Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E44: Quantum Characterization, Verification and Validation IIFocus
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Sponsoring Units: GQI Chair: Barry Sanders, University of Calgary Room: 347 |
Tuesday, March 15, 2016 8:00AM - 8:36AM |
E44.00001: Bounding quantum gate error rate based on reported average fidelity Invited Speaker: Yuval Sanders Remarkable experimental advances in quantum computing are exemplified by recent announcements of impressive average gate fidelities exceeding 99.9\% for single-qubit gates and 99\% for two-qubit gates. Although these high numbers engender optimism that fault-tolerant quantum computing is within reach, the connection of average gate fidelity with fault-tolerance requirements is not direct. Here we use reported average gate fidelity to determine an upper bound on the quantum-gate error rate, which is the appropriate metric for assessing progress towards fault-tolerant quantum computation, and we demonstrate that this bound is asymptotically tight for general noise. Although this bound is unlikely to be saturated by experimental noise, we demonstrate using explicit examples that the bound indicates a realistic deviation between the true error rate and the reported average fidelity. We introduce the Pauli-distance as a measure of this deviation, and we show that knowledge of the Pauli-distance enables tighter estimates of the error rate of quantum gates. [Preview Abstract] |
Tuesday, March 15, 2016 8:36AM - 8:48AM |
E44.00002: Entanglement verification with detection efficiency mismatch Yanbao Zhang, Norbert L\"utkenhaus Entanglement is a necessary condition for secure quantum key distribution (QKD). When there is an efficiency mismatch between various detectors used in the QKD system, it is still an open problem how to verify entanglement. Here we present a method to address this problem, given that the detection efficiency mismatch is characterized and known. The method works without assuming an upper bound on the number of photons going to each threshold detector. Our results suggest that the efficiency mismatch affects the ability to verify entanglement: the larger the efficiency mismatch is, the smaller the set of entangled states that can be verified becomes. When there is no mismatch, our method can verify entanglement even if the method based on squashing maps [PRL 101, 093601 (2008)] fails. [Preview Abstract] |
Tuesday, March 15, 2016 8:48AM - 9:00AM |
E44.00003: Machine Learning for Quantum Metrology and Quantum Control Barry Sanders, Ehsan Zahedinejad, Pantita Palittapongarnpim Generating quantum metrological procedures and quantum gate designs, subject to constraints such as temporal or particle-number bounds or limits on the number of control parameters, are typically hard computationally. Although greedy machine learning algorithms are ubiquitous for tackling these problems, the severe constraints listed above limit the efficacy of such approaches. Our aim is to devise heuristic machine learning techniques to generate tractable procedures for adaptive quantum metrology and quantum gate design. In particular we have modified differential evolution to generate adaptive interferometric-phase quantum metrology procedures for up to 100 photons including loss and noise, and we have generated policies for designing single-shot high-fidelity three-qubit gates in superconducting circuits by avoided level crossings. Although quantum metrology and quantum control are regarded as disparate, we have developed a unified framework for these two subjects, and this unification enables us to transfer insights and breakthroughs from one of the topics to the other. [Preview Abstract] |
Tuesday, March 15, 2016 9:00AM - 9:12AM |
E44.00004: Ultimate precision limit and optimal probe states for quantum metrology Haidong Yuan, Chi-Hang Fred Fung An important task in science and technology is to find out the highest achievable precision in measuring and estimating parameters of interest with given resources, and design schemes to reach it. Quantum metrology, which exploits quantum mechanical effects to achieve high precision, has gained increasing attention in recent years. Here we present a general framework for quantum metrology which relates the ultimate precision limit directly to the underlying dynamics, this framework provides efficient methods for computing the ultimate precision limit and optimal probe states. We further demonstrate the power of the framework by deriving a sufficient condition on when ancillary systems are not useful for improving the precision limit. [Preview Abstract] |
Tuesday, March 15, 2016 9:12AM - 9:24AM |
E44.00005: Improving the precision of weak measurement by nonclassical states Shengshi Pang, Todd A. Brun Weak value amplification is a useful protocol to amplify tiny physical effects by postselecting the system in a weak measurement. However, there has been controversy over its precision advantage in parameter estimation recently, since it discards unselected results of the postselection measurement on the system, which may take away useful information. While it is now clear that retaining failed postselections can yield more Fisher information than discarding them, the advantage of postselection measurement itself still remains to be clarified. If a weak measurement with postselection measurement cannot not produce higher precision than without postselection measurement, it would be meaningless to discuss the use of postselection results. In this work, we address this problem by studying the optimal signal-to-noise ratio (SNR) of postselected weak measurement. We find a surprising result that when the probe is initially prepared in a proper squeezed coherent state, the postselected weak measurement can give a higher SNR than the standard weak measurement, while such an advantage vanishes when the probe is prepared in a normal coherent state. This suggests that raising the precision of weak measurement by postselection calls for the presence of “nonclassicality” in the probe state. [Preview Abstract] |
Tuesday, March 15, 2016 9:24AM - 9:36AM |
E44.00006: Robust Calibration of a Universal Single-Qubit Gate-Set via Robust Phase Estimation Shelby Kimmel, Guang Hao Low, Theodore J. Yoder An important step in building a quantum computer is calibrating experimentally implemented quantum gates to produce operations that are close to ideal unitaries. The calibration step involves estimating the systematic errors in gates and then using controls to correct the implementation. Quantum process tomography is a standard technique for estimating these errors, but is both time consuming, (when one only wants to learn a few key parameters), and is usually inaccurate without resources like perfect state preparation and measurement, which might not be available. With the goal of efficiently and accurately estimating specific errors using minimal resources, we develop a parameter estimation technique, which can gauge key systematic parameters (specifically, amplitude and off-resonance errors) in a universal single-qubit gate-set with provable robustness and efficiency. In particular, our estimates achieve the optimal efficiency, Heisenberg scaling, and do so without entanglement and entirely within a single-qubit Hilbert space. Our main theorem making this possible is a robust version of the phase estimation procedure of Higgins et al. [B. L. Higgins et al., New J. Phys. 11 073023 (2009)]. [Preview Abstract] |
Tuesday, March 15, 2016 9:36AM - 9:48AM |
E44.00007: Numerical Analysis of Robust Phase Estimation Kenneth Rudinger, Shelby Kimmel Robust phase estimation (RPE) is a new technique for estimating rotation angles and axes of single-qubit operations, steps necessary for developing useful quantum gates [arXiv:1502.02677]. As RPE only diagnoses a few parameters of a set of gate operations while at the same time achieving Heisenberg scaling, it requires relatively few resources compared to traditional tomographic procedures. In this talk, we present numerical simulations of RPE that show both Heisenberg scaling and robustness against state preparation and measurement errors, while also demonstrating numerical bounds on the procedure's efficacy. We additionally compare RPE to gate set tomography (GST), another Heisenberg-limited tomographic procedure. While GST provides a full gate set description, it is more resource-intensive than RPE, leading to potential tradeoffs between the procedures. We explore these tradeoffs and numerically establish criteria to guide experimentalists in deciding when to use RPE or GST to characterize their gate sets.\\\\ Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 15, 2016 9:48AM - 10:00AM |
E44.00008: Formal Computer Validation of the Quantum Phase Estimation Algorithm Wayne Witzel, Kenneth Rudinger, Mohan Sarovar, Robert Carr While peer review and scientific consensus provide some assurance to the validity of ideas, people do make mistakes that can slip through the cracks. A plethora of formal methods tools exist and are in use in a variety of settings where high assurance is demanded. Existing tools, however, require a great deal of expertise and lack versatility, demanding a non-trivial translation between a high-level description of a problem and the formal system. Our software, called Prove-It, allows a nearly direct translation between human-recognizable formulations and the underlying formal system. While Prove-It is not designed for particularly efficient automation, a primary goal of other formal methods tools, it is extremely flexible in following a desired line of reasoning (proof structure). This approach is particularly valuable for validating proofs that are already known. We will demonstrate a validation of the Quantum Phase Estimation Algorithm using Prove-It. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 15, 2016 10:00AM - 10:12AM |
E44.00009: Efficient Bayesian Phase Estimation Nathan Wiebe, Christopher Granade We provide a new efficient adaptive algorithm for performing phase estimation that does not require that the user infer the bits of the eigenphase in reverse order; rather it directly infers the phase and estimates the uncertainty in the phase directly from experimental data. Our method is highly flexible, recovers from failures, and can be run in the presence of substantial decoherence and other experimental imperfections and is as fast or faster than existing algorithms. [Preview Abstract] |
Tuesday, March 15, 2016 10:12AM - 10:24AM |
E44.00010: ABSTRACT MOVED TO TALK 1 |
Tuesday, March 15, 2016 10:24AM - 10:36AM |
E44.00011: Percolation bounds for decoding thresholds with correlated erasures in quantum LDPC codes Kathleen Hamilton, Leonid Pryadko Correlations between errors can dramatically affect decoding thresholds, in some cases eliminating the threshold altogether. We analyze the existence of a threshold for quantum low-density parity-check (LDPC) codes in the case of correlated erasures. When erasures are positively correlated, the corresponding multi-variate Bernoulli distribution can be modeled in terms of cluster errors, where qubits in clusters of various size can be marked all at once. In a code family with distance scaling as a power law of the code length, erasures can be always corrected below percolation on a qubit adjacency graph associated with the code. We bound this correlated percolation transition by weighted (uncorrelated) percolation on a specially constructed cluster connectivity graph, and apply our recent results [1] to construct several bounds for the latter.% \smallskip\\[0pt] [1] K. E. Hamilton and L. P. Pryadko, ``\emph{Algebraic bounds for weighted percolation on directed and undirected graphs},'' arXiv:1505.03963 (2015). [Preview Abstract] |
Tuesday, March 15, 2016 10:36AM - 10:48AM |
E44.00012: Robustness and performance scaling of quantum information processors with respect to gate removal and defects Yunseong Nam, Reinhold Bl\"umel A single logical gate, when removed from a classical computer, can completely destroy its information processing capability. For a quantum processor, the story is quite different. We find that the processing capability of a quantum information processor is robust with respect to the removal of a large number of quantum logical gates. In fact, even when most of the quantum processor's gates are removed, quantum processors, such as the universally applicable quantum Fourier transform or the quantum adder, work with satisfactory performance. In this talk, we present our numerical and analytical results detailing the performance scaling of quantum processors with respect to gate pruning operations. We also present the performance scaling of pruned quantum processors subjected to gate defects in the remaining gates. [Preview Abstract] |
Tuesday, March 15, 2016 10:48AM - 11:00AM |
E44.00013: Assessing the performance of quantum repeaters for all phase-insensitive Gaussian bosonic channels Kenneth Goodenough, David Elkouss, Stephanie Wehner One of the most sought-after goals in experimental quantum communication is the implementation of a quantum repeater. Quantum repeaters can be assessed by comparing their performance with the quantum- and private capacity of a direct transmission, assisted by unlimited classical two-way communication. Calculating these quantities is hard to compute however, motivating the search for upper bounds on these capacities. Takeoka, Guha and Wilde found the squashed entanglement of a quantum channel to be an upper bound on these capacities. In general it is hard to find the exact value of the squashed entanglement of a quantum channel, but clever, sub-optimal “squashing maps” still allow one to upper bound this quantity, and thus also the corresponding capacities. We follow this approach to upper bound the capacity of some specific channels, where in particular we extend the analysis of Takeoka et al. on the pure-loss channel to the general case of any Gaussian bosonic channel with equal noise in each quadrature. This bound is of practical importance, since it allows one to benchmark the implementation of quantum repeaters in quantum key distribution networks for a large class of channels. [Preview Abstract] |
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