@article {3482, title = {Integrability of Goldilocks quantum cellular automata}, year = {2024}, month = {4/3/2024}, abstract = {

Goldilocks quantum cellular automata (QCA) have been simulated on quantum hardware and produce emergent small-world correlation networks. In Goldilocks QCA, a single-qubit unitary is applied to each qubit in a one-dimensional chain subject to a balance constraint: a qubit is updated if its neighbors are in opposite basis states. Here, we prove that a subclass of Goldilocks QCA -- including the one implemented experimentally -- map onto free fermions and therefore can be classically simulated efficiently. We support this claim with two independent proofs, one involving a Jordan--Wigner transformation and one mapping the integrable six-vertex model to QCA. We compute local conserved quantities of these QCA and predict experimentally measurable expectation values. These calculations can be applied to test large digital quantum computers against known solutions. In contrast, typical Goldilocks QCA have equilibration properties and quasienergy-level statistics that suggest nonintegrability. Still, the latter QCA conserve one quantity useful for error mitigation. Our work provides a parametric quantum circuit with tunable integrability properties with which to test quantum hardware.

}, url = {https://arxiv.org/abs/2404.02994}, author = {Logan E. Hillberry and Lorenzo Piroli and Eric Vernier and Nicole Yunger Halpern and Toma{\v z} Prosen and Lincoln D. Carr} } @article {3502, title = {Ion trap with in-vacuum high numerical aperture imaging for a dual-species modular quantum computer}, journal = {Review of Scientific Instruments}, volume = {95}, year = {2024}, month = {3/26/2024}, abstract = {

Photonic interconnects between quantum systems will play a central role in both scalable quantum computing and quantum networking. Entanglement of remote qubits via photons has been demonstrated in many platforms; however, improving the rate of entanglement generation will be instrumental for integrating photonic links into modular quantum computers. We present an ion trap system that has the highest reported free-space photon collection efficiency for quantum networking. We use a pair of in-vacuum aspheric lenses, each with a numerical aperture of 0.8, to couple 10\% of the 493 nm photons emitted from a 138Ba+ ion into single-mode fibers. We also demonstrate that proximal effects of the lenses on the ion position and motion can be mitigated.

}, issn = {1089-7623}, doi = {http://dx.doi.org/10.1063/5.0180732}, url = {https://arxiv.org/abs/2310.07058}, author = {Carter, Allison L. and O{\textquoteright}Reilly, Jameson and Toh, George and Saha, Sagnik and Shalaev, Mikhail and Goetting, Isabella and Monroe, Christopher} } @article {3423, title = {Improved Digital Quantum Simulation by Non-Unitary Channels}, year = {2023}, month = {7/24/2023}, abstract = {

Simulating quantum systems is one of the most promising avenues to harness the computational power of quantum computers. However, hardware errors in noisy near-term devices remain a major obstacle for applications. Ideas based on the randomization of Suzuki-Trotter product formulas have been shown to be a powerful approach to reducing the errors of quantum simulation and lowering the gate count. In this paper, we study the performance of non-unitary simulation channels and consider the error structure of channels constructed from a weighted average of unitary circuits. We show that averaging over just a few simulation circuits can significantly reduce the Trotterization error for both single-step short-time and multi-step long-time simulations. We focus our analysis on two approaches for constructing circuit ensembles for averaging: (i) permuting the order of the terms in the Hamiltonian and (ii) applying a set of global symmetry transformations. We compare our analytical error bounds to empirical performance and show that empirical error reduction surpasses our analytical estimates in most cases. Finally, we test our method on an IonQ trapped-ion quantum computer accessed via the Amazon Braket cloud platform, and benchmark the performance of the averaging approach.

}, url = {https://arxiv.org/abs/2307.13028}, author = {W. Gong and Yaroslav Kharkov and Minh C. Tran and Przemyslaw Bienias and Alexey V. Gorshkov} } @article {2607, title = {Implementing a Fast Unbounded Quantum Fanout Gate Using Power-Law Interactions}, journal = {Phys. Rev. Research}, volume = {4}, year = {2022}, month = {10/27/2022}, abstract = {

The standard circuit model for quantum computation presumes the ability to directly perform gates between arbitrary pairs of qubits, which is unlikely to be practical for large-scale experiments. Power-law interactions with strength decaying as 1/rα in the distance r provide an experimentally realizable resource for information processing, whilst still retaining long-range connectivity. We leverage the power of these interactions to implement a fast quantum fanout gate with an arbitrary number of targets. Our implementation allows the quantum Fourier transform (QFT) and Shor\&$\#$39;s algorithm to be performed on a D-dimensional lattice in time logarithmic in the number of qubits for interactions with α\≤D. As a corollary, we show that power-law systems with α\≤D are difficult to simulate classically even for short times, under a standard assumption that factoring is classically intractable. Complementarily, we develop a new technique to give a general lower bound, linear in the size of the system, on the time required to implement the QFT and the fanout gate in systems that are constrained by a linear light cone. This allows us to prove an asymptotically tighter lower bound for long-range systems than is possible with previously available techniques.\ 

}, doi = {https://doi.org/10.1103/PhysRevResearch.4.L042016}, url = {https://arxiv.org/abs/2007.00662}, author = {Andrew Y. Guo and Abhinav Deshpande and Su-Kuan Chu and Zachary Eldredge and Przemyslaw Bienias and Dhruv Devulapalli and Yuan Su and Andrew M. Childs and Alexey V. Gorshkov} } @article {3188, title = {Importance of the Spectral gap in Estimating Ground-State Energies}, journal = {PRX Quantum}, volume = {3}, year = {2022}, month = {12/9/2022}, abstract = {

The field of quantum Hamiltonian complexity lies at the intersection of quantum many-body physics and computational complexity theory, with deep implications to both fields. The main object of study is the LocalHamiltonian problem, which is concerned with estimating the ground-state energy of a local Hamiltonian and is complete for the class QMA, a quantum generalization of the class NP. A major challenge in the field is to understand the complexity of the LocalHamiltonian problem in more physically natural parameter regimes. One crucial parameter in understanding the ground space of any Hamiltonian in many-body physics is the spectral gap, which is the difference between the smallest two eigenvalues. Despite its importance in quantum many-body physics, the role played by the spectral gap in the complexity of the LocalHamiltonian is less well-understood. In this work, we make progress on this question by considering the precise regime, in which one estimates the ground-state energy to within inverse exponential precision. Computing ground-state energies precisely is a task that is important for quantum chemistry and quantum many-body physics.
In the setting of inverse-exponential precision, there is a surprising result that the complexity of LocalHamiltonian is magnified from QMA to PSPACE, the class of problems solvable in polynomial space. We clarify the reason behind this boost in complexity. Specifically, we show that the full complexity of the high precision case only comes about when the spectral gap is exponentially small. As a consequence of the proof techniques developed to show our results, we uncover important implications for the representability and circuit complexity of ground states of local Hamiltonians, the theory of uniqueness of quantum witnesses, and techniques for the amplification of quantum witnesses in the presence of postselection.

}, doi = {10.1103/prxquantum.3.040327}, url = {https://arxiv.org/abs/2007.11582}, author = {Abhinav Deshpande and Alexey V. Gorshkov and Bill Fefferman} } @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 {3135, title = {Isolation and manipulation of a single-donor detector in a silicon quantum dot}, journal = {Phys. Rev. B}, volume = {106}, year = {2022}, month = {9/27/2022}, pages = {125423}, abstract = {

We demonstrate the isolation and electrostatic control of a single phosphorus donor in a silicon quantum dot by making use of source-drain bias during cooldown and biases applied to capacitively coupled gates. Characterization of the device at low temperatures and in magnetic fields shows single donors can be electrostatically isolated near one of the quantum dot\&$\#$39;s tunnel barriers with either single or double occupancy. This model is well supported by capacitance-based simulations. The ability to use the\ D\ 0\ state of such isolated donors as a charge detector is demonstrated by observing the charge stability diagram of a nearby and capacitively coupled semiconnected double quantum dot.

}, doi = {10.1103/PhysRevB.106.125423}, url = {https://link.aps.org/doi/10.1103/PhysRevB.106.125423}, author = {Lasek, A. A. and Barnes, C. H. W. and Ferrus, T.} } @article {2772, title = {On the Impossibility of Post-Quantum Black-Box Zero-Knowledge in Constant Rounds}, year = {2021}, month = {3/20/2021}, abstract = {

We investigate the existence of constant-round post-quantum black-box zero-knowledge protocols for NP. As a main result, we show that there is no constant-round post-quantum black-box zero-knowledge argument for NP unless NP\⊆BQP. As constant-round black-box zero-knowledge arguments for NP exist in the classical setting, our main result points out a fundamental difference between post-quantum and classical zero-knowledge protocols. Combining previous results, we conclude that unless NP\⊆BQP, constant-round post-quantum zero-knowledge protocols for NP exist if and only if we use non-black-box techniques or relax certain security requirements such as relaxing standard zero-knowledge to ϵ-zero-knowledge. Additionally, we also prove that three-round and public-coin constant-round post-quantum black-box ϵ-zero-knowledge arguments for NP do not exist unless NP\⊆BQP.

}, url = {https://arxiv.org/abs/2103.11244}, author = {Nai-Hui Chia and Kai-Min Chung and Qipeng Liu and Takashi Yamakawa} } @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 {2629, title = {The impossibility of efficient quantum weak coin flipping}, journal = {STOC 2020: Proceedings of the 52nd Annual ACM SIGACT Symposium on Theory of Computing }, year = {2020}, month = {6/2020}, pages = {916-929}, abstract = {

How can two parties with competing interests carry out a fair coin flip across a quantum communication channel? This problem (quantum weak coin-flipping) was formalized more than 15 years ago, and, despite some phenomenal theoretical progress, practical quantum coin-flipping protocols with vanishing bias have proved hard to find. In the current work we show that there is a reason that practical weak quantum coin-flipping is difficult: any quantum weak coin-flipping protocol with bias є must use at least exp( Ω (1/\√є )) rounds of communication. This is a large improvement over the previous best known lower bound of Ω ( log log(1/є )) due to Ambainis from 2004. Our proof is based on a theoretical construction (the two-variable profile function) which may find further applications.

}, doi = {https://doi.org/10.1145/3357713.3384276}, author = {Carl Miller} } @article {2625, title = {Impossibility of Quantum Virtual Black-Box Obfuscation of Classical Circuits}, year = {2020}, month = {5/13/2020}, abstract = {

Virtual black-box obfuscation is a strong cryptographic primitive: it encrypts a circuit while maintaining its full input/output functionality. A remarkable result by Barak et al. (Crypto 2001) shows that a general obfuscator that obfuscates classical circuits into classical circuits cannot exist. A promising direction that circumvents this impossibility result is to obfuscate classical circuits into quantum states, which would potentially be better capable of hiding information about the obfuscated circuit. We show that, under the assumption that learning-with-errors (LWE) is hard for quantum computers, this quantum variant of virtual black-box obfuscation of classical circuits is generally impossible. On the way, we show that under the presence of dependent classical auxiliary input, even the small class of classical point functions cannot be quantum virtual black-box obfuscated.

}, url = {https://arxiv.org/abs/2005.06432}, author = {Gorjan Alagic and Zvika Brakerski and Yfke Dulek and Christian Schaffner} } @article {2602, title = {Information scrambling at finite temperature in local quantum systems}, year = {2020}, month = {5/21/2020}, abstract = {

This paper investigates the temperature dependence of quantum information scrambling in local systems with an energy gap, m, above the ground state. We study the speed and shape of growing Heisenberg operators as quantified by out-of-time-order correlators, with particular attention paid to so-called contour dependence, i.e. dependence on the way operators are distributed around the thermal circle. We report large scale tensor network numerics on a gapped chaotic spin chain down to temperatures comparable to the gap which show that the speed of operator growth is strongly contour dependent. The numerics also show a characteristic broadening of the operator wavefront at finite temperature T. To study the behavior at temperatures much below the gap, we perform a perturbative calculation in the paramagnetic phase of a 2+1D O(N) non-linear sigma model, which is analytically tractable at large N. Using the ladder diagram technique, we find that operators spread at a speed T/m\−\−\−\−\√ at low temperatures, T<}, url = {https://arxiv.org/abs/2005.10814}, author = {Subhayan Sahu and Brian Swingle} } @article {2643, title = {Information scrambling at finite temperature in local quantum systems}, year = {2020}, month = {5/21/2020}, abstract = {

This paper investigates the temperature dependence of quantum information scrambling in local systems with an energy gap, m, above the ground state. We study the speed and shape of growing Heisenberg operators as quantified by out-of-time-order correlators, with particular attention paid to so-called contour dependence, i.e. dependence on the way operators are distributed around the thermal circle. We report large scale tensor network numerics on a gapped chaotic spin chain down to temperatures comparable to the gap which show that the speed of operator growth is strongly contour dependent. The numerics also show a characteristic broadening of the operator wavefront at finite temperature T. To study the behavior at temperatures much below the gap, we perform a perturbative calculation in the paramagnetic phase of a 2+1D O(N) non-linear sigma model, which is analytically tractable at large N. Using the ladder diagram technique, we find that operators spread at a speed T/m\−\−\−\−\√ at low temperatures, T<}, url = {https://arxiv.org/abs/2005.10814}, author = {Subhayan Sahu and Brian Swingle} } @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 {2366, title = {Interaction-induced transition in the quantum chaotic dynamics of a disordered metal}, journal = {Ann. Phys. }, volume = {405}, year = {2019}, month = {03/25/2019}, abstract = {

We demonstrate that a weakly disordered metal with short-range interactions exhibits a transition in the quantum chaotic dynamics when changing the temperature or the interaction strength. For weak interactions, the system displays exponential growth of the out-of-time-ordered correlator (OTOC) of the current operator. The Lyapunov exponent of this growth is temperature-independent in the limit of vanishing interaction. With increasing the temperature or the interaction strength, the system undergoes a transition to a non-chaotic behaviour, for which the exponential growth of the OTOC is absent. We conjecture that the transition manifests itself in the quasiparticle energy-level statistics and also discuss ways of its explicit observation in cold-atom setups.

}, doi = {https://doi.org/10.1016/j.aop.2019.03.008}, url = {https://arxiv.org/abs/1709.09296}, author = {S. V. Syzranov and Alexey V. Gorshkov and V. M. Galitski} } @article {2459, title = {Interference of Temporally Distinguishable Photons Using Frequency-Resolved Detection}, journal = {Phys. Rev. Lett. }, volume = {123}, year = {2019}, month = {9/24/2019}, abstract = {

We demonstrate quantum interference of three photons that are distinguishable in time, by resolving them in the conjugate parameter, frequency. We show that the multiphoton interference pattern in our setup can be manipulated by tuning the relative delays between the photons, without the need for reconfiguring the optical network. Furthermore, we observe that the symmetries of our optical network and the spectral amplitude of the input photons are manifested in the interference pattern. Moreover, we demonstrate time-reversed HOM-like interference in the spectral correlations using time-bin entangled photon pairs. By adding a time-varying dispersion using a phase modulator, our setup can be used to realize dynamically reconfigurable and scalable boson sampling in the time domain as well as frequency-resolved multiboson correlation sampling.

}, doi = {https://doi.org/10.1103/PhysRevLett.123.123603}, url = {https://arxiv.org/abs/1904.03222}, author = {Venkata Vikram Orre and Elizabeth A. Goldschmidt and Abhinav Deshpande and Alexey V. Gorshkov and Vincenzo Tamma and Mohammad Hafezi and Sunil Mittal} } @article {2370, title = {Interpreting Neural Networks Using Flip Points}, year = {2019}, month = {03/20/2019}, abstract = {

Neural networks have been criticized for their lack of easy interpretation, which undermines confidence in their use for important applications. Here, we introduce a novel technique, interpreting a trained neural network by investigating its flip points. A flip point is any point that lies on the boundary between two output classes: e.g. for a neural network with a binary yes/no output, a flip point is any input that generates equal scores for \"yes\" and \"no\". The flip point closest to a given input is of particular importance, and this point is the solution to a well-posed optimization problem. This paper gives an overview of the uses of flip points and how they are computed. Through results on standard datasets, we demonstrate how flip points can be used to provide detailed interpretation of the output produced by a neural network. Moreover, for a given input, flip points enable us to measure confidence in the correctness of outputs much more effectively than softmax score. They also identify influential features of the inputs, identify bias, and find changes in the input that change the output of the model. We show that distance between an input and the closest flip point identifies the most influential points in the training data. Using principal component analysis (PCA) and rank-revealing QR factorization (RR-QR), the set of directions from each training input to its closest flip point provides explanations of how a trained neural network processes an entire dataset: what features are most important for classification into a given class, which features are most responsible for particular misclassifications, how an adversary might fool the network, etc. Although we investigate flip points for neural networks, their usefulness is actually model-agnostic.

}, url = {https://arxiv.org/abs/1903.08789}, author = {Roozbeh Yousefzadeh and Dianne P. O{\textquoteright}Leary} } @article {2454, title = {Investigating Decision Boundaries of Trained Neural Networks}, year = {2019}, month = {8/7/2019}, abstract = {

Deep learning models have been the subject of study from various perspectives, for example, their training process, interpretation, generalization error, robustness to adversarial attacks, etc. A trained model is defined by its decision boundaries, and therefore, many of the studies about deep learning models speculate about the decision boundaries, and sometimes make simplifying assumptions about them. So far, finding exact points on the decision boundaries of trained deep models has been considered an intractable problem. Here, we compute exact points on the decision boundaries of these models and provide mathematical tools to investigate the surfaces that define the decision boundaries. Through numerical results, we confirm that some of the speculations about the decision boundaries are accurate, some of the computational methods can be improved, and some of the simplifying assumptions may be unreliable, for models with nonlinear activation functions. We advocate for verification of simplifying assumptions and approximation methods, wherever they are used. Finally, we demonstrate that the computational practices used for finding adversarial examples can be improved and computing the closest point on the decision boundary reveals the weakest vulnerability of a model against adversarial attack.

}, url = {https://arxiv.org/abs/1908.02802}, author = {Roozbeh Yousefzadeh and Dianne P O{\textquoteright}Leary} } @article {2258, title = {Implicit regularization and solution uniqueness in over-parameterized matrix sensing}, year = {2018}, abstract = {

We consider whether algorithmic choices in over-parameterized linear matrix factorization introduce implicit regularization. We focus on noiseless matrix sensing over rank-r positive semi-definite (PSD) matrices in Rn\×n, with a sensing mechanism that satisfies the restricted isometry property (RIP). The algorithm we study is that of \emph{factored gradient descent}, where we model the low-rankness and PSD constraints with the factorization UU⊤, where U\∈Rn\×r. Surprisingly, recent work argues that the choice of r\≤n is not pivotal: even setting U\∈Rn\×n is sufficient for factored gradient descent to find the rank-r solution, which suggests that operating over the factors leads to an implicit regularization. In this note, we provide a different perspective. We show that, in the noiseless case, under certain conditions, the PSD constraint by itself is sufficient to lead to a unique rank-r matrix recovery, without implicit or explicit low-rank regularization. \emph{I.e.}, under assumptions, the set of PSD matrices, that are consistent with the observed data, is a singleton, irrespective of the algorithm used.

}, url = {https://arxiv.org/abs/1806.02046}, author = {Anastasios Kyrillidis and Amir Kalev} } @article {2250, title = {In Defense of a "Single-World" Interpretation of Quantum Mechanics}, journal = {forthcoming in Studies in History and Philosophy of Modern Physics}, year = {2018}, pages = {15}, abstract = {

In a recent result, Frauchiger and Renner argue that if quantum theory accurately describes complex systems like observers who perform measurements, then \"we are forced to give up the view that there is one single reality.\" Following a review of the Frauchiger-Renner argument, I argue that quantum mechanics should be understood probabilistically, as a new sort of non-Boolean probability theory, rather than representationally, as a theory about the elementary constituents of the physical world and how these elements evolve dynamically over time. I show that this way of understanding quantum mechanics is not in conflict with a consistent \"single-world\" interpretation of the theory.

}, url = {https://arxiv.org/abs/1804.03267}, author = {Jeffrey Bub} } @article {2368, title = {Information-Theoretic Privacy For Distributed Average Consensus: Bounded Integral Inputs}, year = {2018}, month = {03/28/2019}, abstract = {

We propose an asynchronous distributed average consensus algorithm that guarantees information-theoretic privacy of honest agents\&$\#$39; inputs against colluding passive adversarial agents, as long as the set of colluding passive adversarial agents is not a vertex cut in the underlying communication network. This implies that a network with (t+1)-connectivity guarantees information-theoretic privacy of honest agents\&$\#$39; inputs against any t colluding agents. The proposed protocol is formed by composing a distributed privacy mechanism we provide with any (non-private) distributed average consensus algorithm. The agent\&$\#$39; inputs are bounded integers, where the bounds are apriori known to all the agents.

}, url = {https://arxiv.org/abs/1809.01794}, author = {Nirupam Gupta and Jonathan Katz and Nikhil Chopra} } @article {2212, title = {Information-Theoretic Privacy in Distributed Average Consensus}, year = {2018}, abstract = {

We propose an asynchronous distributed average consensus algorithm that guarantees information-theoretic privacy of honest agents\&$\#$39; inputs against colluding semi-honest (passively adversarial) agents, as long as the set of colluding semi-honest agents is not a vertex cut in the underlying communication network. This implies that a network with\ (t+1)-connectivity guarantees information-theoretic privacy of honest agents\&$\#$39; inputs against any\ t\ colluding semi-honest agents. The proposed protocol is formed by composing a distributed privacy mechanism we provide with any (non-private) distributed average consensus algorithm.\ 

}, url = {https://arxiv.org/abs/1809.01794}, author = {Nirupam Gupta and Jonathan Katz and Nikhil Chopra} } @article {2149, title = {Input-output theory for spin-photon coupling in Si double quantum dots}, journal = {Physical Review B}, volume = {96}, year = {2017}, month = {2017/12/22}, pages = {235434}, abstract = {

The interaction of qubits via microwave frequency photons enables long-distance qubit-qubit coupling and facilitates the realization of a large-scale quantum processor. However, qubits based on electron spins in semiconductor quantum dots have proven challenging to couple to microwave photons. In this theoretical work we show that a sizable coupling for a single electron spin is possible via spin-charge hybridization using a magnetic field gradient in a silicon double quantum dot. Based on parameters already shown in recent experiments, we predict optimal working points to achieve a coherent spin-photon coupling, an essential ingredient for the generation of long-range entanglement. Furthermore, we employ input-output theory to identify observable signatures of spin-photon coupling in the cavity output field, which may provide guidance to the experimental search for strong coupling in such spin-photon systems and opens the way to cavity-based readout of the spin qubit.

}, doi = {10.1103/PhysRevB.96.235434}, url = {https://link.aps.org/doi/10.1103/PhysRevB.96.235434}, author = {Benito, M. and Mi, X. and J. M. Taylor and Petta, J. R. and Burkard, Guido} } @article {1736, title = {Interacting atomic interferometry for rotation sensing approaching the Heisenberg Limit}, journal = {Physical Review Letters}, volume = {117}, year = {2016}, month = {2016/11/11}, pages = {203002}, abstract = {

Atom interferometers provide exquisite measurements of the properties of non-inertial frames. While atomic interactions are typically detrimental to good sensing, efforts to harness entanglement to improve sensitivity remain tantalizing. Here we explore the role of interactions in an analogy between atomic gyroscopes and SQUIDs, motivated by recent experiments realizing ring shaped traps for ultracold atoms. We explore the one-dimensional limit of these ring systems with a moving weak barrier, such as that provided by a blue-detuned laser beam. In this limit, we employ Luttinger liquid theory and find an analogy with the superconducting phase-slip qubit, in which the topological charge associated with persistent currents can be put into superposition. In particular, we find that strongly-interacting atoms in such a system could be used for precision rotation sensing. We compare the performance of this new sensor to an equivalent non-interacting atom interferometer, and find improvements in sensitivity and bandwidth beyond the atomic shot-noise limit.

}, doi = {10.1103/PhysRevLett.117.203002}, url = {https://doi.org/10.1103/PhysRevLett.117.203002}, author = {Stephen Ragole and J. M. Taylor} } @article {1500, title = {Injection Locking of a Semiconductor Double Quantum Dot Micromaser}, journal = {Physical Review A}, volume = {92}, year = {2015}, month = {2015/11/02}, pages = {053802}, abstract = { Emission linewidth is an important figure of merit for masers and lasers. We recently demonstrated a semiconductor double quantum dot (DQD) micromaser where photons are generated through single electron tunneling events. Charge noise directly couples to the DQD energy levels, resulting in a maser linewidth that is more than 100 times larger than the Schawlow-Townes prediction. Here we demonstrate a linewidth narrowing of more than a factor 10 by locking the DQD emission to a coherent tone that is injected to the input port of the cavity. We measure the injection locking range as a function of cavity input power and show that it is in agreement with the Adler equation. The position and amplitude of distortion sidebands that appear outside of the injection locking range are quantitatively examined. Our results show that this unconventional maser, which is impacted by strong charge noise and electron-phonon coupling, is well described by standard laser models. }, doi = {10.1103/PhysRevA.92.053802}, url = {http://arxiv.org/abs/1508.04147}, author = {Y. -Y. Liu and J. Stehlik and Michael Gullans and J. M. Taylor and J. R. Petta} } @article {1486, title = {Individual Addressing in Quantum Computation through Spatial Refocusing}, journal = {Physical Review A}, volume = {88}, year = {2013}, month = {2013/11/21}, abstract = { Separate addressing of individual qubits is a challenging requirement for scalable quantum computation, and crosstalk between operations on neighboring qubits remains as a significant source of noise for current experimental implementation of multi-qubit platforms. We propose a scheme based on spatial refocusing from interference of several coherent laser beams to significantly reduce the crosstalk noise for any type of quantum gates. A general framework is developed for the spatial refocusing technique, in particular with practical Gaussian beams, and we show under typical experimental conditions, the crosstalk-induced infidelity of quantum gates can be reduced by several orders of magnitude with a moderate cost of a few correction laser beams. }, doi = {10.1103/PhysRevA.88.052325}, url = {http://arxiv.org/abs/1305.2798v3}, author = {Chao Shen and Zhe-Xuan Gong and Luming Duan} } @article {1246, title = {Interpolatability distinguishes LOCC from separable von Neumann measurements}, journal = {Journal of Mathematical Physics}, volume = {54}, year = {2013}, month = {2013/06/25}, pages = {112204}, abstract = { Local operations with classical communication (LOCC) and separable operations are two classes of quantum operations that play key roles in the study of quantum entanglement. Separable operations are strictly more powerful than LOCC, but no simple explanation of this phenomenon is known. We show that, in the case of von Neumann measurements, the ability to interpolate measurements is an operational principle that sets apart LOCC and separable operations. }, doi = {10.1063/1.4830335}, url = {http://arxiv.org/abs/1306.5992v1}, author = {Andrew M. Childs and Debbie Leung and Laura Mancinska and Maris Ozols} } @article {1536, title = {An Introduction to Quantum Programming in Quipper}, journal = {Lecture Notes in Computer Science}, volume = {7948}, year = {2013}, month = {2013/07/05}, pages = {110-124}, abstract = { Quipper is a recently developed programming language for expressing quantum computations. This paper gives a brief tutorial introduction to the language, through a demonstration of how to make use of some of its key features. We illustrate many of Quipper{\textquoteright}s language features by developing a few well known examples of Quantum computation, including quantum teleportation, the quantum Fourier transform, and a quantum circuit for addition. }, isbn = {978-3-642-38986-3}, doi = {10.1007/978-3-642-38986-3_10}, url = {http://arxiv.org/abs/1304.5485v1}, author = {Alexander S. Green and Peter LeFanu Lumsdaine and Neil J. Ross and Peter Selinger and Beno{\^\i}t Valiron} } @article {1529, title = {Implications of the Babinet Principle for Casimir Interactions}, journal = {Physical Review D}, volume = {84}, year = {2011}, month = {2011/9/1}, abstract = { We formulate the Babinet Principle (BP) as a relation between the scattering amplitudes for electromagnetic waves, and combine it with multiple scattering techniques to derive new properties of Casimir forces. We show that the Casimir force exerted by a planar conductor or dielectric on a self- complementary perforated planar mirror is approximately half that on a uniform mirror independent of the distance between them. The BP suggests that Casimir edge effects are anomalously small, supporting results obtained earlier in special cases. Finally, we illustrate how the BP can be used to estimate Casimir forces between perforated planar mirrors. }, doi = {10.1103/PhysRevD.84.061701}, url = {http://arxiv.org/abs/1103.5395v1}, author = {Mohammad F. Maghrebi and Ronen Abravanel and Robert L. Jaffe} } @article {1352, title = {Interferometry with Synthetic Gauge Fields}, journal = {Physical Review A}, volume = {83}, year = {2011}, month = {2011/3/3}, abstract = {We propose a compact atom interferometry scheme for measuring weak, time-dependent accelerations. Our proposal uses an ensemble of dilute trapped bosons with two internal states that couple to a synthetic gauge field with opposite charges. The trapped gauge field couples spin to momentum to allow time dependent accelerations to be continuously imparted on the internal states. We generalize this system to reduce noise and estimate the sensitivity of such a system to be S~10^-7 m / s^2 / Hz^1/2. }, doi = {10.1103/PhysRevA.83.031602}, url = {http://arxiv.org/abs/1008.3910v2}, author = {Brandon M. Anderson and J. M. Taylor and Victor M. Galitski} } @article {1253, title = {Improved quantum algorithms for the ordered search problem via semidefinite programming }, journal = {Physical Review A}, volume = {75}, year = {2007}, month = {2007/3/26}, abstract = { One of the most basic computational problems is the task of finding a desired item in an ordered list of N items. While the best classical algorithm for this problem uses log_2 N queries to the list, a quantum computer can solve the problem using a constant factor fewer queries. However, the precise value of this constant is unknown. By characterizing a class of quantum query algorithms for ordered search in terms of a semidefinite program, we find new quantum algorithms for small instances of the ordered search problem. Extending these algorithms to arbitrarily large instances using recursion, we show that there is an exact quantum ordered search algorithm using 4 log_{605} N \approx 0.433 log_2 N queries, which improves upon the previously best known exact algorithm. }, doi = {10.1103/PhysRevA.75.032335}, url = {http://arxiv.org/abs/quant-ph/0608161v1}, author = {Andrew M. Childs and Andrew J. Landahl and Pablo A. Parrilo} }