@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 {3074, title = {Decoherence from Long-Range Forces in Atom Interferometry}, journal = {Phys. Rev. A }, volume = {107}, year = {2023}, month = {3/17/2023}, doi = {https://doi.org/10.1103/PhysRevA.107.033319}, url = {https://arxiv.org/abs/2205.03006}, author = {Jonathan Kunjummen and Daniel Carney and Jacob M. Taylor} } @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 {3426, title = {The discrete adiabatic quantum linear system solver has lower constant factors than the randomized adiabatic solver}, year = {2023}, month = {12/12/2023}, abstract = {

The solution of linear systems of equations is the basis of many other quantum algorithms, and recent results provided an algorithm with optimal scaling in both the condition number κ and the allowable error ϵ [PRX Quantum \textbf{3}, 0403003 (2022)]. That work was based on the discrete adiabatic theorem, and worked out an explicit constant factor for an upper bound on the complexity. Here we show via numerical testing on random matrices that the constant factor is in practice about 1,500 times smaller than the upper bound found numerically in the previous results. That means that this approach is far more efficient than might naively be expected from the upper bound. In particular, it is over an order of magnitude more efficient than using a randomised approach from [arXiv:2305.11352] that claimed to be more efficient.

}, url = {https://arxiv.org/abs/2312.07690}, author = {Pedro C. S. Costa and Dong An and Ryan Babbush and Dominic Berry} } @article {3318, title = {DiVincenzo-like criteria for autonomous quantum machines}, year = {2023}, month = {7/17/2023}, abstract = {

Controlled quantum machines have matured significantly. A natural next step is to grant them autonomy, freeing them from timed external control. For example, autonomy could unfetter quantum computers from classical control wires that heat and decohere them; and an autonomous quantum refrigerator recently reset superconducting qubits to near their ground states, as is necessary before a computation. What conditions are necessary for realizing useful autonomous quantum machines? Inspired by recent quantum thermodynamics and chemistry, we posit conditions analogous to DiVincenzo\&$\#$39;s criteria for quantum computing. Our criteria are intended to foment and guide the development of useful autonomous quantum machines.

}, url = {https://arxiv.org/abs/2307.08739}, author = {Jos{\'e} Antonio Mar{\'\i}n Guzm{\'a}n and Paul Erker and Simone Gasparinetti and Marcus Huber and Nicole Yunger Halpern} } @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 {3140, title = {Demonstration of three- and four-body interactions between trapped-ion spins}, year = {2022}, month = {9/12/2022}, abstract = {

Quantum processors use the native interactions between effective spins to simulate Hamiltonians or execute quantum gates. In most processors, the native interactions are pairwise, limiting the efficiency of controlling entanglement between many qubits. Here we experimentally demonstrate a new class of native interactions between trapped-ion qubits, extending conventional pairwise interactions to higher order. We realize three- and four-body spin interactions as examples, showing that high-order spin polynomials may serve as a new toolbox for quantum information applications.

}, keywords = {Atomic Physics (physics.atom-ph), FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2209.05691}, url = {https://arxiv.org/abs/2209.05691}, author = {Katz, Or and Feng, Lei and Risinger, Andrew and Monroe, Christopher and Cetina, Marko} } @article {3195, title = {Differentiable Quantum Programming with Unbounded Loops}, year = {2022}, month = {11/8/2022}, abstract = {

The emergence of variational quantum applications has led to the development of automatic differentiation techniques in quantum computing. Recently, Zhu et al. (PLDI 2020) have formulated differentiable quantum programming with bounded loops, providing a framework for scalable gradient calculation by quantum means for training quantum variational applications. However, promising parameterized quantum applications, e.g., quantum walk and unitary implementation, cannot be trained in the existing framework due to the natural involvement of unbounded loops. To fill in the gap, we provide the first differentiable quantum programming framework with unbounded loops, including a newly designed differentiation rule, code transformation, and their correctness proof. Technically, we introduce a randomized estimator for derivatives to deal with the infinite sum in the differentiation of unbounded loops, whose applicability in classical and probabilistic programming is also discussed. We implement our framework with Python and Q$\#$, and demonstrate a reasonable sample efficiency. Through extensive case studies, we showcase an exciting application of our framework in automatically identifying close-to-optimal parameters for several parameterized quantum applications.

}, keywords = {FOS: Computer and information sciences, FOS: Physical sciences, Machine Learning (cs.LG), Programming Languages (cs.PL), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2211.04507}, url = {https://arxiv.org/abs/2211.04507}, author = {Fang, Wang and Ying, Mingsheng and Wu, Xiaodi} } @article {3127, title = {Disordered Lieb-Robinson bounds in one dimension}, year = {2022}, month = {8/10/2022}, abstract = {

By tightening the conventional Lieb-Robinson bounds to better handle systems which lack translation invariance, we determine the extent to which \"weak links\" suppress operator growth in disordered one-dimensional spin chains. In particular, we prove that ballistic growth is impossible when the distribution of coupling strengths μ(J) has a sufficiently heavy tail at small J, and identify the correct dynamical exponent to use instead. Furthermore, through a detailed analysis of the special case in which the couplings are genuinely random and independent, we find that the standard formulation of Lieb-Robinson bounds is insufficient to capture the complexity of the dynamics -- we must distinguish between bounds which hold for all sites of the chain and bounds which hold for a subsequence of sites, and we show by explicit example that these two can have dramatically different behaviors. All the same, our result for the dynamical exponent is tight, in that we prove by counterexample that there cannot exist any Lieb-Robinson bound with a smaller exponent. We close by discussing the implications of our results, both major and minor, for numerous applications ranging from quench dynamics to the structure of ground states.

}, keywords = {Disordered Systems and Neural Networks (cond-mat.dis-nn), FOS: Physical sciences, Mathematical Physics (math-ph), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2208.05509}, url = {https://arxiv.org/abs/2208.05509}, author = {Baldwin, Christopher L. and Ehrenberg, Adam and Guo, Andrew Y. and Alexey V. Gorshkov} } @article {2817, title = {Decoding conformal field theories: from supervised to unsupervised learning}, year = {2021}, month = {7/10/2021}, abstract = {

We use machine learning to classify rational two-dimensional conformal field theories. We first use the energy spectra of these minimal models to train a supervised learning algorithm. We find that the machine is able to correctly predict the nature and the value of critical points of several strongly correlated spin models using only their energy spectra. This is in contrast to previous works that use machine learning to classify different phases of matter, but do not reveal the nature of the critical point between phases. Given that the ground-state entanglement Hamiltonian of certain topological phases of matter is also described by conformal field theories, we use supervised learning on R{\'e}yni entropies and find that the machine is able to identify which conformal field theory describes the entanglement Hamiltonian with only the lowest few R{\'e}yni entropies to a high degree of accuracy. Finally, using autoencoders, an unsupervised learning algorithm, we find a hidden variable that has a direct correlation with the central charge and discuss prospects for using machine learning to investigate other conformal field theories, including higher-dimensional ones. Our results highlight that machine learning can be used to find and characterize critical points and also hint at the intriguing possibility to use machine learning to learn about more complex conformal field theories.

}, url = {https://arxiv.org/abs/2106.13485}, author = {En-Jui Kuo and Alireza Seif and Rex Lundgren and Seth Whitsitt and Mohammad Hafezi} } @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 {2916, title = {Discovering hydrodynamic equations of many-body quantum systems}, year = {2021}, month = {11/3/2021}, abstract = {

Simulating and predicting dynamics of quantum many-body systems is extremely challenging, even for state-of-the-art computational methods, due to the spread of entanglement across the system. However, in the long-wavelength limit, quantum systems often admit a simplified description, which involves a small set of physical observables and requires only a few parameters such as sound velocity or viscosity. Unveiling the relationship between these hydrodynamic equations and the underlying microscopic theory usually requires a great effort by condensed matter theorists. In the present paper, we develop a new machine-learning framework for automated discovery of effective equations from a limited set of available data, thus bypassing complicated analytical derivations. The data can be generated from numerical simulations or come from experimental quantum simulator platforms. Using integrable models, where direct comparisons can be made, we reproduce previously known hydrodynamic equations, strikingly discover novel equations and provide their derivation whenever possible. We discover new hydrodynamic equations describing dynamics of interacting systems, for which the derivation remains an outstanding challenge. Our approach provides a new interpretable method to study properties of quantum materials and quantum simulators in non-perturbative regimes.

}, url = {https://arxiv.org/abs/2111.02385}, author = {Yaroslav Kharkov and Oles Shtanko and Alireza Seif and Przemyslaw Bienias and Mathias Van Regemortel and Mohammad Hafezi and Alexey V. Gorshkov} } @article {2514, title = {Destructive Error Interference in Product-Formula Lattice Simulation}, journal = {Phys. Rev. Lett. }, volume = {124}, year = {2020}, month = {6/4/2020}, abstract = {

Quantum computers can efficiently simulate the dynamics of quantum systems. In this paper, we study the cost of digitally simulating the dynamics of several physically relevant systems using the first-order product formula algorithm. We show that the errors from different Trotterization steps in the algorithm can interfere destructively, yielding a much smaller error than previously estimated. In particular, we prove that the total error in simulating a nearest-neighbor interacting system of n sites for time t using the first-order product formula with r time slices is O(nt/r+nt3/r2) when nt2/r is less than a small constant. Given an error tolerance ε, the error bound yields an estimate of max{O(n2t/ε),O(n2t3/2/ε1/2)} for the total gate count of the simulation. The estimate is tighter than previous bounds and matches the empirical performance observed in Childs et al. [PNAS 115, 9456-9461 (2018)]. We also provide numerical evidence for potential improvements and conjecture an even tighter estimate for the gate count.\ 

}, doi = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.220502}, url = {https://arxiv.org/abs/1912.11047}, author = {Minh C. Tran and Su-Kuan Chu and Yuan Su and Andrew M. Childs and Alexey V. Gorshkov} } @article {2411, title = {Discrete Time Crystals}, journal = {Annual Review of Condensed Matter Physics }, volume = {11}, year = {2020}, month = {3/10/2020}, pages = {467-499}, abstract = {

Experimental advances have allowed for the exploration of nearly isolated quantum many-body systems whose coupling to an external bath is very weak. A particularly interesting class of such systems is those which do not thermalize under their own isolated quantum dynamics. In this review, we highlight the possibility for such systems to exhibit new non-equilibrium phases of matter. In particular, we focus on \"discrete time crystals\", which are many-body phases of matter characterized by a spontaneously broken discrete time translation symmetry. We give a definition of discrete time crystals from several points of view, emphasizing that they are a non-equilibrium phenomenon, which is stabilized by many-body interactions, with no analog in non-interacting systems. We explain the theory behind several proposed models of discrete time crystals, and compare a number of recent realizations, in different experimental contexts.\ 

}, doi = {https://doi.org/10.1146/annurev-conmatphys-031119-050658}, url = {https://arxiv.org/abs/1905.13232}, author = {Dominic V. Else and Christopher Monroe and Chetan Nayak and Norman Y. Yao} } @article {2688, title = {Distinct Critical Behaviors from the Same State in Quantum Spin and Population Dynamics Perspectives}, year = {2020}, month = {9/10/2020}, abstract = {

There is a deep connection between the ground states of transverse-field spin systems and the late-time distributions of evolving viral populations -- within simple models, both are obtained from the principal eigenvector of the same matrix. However, that vector is the wavefunction amplitude in the quantum spin model, whereas it is the probability itself in the population model. We show that this seemingly minor difference has significant consequences: phase transitions which are discontinuous in the spin system become continuous when viewed through the population perspective, and transitions which are continuous become governed by new critical exponents. We introduce a more general class of models which encompasses both cases, and that can be solved exactly in a mean-field limit. Numerical results are also presented for a number of one-dimensional chains with power-law interactions. We see that well-worn spin models of quantum statistical mechanics can contain unexpected new physics and insights when treated as population-dynamical models and beyond, motivating further studies.\ 

}, url = {https://arxiv.org/abs/2009.05064}, author = {Christopher L. Baldwin and S. Shivam and S. L. Sondhi and M. Kardar} } @article {2388, title = {Distributional property testing in a quantum world}, journal = {Proceedings of ITCS 2020}, volume = {25}, year = {2020}, month = {02/02/2019}, pages = {1-25}, abstract = {

A fundamental problem in statistics and learning theory is to test properties of distributions. We show that quantum computers can solve such problems with significant speed-ups. In particular, we give fast quantum algorithms for testing closeness between unknown distributions, testing independence between two distributions, and estimating the Shannon / von Neumann entropy of distributions. The distributions can be either classical or quantum, however our quantum algorithms require coherent quantum access to a process preparing the samples. Our results build on the recent technique of quantum singular value transformation, combined with more standard tricks such as divide-and-conquer. The presented approach is a natural fit for distributional property testing both in the classical and the quantum case, demonstrating the first speed-ups for testing properties of density operators that can be accessed coherently rather than only via sampling; for classical distributions our algorithms significantly improve the precision dependence of some earlier results.

}, doi = {http://dx.doi.org/10.4230/LIPIcs.ITCS.2020.25}, url = {https://arxiv.org/abs/1902.00814}, author = {Andras Gilyen and Tongyang Li} } @article {2778, title = {Dynamical Purification Phase Transition Induced by Quantum Measurements}, journal = {Physical Review X}, volume = {10}, year = {2020}, month = {7/30/2020}, abstract = {

Continuously monitoring the environment of a quantum many-body system reduces the entropy of (purifies) the reduced density matrix of the system, conditional on the outcomes of the measurements. We show that, for mixed initial states, a balanced competition between measurements and entangling interactions within the system can result in a dynamical purification phase transition between (i) a phase that locally purifies at a constant system-size-independent rate, and (ii) a \"mixed\" phase where the purification time diverges exponentially in the system size. The residual entropy density in the mixed phase implies the existence of a quantum error-protected subspace where quantum information is reliably encoded against the future non-unitary evolution of the system. We show that these codes are of potential relevance to fault-tolerant quantum computation as they are often highly degenerate and satisfy optimal tradeoffs between encoded information densities and error thresholds. In spatially local models in 1+1 dimensions, this phase transition for mixed initial states occurs concurrently with a recently identified class of entanglement phase transitions for pure initial states. The mutual information of an initially completely-mixed state in 1+1 dimensions grows sublinearly in time due to the formation of the error protected subspace. The purification transition studied here also generalizes to systems with long-range interactions, where conventional notions of entanglement transitions have to be reformulated. Purification dynamics is likely a more robust probe of the transition in experiments, where imperfections generically reduce entanglement and drive the system towards mixed states. We describe the motivations for studying this novel class of non-equilibrium quantum dynamics in the context of advanced quantum computing platforms and fault-tolerant quantum computation.

}, doi = {10.1103/PhysRevX.10.041020}, url = {https://arxiv.org/abs/1905.05195}, author = {Michael Gullans and Huse, David A.} } @article {2533, title = {Development of Quantum InterConnects for Next-Generation Information Technologies}, year = {2019}, month = {12/13/2019}, abstract = {

Just as classical information technology rests on a foundation built of interconnected information-processing systems, quantum information technology (QIT) must do the same. A critical component of such systems is the interconnect, a device or process that allows transfer of information between disparate physical media, for example, semiconductor electronics, individual atoms, light pulses in optical fiber, or microwave fields. While interconnects have been well engineered for decades in the realm of classical information technology, quantum interconnects (QuICs) present special challenges, as they must allow the transfer of fragile quantum states between different physical parts or degrees of freedom of the system. The diversity of QIT platforms (superconducting, atomic, solid-state color center, optical, etc.) that will form a quantum internet poses additional challenges. As quantum systems scale to larger size, the quantum interconnect bottleneck is imminent, and is emerging as a grand challenge for QIT. For these reasons, it is the position of the community represented by participants of the NSF workshop on Quantum Interconnects that accelerating QuIC research is crucial for sustained development of a national quantum science and technology program. Given the diversity of QIT platforms, materials used, applications, and infrastructure required, a convergent research program including partnership between academia, industry and national laboratories is required. This document is a summary from a U.S. National Science Foundation supported workshop held on 31 October - 1 November 2019 in Alexandria, VA. Attendees were charged to identify the scientific and community needs, opportunities, and significant challenges for quantum interconnects over the next 2-5 years.\ 

}, url = {https://arxiv.org/abs/1912.06642}, author = {David Awschalom and Karl K. Berggren and Hannes Bernien and Sunil Bhave and Lincoln D. Carr and Paul Davids and Sophia E. Economou and Dirk Englund and Andrei Faraon and Marty Fejer and Saikat Guha and Martin V. Gustafsson and Evelyn Hu and Liang Jiang and Jungsang Kim and Boris Korzh and Prem Kumar and Paul G. Kwiat and Marko Lon{\v c}ar and Mikhail D. Lukin and David A. B. Miller and Christopher Monroe and Sae Woo Nam and Prineha Narang and Jason S. Orcutt} } @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 {2288, title = {Demonstration of Bayesian quantum game on an ion trap quantum computer}, year = {2018}, abstract = {

We demonstrate a Bayesian quantum game on an ion trap quantum computer with five qubits. The players share an entangled pair of qubits and perform rotations on their qubit as the strategy choice. Two five-qubit circuits are sufficient to run all 16 possible strategy choice sets in a game with four possible strategies. The data are then parsed into player types randomly in order to combine them classically into a Bayesian framework. We exhaustively compute the possible strategies of the game so that the experimental data can be used to solve for the Nash equilibria of the game directly. Then we compare the payoff at the Nash equilibria and location of phase-change-like transitions obtained from the experimental data to the theory, and study how it changes as a function of the amount of entanglement.

}, url = {https://arxiv.org/abs/1802.08116}, author = {Neal Solmeyer and Norbert M. Linke and Caroline Figgatt and Kevin A. Landsman and Radhakrishnan Balu and George Siopsis and Christopher Monroe} } @article {2047, title = {Diffusion Monte Carlo Versus Adiabatic Computation for Local Hamiltonians}, journal = {Physical Review A}, volume = {97}, year = {2018}, month = {2018/02/15}, pages = {022323}, abstract = {

Most research regarding quantum adiabatic optimization has focused on stoquastic Hamiltonians, whose ground states can be expressed with only real, nonnegative amplitudes. This raises the question of whether classical Monte Carlo algorithms can efficiently simulate quantum adiabatic optimization with stoquastic Hamiltonians. Recent results have given counterexamples in which path integral and diffusion Monte Carlo fail to do so. However, most adiabatic optimization algorithms, such as for solving MAX-k-SAT problems, use k-local Hamiltonians, whereas our previous counterexample for diffusion Monte Carlo involved n-body interactions. Here we present a new 6-local counterexample which demonstrates that even for these local Hamiltonians there are cases where diffusion Monte Carlo cannot efficiently simulate quantum adiabatic optimization. Furthermore, we perform empirical testing of diffusion Monte Carlo on a standard well-studied class of permutation-symmetric tunneling problems and similarly find large advantages for quantum optimization over diffusion Monte Carlo.

}, doi = {10.1103/PhysRevA.97.022323}, url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.97.022323}, author = {Jacob Bringewatt and William Dorland and Stephen P. Jordan and Alan Mink} } @article {2142, title = {Dissipation induced dipole blockade and anti-blockade in driven Rydberg systems}, journal = {Phys. Rev. A}, volume = {97}, year = {2018}, month = {2018/02/28}, pages = {023424}, abstract = {

We study theoretically and experimentally the competing blockade and antiblockade effects induced by spontaneously generated contaminant Rydberg atoms in driven Rydberg systems. These contaminant atoms provide a source of strong dipole-dipole interactions and play a crucial role in the system\&$\#$39;s behavior. We study this problem theoretically using two different approaches. The first is a cumulant expansion approximation, in which we ignore third-order and higher connected correlations. Using this approach for the case of resonant drive, a many-body blockade radius picture arises, and we find qualitative agreement with previous experimental results. We further predict that as the atomic density is increased, the Rydberg population\&$\#$39;s dependence on Rabi frequency will transition from quadratic to linear dependence at lower Rabi frequencies. We study this behavior experimentally by observing this crossover at two different atomic densities. We confirm that the larger density system has a smaller crossover Rabi frequency than the smaller density system. The second theoretical approach is a set of phenomenological inhomogeneous rate equations. We compare the results of our rate-equation model to the experimental observations [E. A. Goldschmidt\ et al.,\ Phys. Rev. Lett.116, 113001 (2016)] and find that these rate equations provide quantitatively good scaling behavior of the steady-state Rydberg population for both resonant and off-resonant drives.

}, doi = {10.1103/PhysRevA.97.023424}, url = {https://link.aps.org/doi/10.1103/PhysRevA.97.023424}, author = {Jeremy T. Young and Thomas Boulier and Eric Magnan and Elizabeth A. Goldschmidt and Ryan M. Wilson and Steven L. Rolston and James V. Porto and Alexey V. Gorshkov} } @article {2137, title = {Distributed Quantum Metrology and the Entangling Power of Linear Networks}, year = {2018}, month = {2018/07/25}, abstract = {

We derive a bound on the ability of a linear optical network to estimate a linear combination of independent phase shifts by using an arbitrary non-classical but unentangled input state, thereby elucidating the quantum resources required to obtain the Heisenberg limit with a multi-port interferometer. Our bound reveals that while linear networks can generate highly entangled states, they cannot effectively combine quantum resources that are well distributed across multiple modes for the purposes of metrology: in this sense linear networks endowed with well-distributed quantum resources behave classically. Conversely, our bound shows that linear networks can achieve the Heisenberg limit for distributed metrology when the input photons are hoarded in a small number of input modes, and we present an explicit scheme for doing so. Our results also have implications for measures of non-classicality.

}, doi = {https://doi.org/10.1103/PhysRevLett.121.043604}, url = {https://arxiv.org/abs/1707.06655}, author = {Wenchao Ge and Kurt Jacobs and Zachary Eldredge and Alexey V. Gorshkov and Michael Foss-Feig} } @article {2278, title = {Distributed Quantum Metrology and the Entangling Power of Linear Networks}, journal = {Phys. Rev. Lett. 121, 043604}, year = {2018}, month = {2018/07/25}, abstract = {

We derive a bound on the ability of a linear optical network to estimate a linear combination of independent phase shifts by using an arbitrary non-classical but unentangled input state, thereby elucidating the quantum resources required to obtain the Heisenberg limit with a multi-port interferometer. Our bound reveals that while linear networks can generate highly entangled states, they cannot effectively combine quantum resources that are well distributed across multiple modes for the purposes of metrology: in this sense linear networks endowed with well-distributed quantum resources behave classically. Conversely, our bound shows that linear networks can achieve the Heisenberg limit for distributed metrology when the input photons are hoarded in a small number of input modes, and we present an explicit scheme for doing so. Our results also have implications for measures of non-classicality.\ 

}, doi = {https://doi.org/10.1103/PhysRevLett.121.043604}, url = {https://arxiv.org/abs/1707.06655}, author = {Wenchao Ge and Kurt Jacobs and Zachary Eldredge and Alexey V. Gorshkov and Michael Foss-Feig} } @article {2273, title = {Dynamic suppression of Rayleigh light scattering in dielectric resonators}, year = {2018}, abstract = {

The ultimate limits of performance for any classical optical system are set by sub-wavelength fluctuations within the host material, that may be frozen-in or even dynamically induced. The most common manifestation of such sub-wavelength disorder is Rayleigh light scattering, which is observed in nearly all wave-guiding technologies today and can lead to both irreversible radiative losses as well as undesirable intermodal coupling. While it has been shown that backscattering from disorder can be suppressed by breaking time-reversal symmetry in magneto-optic and topological insulator materials, common optical dielectrics possess neither of these properties. Here we demonstrate an optomechanical approach for dynamically suppressing Rayleigh backscattering within dielectric resonators. We achieve this by locally breaking time-reversal symmetry in a silica resonator through a Brillouin scattering interaction that is available in all materials. Near-complete suppression of Rayleigh backscattering is experimentally confirmed through three independent measurements -- the reduction of the back-reflections caused by scatterers, the elimination of a commonly seen normal-mode splitting effect, and by measurement of the reduction in intrinsic optical loss. More broadly, our results provide new evidence that it is possible to dynamically suppress Rayleigh backscattering within any optical dielectric medium, for achieving robust light propagation in nanophotonic devices in spite of the presence of scatterers or defects.

}, url = {https://arxiv.org/abs/1803.02366}, author = {Seunghwi Kim and J. M. Taylor and Gaurav Bahl} } @article {2210, title = {Dynamical phase transitions in sampling complexity}, journal = {Phys. Rev. Lett.}, volume = {121}, year = {2018}, pages = {12 pages, 4 figures. v3: published version}, abstract = {

We make the case for studying the complexity of approximately simulating (sampling) quantum systems for reasons beyond that of quantum computational supremacy, such as diagnosing phase transitions. We consider the sampling complexity as a function of time\ t\ due to evolution generated by spatially local quadratic bosonic Hamiltonians. We obtain an upper bound on the scaling of\ t\ with the number of bosons\ n\ for which approximate sampling is classically efficient. We also obtain a lower bound on the scaling of\ t\ with\ n\ for which any instance of the boson sampling problem reduces to this problem and hence implies that the problem is hard, assuming the conjectures of Aaronson and Arkhipov [Proc. 43rd Annu. ACM Symp. Theory Comput. STOC \&$\#$39;11]. This establishes a dynamical phase transition in sampling complexity. Further, we show that systems in the Anderson-localized phase are always easy to sample from at arbitrarily long times. We view these results in the light of classifying phases of physical systems based on parameters in the Hamiltonian. In doing so, we combine ideas from mathematical physics and computational complexity to gain insight into the behavior of condensed matter, atomic, molecular and optical systems.

}, doi = {https://doi.org/10.1103/PhysRevLett.121.030501}, url = {https://arxiv.org/abs/1703.05332}, author = {Abhinav Deshpande and Bill Fefferman and Minh C. Tran and Michael Foss-Feig and Alexey V. Gorshkov} } @article {2305, title = {Development of a new UHV/XHV pressure standard (cold atom vacuum standard)}, journal = {Metrologia}, volume = {54}, year = {2017}, month = {2017/11/3}, abstract = {

The National Institute of Standards and Technology has recently begun a program to develop a primary pressure standard that is based on ultra-cold atoms, covering a pressure range of 1 x 10-6 to 1 x 10-10 Pa and possibly lower. These pressures correspond to the entire ultra-high vacuum range and extend into the extreme-high vacuum. This cold-atom vacuum standard (CAVS) is both a primary standard and absolute sensor of vacuum. The CAVS is based on the loss of cold, sensor atoms (such as the alkali-metal lithium) from a magnetic trap due to collisions with the background gas (primarily H2) in the vacuum. The pressure is determined from a thermally-averaged collision cross section, which is a fundamental atomic property, and the measured loss rate. The CAVS is primary because it will use collision cross sections determined from ab initio calculations for the Li + H2 system. Primary traceability is transferred to other systems of interest using sensitivity coefficients.

}, doi = {https://doi.org/10.1088/1681-7575/aa8a7b}, url = {https://arxiv.org/abs/1801.10120}, author = {Julia Scherschligt and James A Fedchak and Daniel S Barker and Stephen Eckel and Nikolai Klimov and Constantinos Makrides and Eite Tiesinga} } @article {2062, title = {Disorder induced transitions in resonantly driven Floquet Topological Insulators}, journal = {Physical Review B}, volume = {96}, year = {2017}, month = {2017/08/16}, pages = {054207}, abstract = {

We investigate the effects of disorder in Floquet topological insulators (FTIs) occurring in semiconductor quantum wells. Such FTIs are induced by resonantly driving a transition between the valence and conduction band. We show that when disorder is added, the topological nature of such FTIs persists as long as there is a mobility gap at the resonant quasi-energy. For strong enough disorder, this gap closes and all the states become localized as the system undergoes a transition to a trivial insulator. Interestingly, the effects of disorder are not necessarily adverse: we show that in the same quantum well, disorder can also induce a transition from a trivial to a topological system, thereby establishing a Floquet Topological Anderson Insulator (FTAI). We identify the conditions on the driving field necessary for observing such a transition.

}, doi = {10.1103/PhysRevB.96.054207}, url = {https://arxiv.org/abs/1702.02956}, author = {Paraj Titum and Netanel H. Lindner and Gil Refael} } @article {1959, title = {Dispersive optical detection of magnetic Feshbach resonances in ultracold gases}, journal = {Physical Review A}, volume = {96}, year = {2017}, month = {2017/08/18}, pages = {022705}, abstract = {

Magnetically tunable Feshbach resonances in ultracold atomic systems are chiefly identified and characterized through time consuming atom loss spectroscopy. We describe an off-resonant dispersive optical probing technique to rapidly locate Feshbach resonances and demonstrate the method by locating four resonances of\ 87Rb, between the\ |F=1,mF=1\⟩\ and\ |F=2,mF=0\⟩\ states. Despite the loss features being\ 100\ mG wide, we require only 21 experimental runs to explore a magnetic field range \>18 G. The resonances consist of two known s-wave features in the vicinity of 9 G and 18 G and two previously unobserved p-wave features near 5 G and 10 G. We further utilize the dispersive approach to directly characterize the two-body loss dynamics for each Feshbach resonance.

}, doi = {10.1103/PhysRevA.96.022705}, url = {https://arxiv.org/abs/1702.02216}, author = {Bianca J. Sawyer and Milena S. J. Horvath and Eite Tiesinga and Amita B. Deb and Niels Kj{\ae}rgaard} } @article {1985, title = {Domination with decay in triangular matchstick arrangement graphs}, journal = {Involve, a Journal of Mathematics}, volume = {10}, year = {2017}, month = {2017/05/14}, pages = {749 - 766}, abstract = {

We provide results for the exponential dominating numbers and total exponential dominating numbers of a family of triangular grid graphs. We then prove inequalities for these numbers and compare them with inequalities that hold more generally for exponential dominating numbers of graphs.

}, issn = {1944-4176}, doi = {10.2140/involve10.2140/involve.2017.10-510.2140/involve.2017.10.749}, url = {http://msp.org/involve/http://msp.org/involve/2017/10-5/index.xhtmlhttp://msp.org/involve/2017/10-5/p03.xhtmlhttp://msp.org/involve/2017/10-5/involve-v10-n5-p03-s.pdf}, author = {Jill Cochran and Terry Henderson and Aaron Ostrander and Ron Taylor} } @article {1913, title = {Dynamically induced robust phonon transport and chiral cooling in an optomechanical system}, journal = {Nature Communications}, volume = {8}, year = {2017}, month = {2017/06/19}, pages = {205}, abstract = {

The transport of sound and heat, in the form of phonons, has a fundamental material limit: disorder-induced scattering. In electronic and optical settings, introduction of chiral transport - in which carrier propagation exhibits broken parity symmetry - provides robustness against such disorder by preventing elastic backscattering. Here we experimentally demonstrate a path for achieving robust phonon transport even in the presence of material disorder, by dynamically inducing chirality through traveling-wave optomechanical coupling. Using this approach, we demonstrate dramatic optically-induced chiral transport for clockwise and counterclockwise phonons in a symmetric resonator. This induced chirality also enhances isolation from the thermal bath and leads to gain-free reduction of the intrinsic damping of the phonons. Surprisingly, this passive mechanism is also accompanied by a chiral reduction in heat load leading to a novel optical cooling of the mechanics. This technique has the potential to improve upon the fundamental thermal limits of resonant mechanical sensor, which cannot be otherwise attained through conventional optomechanical cooling.

}, doi = {10.1038/s41467-017-00247-7}, url = {https://arxiv.org/abs/1609.08674}, author = {Seunghwi Kim and Xunnong Xu and J. M. Taylor and Gaurav Bahl} } @article {1915, title = {Demonstration of a small programmable quantum computer with atomic qubits}, journal = {Nature}, volume = {536}, year = {2016}, month = {2016/08/04}, pages = {63-66}, abstract = {

Quantum computers can solve certain problems more efficiently than any possible conventional computer. Small quantum algorithms have been demonstrated on multiple quantum computing platforms, many specifically tailored in hardware to implement a particular algorithm or execute a limited number of computational paths. Here, we demonstrate a five-qubit trapped-ion quantum computer that can be programmed in software to implement arbitrary quantum algorithms by executing any sequence of universal quantum logic gates. We compile algorithms into a fully-connected set of gate operations that are native to the hardware and have a mean fidelity of 98 \%. Reconfiguring these gate sequences provides the flexibility to implement a variety of algorithms without altering the hardware. As examples, we implement the Deutsch-Jozsa (DJ) and Bernstein-Vazirani (BV) algorithms with average success rates of 95 \% and 90 \%, respectively. We also perform a coherent quantum Fourier transform (QFT) on five trappedion qubits for phase estimation and period finding with average fidelities of 62 \% and 84 \%, respectively. This small quantum computer can be scaled to larger numbers of qubits within a single register, and can be further expanded by connecting several such modules through ion shuttling or photonic quantum channels.

}, doi = {10.1038/nature18648}, url = {http://www.nature.com/nature/journal/v536/n7614/full/nature18648.html}, author = {S. Debnath and N. M. Linke and C. Figgatt and K. A. Landsman and K. Wright and C. Monroe} } @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 {1815, title = {Double Quantum Dot Floquet Gain Medium}, journal = {Physical Review X}, volume = {6}, year = {2016}, month = {2016/11/07}, pages = {041027}, abstract = {

Strongly driving a two-level quantum system with light leads to a ladder of Floquet states separated by the photon energy. Nanoscale quantum devices allow the interplay of confined electrons, phonons, and photons to be studied under strong driving conditions. Here we show that a single electron in a periodically driven DQD functions as a \"Floquet gain medium,\" where population imbalances in the DQD Floquet quasi-energy levels lead to an intricate pattern of gain and loss features in the cavity response. We further measure a large intra-cavity photon number n_c in the absence of a cavity drive field, due to equilibration in the Floquet picture. Our device operates in the absence of a dc current -- one and the same electron is repeatedly driven to the excited state to generate population inversion. These results pave the way to future studies of non-classical light and thermalization of driven quantum systems.

}, doi = {10.1103/PhysRevX.6.041027}, url = {http://journals.aps.org/prx/abstract/10.1103/PhysRevX.6.041027}, author = {J. Stehlik and Y.-Y. Liu and C. Eichler and T. R. Hartke and X. Mi and Michael Gullans and J. M. Taylor and J. R. Petta} } @article {1515, title = {Demonstration of Robust Quantum Gate Tomography via Randomized Benchmarking}, journal = {New Journal of Physics}, volume = {17}, year = {2015}, month = {2015/11/05}, pages = {113019}, abstract = { Typical quantum gate tomography protocols struggle with a self-consistency problem: the gate operation cannot be reconstructed without knowledge of the initial state and final measurement, but such knowledge cannot be obtained without well-characterized gates. A recently proposed technique, known as randomized benchmarking tomography (RBT), sidesteps this self-consistency problem by designing experiments to be insensitive to preparation and measurement imperfections. We implement this proposal in a superconducting qubit system, using a number of experimental improvements including implementing each of the elements of the Clifford group in single {\textquoteleft}atomic{\textquoteright} pulses and custom control hardware to enable large overhead protocols. We show a robust reconstruction of several single-qubit quantum gates, including a unitary outside the Clifford group. We demonstrate that RBT yields physical gate reconstructions that are consistent with fidelities obtained by randomized benchmarking. }, doi = {10.1088/1367-2630/17/11/113019}, url = {http://arxiv.org/abs/1505.06686}, author = {Blake R. Johnson and Marcus P. da Silva and Colm A. Ryan and Shelby Kimmel and Jerry M. Chow and Thomas A. Ohki} } @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 {2561, title = {Driving Rabi oscillations at the giant dipole resonance in xenon}, journal = {Phys. Rev. A }, volume = {92}, year = {2015}, month = {11/23/2015}, abstract = {

Free-electron lasers (FELs) produce short and very intense light pulses in the XUV and x-ray regimes. We investigate the possibility to drive Rabi oscillations in xenon with an intense FEL pulse by using the unusually large dipole strength of the giant-dipole resonance (GDR). The GDR decays within less than 30 as due to its position, which is above the 4d ionization threshold. We find that intensities around 1018 W/cm2 are required to induce Rabi oscillations with a period comparable to the lifetime. The pulse duration should not exceed 100 as because xenon will be fully ionized within a few lifetimes. Rabi oscillations reveal themselves also in the photoelectron spectrum in form of Autler-Townes splittings extending over several tens of electronvolt.

}, doi = {https://doi.org/10.1103/PhysRevA.92.053424}, url = {https://arxiv.org/abs/1511.00058}, author = {Stefan Pabst and Daochen Wang and Robin Santra} } @article {1516, title = {Different Strategies for Optimization Using the Quantum Adiabatic Algorithm }, year = {2014}, month = {2014/01/28}, abstract = { We present the results of a numerical study, with 20 qubits, of the performance of the Quantum Adiabatic Algorithm on randomly generated instances of MAX 2-SAT with a unique assignment that maximizes the number of satisfied clauses. The probability of obtaining this assignment at the end of the quantum evolution measures the success of the algorithm. Here we report three strategies which consistently increase the success probability for the hardest instances in our ensemble: decreasing the overall evolution time, initializing the system in excited states, and adding a random local Hamiltonian to the middle of the evolution. }, url = {http://arxiv.org/abs/1401.7320v1}, author = {Elizabeth Crosson and Edward Farhi and Cedric Yen-Yu Lin and Han-Hsuan Lin and Peter Shor} } @article {bian2014discrete, title = {Discrete optimization using quantum annealing on sparse Ising models}, journal = {Frontiers in Physics}, volume = {2}, year = {2014}, month = {2014/09/01}, pages = {56}, publisher = {Frontiers}, abstract = {This paper discusses techniques for solving discrete optimization problems using quantum annealing. Practical issues likely to affect the computation include precision limitations, finite temperature, bounded energy range, sparse connectivity, and small numbers of qubits. To address these concerns we propose a way of finding energy representations with large classical gaps between ground and first excited states, efficient algorithms for mapping non-compatible Ising models into the hardware, and the use of decomposition methods for problems that are too large to fit in hardware. We validate the approach by describing experiments with D-Wave quantum hardware for low density parity check decoding with up to 1000 variables.}, author = {Bian, Zhengbing and Chudak, Fabian and Israel, Robert and Brad Lackey and Macready, William G and Roy, Aidan} } @article {1161, title = {Dissipative Many-body Quantum Optics in Rydberg Media}, journal = {Physical Review Letters}, volume = {110}, year = {2013}, month = {2013/4/9}, abstract = { We develop a theoretical framework for the dissipative propagation of quantized light in interacting optical media under conditions of electromagnetically induced transparency (EIT). The theory allows us to determine the peculiar spatiotemporal structure of the output of two complementary Rydberg-EIT-based light-processing modules: the recently demonstrated single-photon filter and the recently proposed single-photon subtractor, which, respectively, let through and absorb a single photon. In addition to being crucial for applications of these and other optical quantum devices, the theory opens the door to the study of exotic dissipative many-body dynamics of strongly interacting photons in nonlinear nonlocal media. }, doi = {10.1103/PhysRevLett.110.153601}, url = {http://arxiv.org/abs/1211.7060v1}, author = {Alexey V. Gorshkov and Rejish Nath and Thomas Pohl} } @article {1475, title = {Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence }, journal = {New Journal of Physics}, volume = {15}, year = {2013}, month = {2013/11/07}, pages = {113008}, abstract = { Ising models, and the physical systems described by them, play a central role in generating entangled states for use in quantum metrology and quantum information. In particular, ultracold atomic gases, trapped ion systems, and Rydberg atoms realize long-ranged Ising models, which even in the absence of a transverse field can give rise to highly non-classical dynamics and long-range quantum correlations. In the first part of this paper, we present a detailed theoretical framework for studying the dynamics of such systems driven (at time t=0) into arbitrary unentangled non-equilibrium states, thus greatly extending and unifying the work of Ref. [1]. Specifically, we derive exact expressions for closed-time-path ordered correlation functions, and use these to study experimentally relevant observables, e.g. Bloch vector and spin-squeezing dynamics. In the second part, these correlation functions are then used to derive closed-form expressions for the dynamics of arbitrary spin-spin correlation functions in the presence of both T_1 (spontaneous spin relaxation/excitation) and T_2 (dephasing) type decoherence processes. Even though the decoherence is local, our solution reveals that the competition between Ising dynamics and T_1 decoherence gives rise to an emergent non-local dephasing effect, thereby drastically amplifying the degradation of quantum correlations. In addition to identifying the mechanism of this deleterious effect, our solution points toward a scheme to eliminate it via measurement-based coherent feedback. }, doi = {10.1088/1367-2630/15/11/113008}, url = {http://arxiv.org/abs/1306.0172v1}, author = {Michael Foss-Feig and Kaden R A Hazzard and John J Bollinger and Ana Maria Rey and Charles W Clark} } @article {1866, title = {Deciding Unitary Equivalence Between Matrix Polynomials and Sets of Bipartite Quantum States}, journal = {Quantum Information and Computation}, volume = {11}, year = {2011}, month = {2001/09/01}, pages = {813{\textendash}819}, abstract = {

In this brief report, we consider the equivalence between two sets of\ m\ + 1 bipartite quantum states under local unitary transformations. For pure states, this problem corresponds to the matrix algebra question of whether two degree m matrix polynomials are unitarily equivalent; i.e.\ UAiV\† =\ Bi\ for 0 \≤\ i\ \≤\ m\ where\ U\ and\ V\ are unitary and (Ai, Bi) are arbitrary pairs of rectangular matrices. We present a randomized polynomial-time algorithm that solves this problem with an arbitrarily high success probability and outputs transforming matrices\ U\ and\ V.

}, keywords = {matrix polynomials, Schwartz-Zippel lemma, unitary transformations}, issn = {1533-7146}, url = {http://dl.acm.org/citation.cfm?id=2230936.2230942}, author = {Chitambar, Eric and Carl Miller and Shi, Yaoyun} } @article {1303, title = {Detecting paired and counterflow superfluidity via dipole oscillations}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/10/27}, abstract = { We suggest an experimentally feasible procedure to observe paired and counterflow superfluidity in ultra-cold atom systems. We study the time evolution of one-dimensional mixtures of bosonic atoms in an optical lattice following an abrupt displacement of an additional weak confining potential. We find that the dynamic responses of the paired superfluid phase for attractive inter-species interactions and the counterflow superfluid phase for repulsive interactions are qualitatively distinct and reflect the quasi long-range order that characterizes these states. These findings suggest a clear experimental procedure to detect these phases, and give an intuitive insight into their dynamics. }, doi = {10.1103/PhysRevA.84.041609}, url = {http://arxiv.org/abs/1103.3513v3}, author = {Anzi Hu and L. Mathey and Eite Tiesinga and Ippei Danshita and Carl J. Williams and Charles W. Clark} } @article {1520, title = {A diagrammatic expansion of the Casimir energy in multiple reflections: theory and applications }, journal = {Physical Review D}, volume = {83}, year = {2011}, month = {2011/2/2}, abstract = { We develop a diagrammatic representation of the Casimir energy of a multibody configuration. The diagrams represent multiple reflections between the objects and can be organized by a few simple rules. The lowest-order diagrams (or reflections) give the main contribution to the Casimir interaction which proves the usefulness of this expansion. Among some applications of this, we find analytical formulae describing the interaction between "edges", i.e. semi-infinite plates, where we also give a first example of blocking in the context of the Casimir energy. We also find the interaction of edges with a needle and describe analytically a recent model of the repulsion due to the Casimir interaction. }, doi = {10.1103/PhysRevD.83.045004}, url = {http://arxiv.org/abs/1012.1060v1}, author = {Mohammad F. Maghrebi} } @article {1430, title = {Direct Fidelity Estimation from Few Pauli Measurements}, journal = {Physical Review Letters}, volume = {106}, year = {2011}, month = {2011/6/8}, abstract = { We describe a simple method for certifying that an experimental device prepares a desired quantum state rho. Our method is applicable to any pure state rho, and it provides an estimate of the fidelity between rho and the actual (arbitrary) state in the lab, up to a constant additive error. The method requires measuring only a constant number of Pauli expectation values, selected at random according to an importance-weighting rule. Our method is faster than full tomography by a factor of d, the dimension of the state space, and extends easily and naturally to quantum channels. }, doi = {10.1103/PhysRevLett.106.230501}, url = {http://arxiv.org/abs/1104.4695v3}, author = {Steven T. Flammia and Yi-Kai Liu} } @article {1171, title = {d-Wave Superfluidity in Optical Lattices of Ultracold Polar Molecules}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/12/29}, abstract = { Recent work on ultracold polar molecules, governed by a generalization of the t-J Hamiltonian, suggests that molecules may be better suited than atoms for studying d-wave superfluidity due to stronger interactions and larger tunability of the system. We compute the phase diagram for polar molecules in a checkerboard lattice consisting of weakly coupled square plaquettes. In the simplest experimentally realizable case where there is only tunneling and an XX-type spin-spin interaction, we identify the parameter regime where d-wave superfluidity occurs. We also find that the inclusion of a density-density interaction destroys the superfluid phase and that the inclusion of a spin-density or an Ising-type spin-spin interaction can enhance the superfluid phase. We also propose schemes for experimentally realizing the perturbative calculations exhibiting enhanced d-wave superfluidity. }, doi = {10.1103/PhysRevA.84.063639}, url = {http://arxiv.org/abs/1110.5330v2}, author = {Kevin A. Kuns and Ana Maria Rey and Alexey V. Gorshkov} } @article {1488, title = {Dynamics of Overhauser Field under nuclear spin diffusion in a quantum dot }, journal = {New Journal of Physics}, volume = {13}, year = {2011}, month = {2011/03/25}, pages = {033036}, abstract = { The coherence of electron spin can be significantly enhanced by locking the Overhauser field from nuclear spins using the nuclear spin preparation. We propose a theoretical model to calculate the long time dynamics of the Overhauser field under intrinsic nuclear spin diffusion in a quantum dot. We obtain a simplified diffusion equation that can be numerically solved and show quantitatively how the Knight shift and the electron-mediated nuclear spin flip-flop affect the nuclear spin diffusion. The results explain several recent experimental observations, where the decay time of Overhauser field is measured under different configurations, including variation of the external magnetic field, the electron spin configuration in a double dot, and the initial nuclear spin polarization rate. }, doi = {10.1088/1367-2630/13/3/033036}, url = {http://arxiv.org/abs/0912.4322v1}, author = {Zhe-Xuan Gong and Zhang-qi Yin and L. -M. Duan} } @article {1385, title = {On the degeneracy of SU(3)k topological phases}, year = {2010}, month = {2010/09/01}, abstract = {

The ground state degeneracy of an $SU(N)_k$ topological phase with $n$ quasiparticle excitations is relevant quantity for quantum computation, condensed matter physics, and knot theory. It is an open question to find a closed formula for this degeneracy for any $N \> 2$. Here we present the problem in an explicit combinatorial way and analyze the case N=3. While not finding a complete closed-form solution, we obtain generating functions and solve some special cases.

}, url = {http://arxiv.org/abs/1009.0114v1}, author = {Stephen P. Jordan and Toufik Mansour and Simone Severini} } @article {1353, title = {Dynamic Nuclear Polarization in Double Quantum Dots}, journal = {Physical Review Letters}, volume = {104}, year = {2010}, month = {2010/6/4}, abstract = {We theoretically investigate the controlled dynamic polarization of lattice nuclear spins in GaAs double quantum dots containing two electrons. Three regimes of long-term dynamics are identified, including the build up of a large difference in the Overhauser fields across the dots, the saturation of the nuclear polarization process associated with formation of so-called "dark states," and the elimination of the difference field. We show that in the case of unequal dots, build up of difference fields generally accompanies the nuclear polarization process, whereas for nearly identical dots, build up of difference fields competes with polarization saturation in dark states. The elimination of the difference field does not, in general, correspond to a stable steady state of the polarization process. }, doi = {10.1103/PhysRevLett.104.226807}, url = {http://arxiv.org/abs/1003.4508v2}, author = {Michael Gullans and J. J. Krich and J. M. Taylor and H. Bluhm and B. I. Halperin and C. M. Marcus and M. Stopa and A. Yacoby and M. D. Lukin} } @article {1254, title = {Discrete-query quantum algorithm for NAND trees}, journal = {Theory of Computing}, volume = {5}, year = {2009}, month = {2009/07/01}, pages = {119 - 123}, abstract = { Recently, Farhi, Goldstone, and Gutmann gave a quantum algorithm for evaluating NAND trees that runs in time O(sqrt(N log N)) in the Hamiltonian query model. In this note, we point out that their algorithm can be converted into an algorithm using O(N^{1/2 + epsilon}) queries in the conventional quantum query model, for any fixed epsilon > 0. }, doi = {10.4086/toc.2009.v005a005}, url = {http://arxiv.org/abs/quant-ph/0702160v1}, author = {Andrew M. Childs and Richard Cleve and Stephen P. Jordan and David Yeung} } @article {2612, title = {Decoherence in Quantum Walks on the Hypercube }, journal = {Phys. Rev. A }, volume = {76}, year = {2005}, month = {12/5/2005}, pages = {062304}, abstract = {

We study a natural notion of decoherence on quantum random walks over the hypercube. We prove that in this model there is a decoherence threshold beneath which the essential properties of the hypercubic quantum walk, such as linear mixing times, are preserved. Beyond the threshold, we prove that the walks behave like their classical counterparts.

}, doi = {https://doi.org/10.1103/PhysRevA.72.062304}, url = {https://arxiv.org/abs/quant-ph/0501169}, author = {Gorjan Alagic and Alexander Russell} } @article {1361, title = {Dephasing of quantum bits by a quasi-static mesoscopic environment}, year = {2005}, month = {2005/12/07}, abstract = {We examine coherent processes in a two-state quantum system that is strongly coupled to a mesoscopic spin bath and weakly coupled to other environmental degrees of freedom. Our analysis is specifically aimed at understanding the quantum dynamics of solid-state quantum bits such as electron spins in semiconductor structures and superconducting islands. The role of mesoscopic degrees of freedom with long correlation times (local degrees of freedom such as nuclear spins and charge traps) in qubit-related dephasing is discussed in terms of a quasi-static bath. A mathemat- ical framework simultaneously describing coupling to the slow mesoscopic bath and a Markovian environment is developed and the dephasing and decoherence properties of the total system are investigated. The model is applied to several specific examples with direct relevance to current ex- periments. Comparisons to experiments suggests that such quasi-static degrees of freedom play an important role in current qubit implementations. Several methods of mitigating the bath-induced error are considered. }, url = {http://arxiv.org/abs/quant-ph/0512059v2}, author = {J. M. Taylor and M. D. Lukin} } @article {1590, title = {Designing Incentives for Peer-to-Peer Routing}, journal = {Proc. INFOCOM}, year = {2005}, month = {2005/03/13}, pages = {374-385}, abstract = {In a peer-to-peer network, nodes are typically required to route packets for each other. This leads to a problem of {\textquotedblleft}free-loaders,{\textquotedblright} nodes that use the network but refuse to route other nodes{\textquoteright} packets. In this paper we study ways of designing incentives to discourage free-loading. We model the interactions between nodes as a {\textquotedblleft}random matching game,{\textquotedblright} and describe a simple reputation system that provides incentives for good behavior. Under certain assumptions, we obtain a stable subgame-perfect equilibrium. We use simulations to investigate the robustness of this scheme in the presence of noise and malicious nodes, and we examine some of the design trade-offs. We also evaluate some possible adversarial strategies, and discuss how our results might apply to real peer-to-peer systems.}, url = {http://cseweb.ucsd.edu/~vahdat/papers/infocom05.pdf}, author = {Alberto Blanc and Yi-Kai Liu and Amin Vahda} }