {2012}, month = {2012/05/12}, abstract = { Photonic quantum simulators are promising candidates for providing insight into other small- to medium-sized quantum systems. The available photonic quantum technology is reaching the state where significant advantages arise for the quantum simulation of interesting questions in Heisenberg spin systems. Here we experimentally simulate such spin systems with single photons and linear optics. The effective Heisenberg-type interactions among individual single photons are realized by quantum interference at the tunable direction coupler followed by the measurement process. The effective interactions are characterized by comparing the entanglement dynamics using pairwise concurrence of a four-photon quantum system. We further show that photonic quantum simulations of generalized Heisenberg interactions on a four-site square lattice and a six-site checkerboard lattice are in reach of current technology. }, url = {http://arxiv.org/abs/1205.2801v1}, author = {Xiao-song Ma and Borivoje Dakic and Sebastian Kropatsche and William Naylor and Yang-hao Chan and Zhe-Xuan Gong and Lu-ming Duan and Anton Zeilinger and Philip Walther} } @article {1456, title = {Rank Reduction for the Local Consistency Problem}, journal = {Journal of Mathematical Physics}, volume = {53}, year = {2012}, month = {2012/02/09}, pages = {022202}, abstract = { We address the problem of how simple a solution can be for a given quantum local consistency instance. More specifically, we investigate how small the rank of the global density operator can be if the local constraints are known to be compatible. We prove that any compatible local density operators can be satisfied by a low rank global density operator. Then we study both fermionic and bosonic versions of the N-representability problem as applications. After applying the channel-state duality, we prove that any compatible local channels can be obtained through a global quantum channel with small Kraus rank. }, doi = {10.1063/1.3685644}, url = {http://arxiv.org/abs/1106.3235v2}, author = {Jianxin Chen and Zhengfeng Ji and Alexander Klyachko and David W. Kribs and Bei Zeng} } @article {1491, title = {Reply to Comment on "Space-Time Crystals of Trapped Ions}, year = {2012}, month = {2012/10/15}, abstract = { This is a reply to the comment from Patrick Bruno (arXiv:1211.4792) on our paper (Phys. Rev. Lett. 109, 163001 (2012)). }, doi = {http://dx.doi.org/10.1103/PhysRevLett.109.163001}, url = {http://arxiv.org/abs/1212.6959v2}, author = {Tongcang Li and Zhe-Xuan Gong and Zhang-qi Yin and H. T. Quan and Xiaobo Yin and Peng Zhang and L. -M. Duan and Xiang Zhang} } @article {1794, title = {Space-Time Crystals of Trapped Ions}, journal = {Physical Review Letters}, volume = {109}, year = {2012}, month = {2012/10/19}, pages = {163001}, abstract = {Spontaneous symmetry breaking can lead to the formation of time crystals, as well as spatial crystals. Here we propose a space-time crystal of trapped ions and a method to realize it experimentally by confining ions in a ring-shaped trapping potential with a static magnetic field. The ions spontaneously form a spatial ring crystal due to Coulomb repulsion. This ion crystal can rotate persistently at the lowest quantum energy state in magnetic fields with fractional fluxes. The persistent rotation of trapped ions produces the temporal order, leading to the formation of a space-time crystal. We show that these space-time crystals are robust for direct experimental observation. We also study the effects of finite temperatures on the persistent rotation. The proposed space-time crystals of trapped ions provide a new dimension for exploring many-body physics and emerging properties of matter.}, doi = {10.1103/PhysRevLett.109.163001}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.109.163001}, author = {Tongcang Li and Gong, Zhe-Xuan and Yin, Zhang-Qi and Quan, H. T. and Yin, Xiaobo and Zhang, Peng and Duan, L.-M. and Zhang, Xiang} } @article {1510, title = {Super-Polynomial Quantum Speed-ups for Boolean Evaluation Trees with Hidden Structure}, journal = {ITCS {\textquoteright}12 Proceedings of the 3rd Innovations in Theoretical Computer Science Conference}, year = {2012}, month = {2012/01/08}, pages = {249-265}, abstract = { We give a quantum algorithm for evaluating a class of boolean formulas (such as NAND trees and 3-majority trees) on a restricted set of inputs. Due to the structure of the allowed inputs, our algorithm can evaluate a depth $n$ tree using $O(n^{2+\log\omega})$ queries, where $\omega$ is independent of $n$ and depends only on the type of subformulas within the tree. We also prove a classical lower bound of $n^{\Omega(\log\log n)}$ queries, thus showing a (small) super-polynomial speed-up. }, isbn = {978-1-4503-1115-1}, doi = {10.1145/2090236.2090258}, url = {http://arxiv.org/abs/1101.0796v3}, author = {Bohua Zhan and Shelby Kimmel and Avinatan Hassidim} } @article {1201, title = {Topological Flat Bands from Dipolar Spin Systems}, journal = {Physical Review Letters}, volume = {109}, year = {2012}, month = {2012/12/26}, abstract = { We propose and analyze a physical system that naturally admits two-dimensional topological nearly flat bands. Our approach utilizes an array of three-level dipoles (effective S = 1 spins) driven by inhomogeneous electromagnetic fields. The dipolar interactions produce arbitrary uniform background gauge fields for an effective collection of conserved hardcore bosons, namely, the dressed spin-flips. These gauge fields result in topological band structures, whose bandgap can be larger than the corresponding bandwidth. Exact diagonalization of the full interacting Hamiltonian at half-filling reveals the existence of superfluid, crystalline, and supersolid phases. An experimental realization using either ultra-cold polar molecules or spins in the solid state is considered. }, doi = {10.1103/PhysRevLett.109.266804}, url = {http://arxiv.org/abs/1207.4479v3}, author = {Norman Y. Yao and Chris R. Laumann and Alexey V. Gorshkov and Steven D. Bennett and Eugene Demler and Peter Zoller and Mikhail D. Lukin} } @article {1412, title = {Chern numbers hiding in time-of-flight images}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/12/21}, abstract = { We present a technique for detecting topological invariants -- Chern numbers -- from time-of-flight images of ultra-cold atoms. We show that the Chern numbers of integer quantum Hall states of lattice fermions leave their fingerprints in the atoms{\textquoteright} momentum distribution. We analytically demonstrate that the number of local maxima in the momentum distribution is equal to the Chern number in two limiting cases, for large hopping anisotropy and in the continuum limit. In addition, our numerical simulations beyond these two limits show that these local maxima persist for a range of parameters. Thus, an everyday observable in cold atom experiments can serve as a useful tool to characterize and visualize quantum states with non-trivial topology. }, doi = {10.1103/PhysRevA.84.063629}, url = {http://arxiv.org/abs/1105.3100v3}, author = {Erhai Zhao and Noah Bray-Ali and Carl J. Williams and I. B. Spielman and Indubala I. Satija} } @article {1447, title = {No-go Theorem for One-way Quantum Computing on Naturally Occurring Two-level Systems }, journal = {Physical Review A}, volume = {83}, year = {2011}, month = {2011/5/9}, abstract = { One-way quantum computing achieves the full power of quantum computation by performing single particle measurements on some many-body entangled state, known as the resource state. As single particle measurements are relatively easy to implement, the preparation of the resource state becomes a crucial task. An appealing approach is simply to cool a strongly correlated quantum many-body system to its ground state. In addition to requiring the ground state of the system to be universal for one-way quantum computing, we also want the Hamiltonian to have non-degenerate ground state protected by a fixed energy gap, to involve only two-body interactions, and to be frustration-free so that measurements in the course of the computation leave the remaining particles in the ground space. Recently, significant efforts have been made to the search of resource states that appear naturally as ground states in spin lattice systems. The approach is proved to be successful in spin-5/2 and spin-3/2 systems. Yet, it remains an open question whether there could be such a natural resource state in a spin-1/2, i.e., qubit system. Here, we give a negative answer to this question by proving that it is impossible for a genuinely entangled qubit states to be a non-degenerate ground state of any two-body frustration-free Hamiltonian. What is more, we prove that every spin-1/2 frustration-free Hamiltonian with two-body interaction always has a ground state that is a product of single- or two-qubit states, a stronger result that is interesting independent of the context of one-way quantum computing. }, doi = {10.1103/PhysRevA.83.050301}, url = {http://arxiv.org/abs/1004.3787v1}, author = {Jianxin Chen and Xie Chen and Runyao Duan and Zhengfeng Ji and Bei Zeng} } @article {1196, title = {Robust Quantum State Transfer in Random Unpolarized Spin Chains}, journal = {Physical Review Letters}, volume = {106}, year = {2011}, month = {2011/1/27}, abstract = { We propose and analyze a new approach for quantum state transfer between remote spin qubits. Specifically, we demonstrate that coherent quantum coupling between remote qubits can be achieved via certain classes of random, unpolarized (infinite temperature) spin chains. Our method is robust to coupling strength disorder and does not require manipulation or control over individual spins. In principle, it can be used to attain perfect state transfer over arbitrarily long range via purely Hamiltonian evolution and may be particularly applicable in a solid-state quantum information processor. As an example, we demonstrate that it can be used to attain strong coherent coupling between Nitrogen-Vacancy centers separated by micrometer distances at room temperature. Realistic imperfections and decoherence effects are analyzed. }, doi = {10.1103/PhysRevLett.106.040505}, url = {http://arxiv.org/abs/1011.2762v2}, author = {Norman Y. Yao and Liang Jiang and Alexey V. Gorshkov and Zhe-Xuan Gong and Alex Zhai and L. -M. Duan and Mikhail D. Lukin} } @article {1848, title = {Far-field optical imaging and manipulation of individual spins with nanoscale resolution}, journal = {Nature Phys.}, volume = {6}, year = {2010}, pages = {912}, url = {http://www.nature.com/nphys/journal/v6/n11/abs/nphys1774.html}, author = {Maurer, P C and Maze, J R and Stanwix, P L and Jiang, L and Alexey V. Gorshkov and Zibrov, A A and Harke, B and Hodges, J S and Zibrov, A S and Yacoby, A and Twitchen, D and Hell, S W and Walsworth, R L and Lukin, M D} } @article {1455, title = {Principle of Maximum Entropy and Ground Spaces of Local Hamiltonians}, year = {2010}, month = {2010/10/13}, abstract = { The structure of the ground spaces of quantum systems consisting of local interactions is of fundamental importance to different areas of physics. In this Letter, we present a necessary and sufficient condition for a subspace to be the ground space of a k-local Hamiltonian. Our analysis are motivated by the concept of irreducible correlations studied by [Linden et al., PRL 89, 277906] and [Zhou, PRL 101, 180505], which is in turn based on the principle of maximum entropy. It establishes a better understanding of the ground spaces of local Hamiltonians and builds an intimate link of ground spaces to the correlations of quantum states. }, url = {http://arxiv.org/abs/1010.2739v4}, author = {Jianxin Chen and Zhengfeng Ji and Mary Beth Ruskai and Bei Zeng and Duanlu Zhou} } @article {1850, title = {Two-orbital SU(N) magnetism with ultracold alkaline-earth atoms}, journal = {Nature Phys.}, volume = {6}, year = {2010}, pages = {289}, url = {http://www.nature.com/nphys/journal/v6/n4/abs/nphys1535.html}, author = {Alexey V. Gorshkov and Hermele, M and Gurarie, V and Xu, C and Julienne, P S and Ye, J and Zoller, P and Demler, E and Lukin, M D and Rey, A M} } @article {1193, title = {Alkaline-Earth-Metal Atoms as Few-Qubit Quantum Registers}, journal = {Physical Review Letters}, volume = {102}, year = {2009}, month = {2009/3/18}, abstract = { We propose and analyze a novel approach to quantum information processing, in which multiple qubits can be encoded and manipulated using electronic and nuclear degrees of freedom associated with individual alkaline-earth atoms trapped in an optical lattice. Specifically, we describe how the qubits within each register can be individually manipulated and measured with sub-wavelength optical resolution. We also show how such few-qubit registers can be coupled to each other in optical superlattices via conditional tunneling to form a scalable quantum network. Finally, potential applications to quantum computation and precision measurements are discussed. }, doi = {10.1103/PhysRevLett.102.110503}, url = {http://arxiv.org/abs/0812.3660v2}, author = {Alexey V. Gorshkov and Ana Maria Rey and Andrew J. Daley and Martin M. Boyd and Jun Ye and Peter Zoller and Mikhail D. Lukin} } @article {1195, title = {Realization of Coherent Optically Dense Media via Buffer-Gas Cooling}, journal = {Physical Review A}, volume = {79}, year = {2009}, month = {2009/1/6}, abstract = { We demonstrate that buffer-gas cooling combined with laser ablation can be used to create coherent optical media with high optical depth and low Doppler broadening that offers metastable states with low collisional and motional decoherence. Demonstration of this generic technique opens pathways to coherent optics with a large variety of atoms and molecules. We use helium buffer gas to cool 87Rb atoms to below 7 K and slow atom diffusion to the walls. Electromagnetically induced transparency (EIT) in this medium allows for 50\% transmission in a medium with initial OD >70 and for slow pulse propagation with large delay-bandwidth products. In the high-OD regime, we observe high-contrast spectrum oscillations due to efficient four-wave mixing. }, doi = {10.1103/PhysRevA.79.013806}, url = {http://arxiv.org/abs/0805.1416v2}, author = {Tao Hong and Alexey V. Gorshkov and David Patterson and Alexander S. Zibrov and John M. Doyle and Mikhail D. Lukin and Mara G. Prentiss} } @article {1192, title = {Anyonic interferometry and protected memories in atomic spin lattices}, journal = {Nature Physics}, volume = {4}, year = {2008}, month = {2008/4/20}, pages = {482 - 488}, abstract = { Strongly correlated quantum systems can exhibit exotic behavior called topological order which is characterized by non-local correlations that depend on the system topology. Such systems can exhibit remarkable phenomena such as quasi-particles with anyonic statistics and have been proposed as candidates for naturally fault-tolerant quantum computation. Despite these remarkable properties, anyons have never been observed in nature directly. Here we describe how to unambiguously detect and characterize such states in recently proposed spin lattice realizations using ultra-cold atoms or molecules trapped in an optical lattice. We propose an experimentally feasible technique to access non-local degrees of freedom by performing global operations on trapped spins mediated by an optical cavity mode. We show how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit. Furthermore, our technique can be used to probe statistics and dynamics of anyonic excitations. }, doi = {10.1038/nphys943}, url = {http://arxiv.org/abs/0711.1365v1}, author = {Liang Jiang and Gavin K. Brennen and Alexey V. Gorshkov and Klemens Hammerer and Mohammad Hafezi and Eugene Demler and Mikhail D. Lukin and Peter Zoller} } @article {1173, title = {Coherent Quantum Optical Control with Subwavelength Resolution}, journal = {Physical Review Letters}, volume = {100}, year = {2008}, month = {2008/3/7}, abstract = { We suggest a new method for quantum optical control with nanoscale resolution. Our method allows for coherent far-field manipulation of individual quantum systems with spatial selectivity that is not limited by the wavelength of radiation and can, in principle, approach a few nanometers. The selectivity is enabled by the nonlinear atomic response, under the conditions of Electromagnetically Induced Transparency, to a control beam with intensity vanishing at a certain location. Practical performance of this technique and its potential applications to quantum information science with cold atoms, ions, and solid-state qubits are discussed. }, doi = {10.1103/PhysRevLett.100.093005}, url = {http://arxiv.org/abs/0706.3879v2}, author = {Alexey V. Gorshkov and Liang Jiang and Markus Greiner and Peter Zoller and Mikhail D. Lukin} } @article {1855, title = {Suppression of Inelastic Collisions Between Polar Molecules With a Repulsive Shield}, journal = {Phys. Rev. Lett.}, volume = {101}, year = {2008}, pages = {073201}, url = {http://link.aps.org/abstract/PRL/v101/e073201/}, author = {Alexey V. Gorshkov and Rabl, P and Pupillo, G and Micheli, A and Zoller, P and Lukin, M D and B{\"u}chler, H P} } @article {1255, title = {Every NAND formula of size N can be evaluated in time N^{1/2+o(1)} on a quantum computer }, year = {2007}, month = {2007/03/02}, abstract = { For every NAND formula of size N, there is a bounded-error N^{1/2+o(1)}-time quantum algorithm, based on a coined quantum walk, that evaluates this formula on a black-box input. Balanced, or {\textquoteleft}{\textquoteleft}approximately balanced,{\textquoteright}{\textquoteright} NAND formulas can be evaluated in O(sqrt{N}) queries, which is optimal. It follows that the (2-o(1))-th power of the quantum query complexity is a lower bound on the formula size, almost solving in the positive an open problem posed by Laplante, Lee and Szegedy. }, url = {http://arxiv.org/abs/quant-ph/0703015v3}, author = {Andrew M. Childs and Ben W. Reichardt and Robert Spalek and Shengyu Zhang} } @article {1856, title = {Multi-photon Entanglement: From Quantum Curiosity to Quantum Computing and Quantum Repeaters}, journal = {Proc. SPIE}, volume = {6664}, year = {2007}, pages = {66640G}, url = {http://spiedigitallibrary.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PSISDG00666400000166640G000001\&idtype=cvips\&gifs=Yes\&bproc=volrange\&scode=6600\%20-\%206699}, author = {Walther, P and Eisaman, M D and Nemiroski, A and Alexey V. Gorshkov and Zibrov, A S and Zeilinger, A and Lukin, M D} } @article {1362, title = {Solid-state circuit for spin entanglement generation and purification}, journal = {Physical Review Letters}, volume = {94}, year = {2005}, month = {2005/6/15}, abstract = {We show how realistic charge manipulation and measurement techniques, combined with the exchange interaction, allow for the robust generation and purification of four-particle spin entangled states in electrically controlled semiconductor quantum dots. The generated states are immunized to the dominant sources of noise via a dynamical decoherence-free subspace; all additional errors are corrected by a purification protocol. This approach may find application in quantum computation, communication, and metrology. }, doi = {10.1103/PhysRevLett.94.236803}, url = {http://arxiv.org/abs/cond-mat/0503255v2}, author = {J. M. Taylor and W. D{\"u}r and P. Zoller and A. Yacoby and C. M. Marcus and M. D. Lukin} } @article {1365, title = {Quantum information processing using localized ensembles of nuclear spins}, year = {2004}, month = {2004/07/23}, abstract = {We describe a technique for quantum information processing based on localized en sembles of nuclear spins. A qubit is identified as the presence or absence of a collective excitation of a mesoscopic ensemble of nuclear spins surrounding a single quantum dot. All single and two-qubit operations can be effected using hyperfine interactions and single-electron spin rotations, hence the proposed scheme avoids gate errors arising from entanglement between spin and orbital degrees of freedom. Ultra-long coherence times of nuclear spins suggest that this scheme could be particularly well suited for applications where long lived memory is essential. }, url = {http://arxiv.org/abs/cond-mat/0407640v2}, author = {J. M. Taylor and G. Giedke and H. Christ and B. Paredes and J. I. Cirac and P. Zoller and M. D. Lukin and A. Imamoglu} } @article {1260, title = {Universal simulation of Markovian quantum dynamics}, journal = {Physical Review A}, volume = {64}, year = {2001}, month = {2001/11/9}, abstract = { Although the conditions for performing arbitrary unitary operations to simulate the dynamics of a closed quantum system are well understood, the same is not true of the more general class of quantum operations (also known as superoperators) corresponding to the dynamics of open quantum systems. We propose a framework for the generation of Markovian quantum dynamics and study the resources needed for universality. For the case of a single qubit, we show that a single nonunitary process is necessary and sufficient to generate all unital Markovian quantum dynamics, whereas a set of processes parametrized by one continuous parameter is needed in general. We also obtain preliminary results for the unital case in higher dimensions. }, doi = {10.1103/PhysRevA.64.062302}, url = {http://arxiv.org/abs/quant-ph/0008070v2}, author = {Dave Bacon and Andrew M. Childs and Isaac L. Chuang and Julia Kempe and Debbie W. Leung and Xinlan Zhou} }