Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W35: Focus Session: Quantum Computing Architectures and Algorithms: Characterization, Verification, & Validation |
Hide Abstracts |
Sponsoring Units: GQI Chair: Robin Blume-Kohout, Sandia National Laboratories Room: 702 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W35.00001: Weak value amplification considered harmful Christopher Ferrie, Joshua Combes We show using statistically rigorous arguments that the technique of weak value amplification does not perform better than standard statistical techniques for the tasks of parameter estimation and signal detection. We show that using all data and considering the joint distribution of all measurement outcomes yields the optimal estimator. Moreover, we show estimation using the maximum likelihood technique with weak values as small as possible produces better performance for quantum metrology. In doing so, we identify the optimal experimental arrangement to be the one which reveals the maximal eigenvalue of the square of system observables. We also show these conclusions do not change in the presence of technical noise. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W35.00002: The information inequality in postselection on parameter estimation problems Saki Tanaka, Naoki Yamamoto The weak measurement proposed by Aharonov and his co-workers allows the meter to generate an infinitely large measurement result. The essence of this measurement is the operation called postselection; more precisely, we read the measurement result displayed by the probe only when we get a particular postselected state of the system, after the system-probe interaction. When the postelected state is nearly orthogonal to the preselected state, this weak measurement significantly amplifies the amount of displacement of the probe, which is regarded as a signal amplification. However, does the large displacement really mean signal amplification? We show that this is not true in the parameter estimation context. In general, the estimation error of a parameter is lower bounded by the inverse of the SLD-Fisher information, known as the Cram\'er-Rao inequality. We compare the SLD-Fisher information of the states with and without postselection. Taking into account the success rate of the postselection, we then derive an inequality showing that the postselection never decrease the estimation error. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W35.00003: Quantum Limits on Probabilistic Amplifiers Shashank Pandey, Zhang Jiang, Joshua Combes, Carlton Caves An ideal phase-preserving linear amplifier is a deterministic device that adds to an input signal the minimal amount of noise consistent with the constraints imposed by quantum mechanics. A noiseless linear amplifier takes an input coherent state to an amplified coherent state, but only works part of the time. Such a device is actually better than noiseless, since the output has less noise than the amplified noise of the input coherent state; we refer to such devices as immaculate. We bound the working probabilities of probabilistic and approximate immaculate amplifiers and construct theoretical models that achieve some of these bounds. Our chief conclusions are the following: (i) the working probability of any phase-insensitive immaculate amplifier is very small in the phase-plane region where the device works with high fidelity;(ii) phase-sensitive immaculate amplifiers that work only on coherent states sparsely distributed on a phase-plane circle centered at the origin can have a reasonably high working probability. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W35.00004: Optimal quantum-enhanced interferometry using a laser power source Matthias Lang, Carlton Caves We consider an interferometer powered by laser light (a coherent state) into one input port and ask the following question: what is the best state to inject into the second input port, given a constraint on the mean number of photons this state can carry, in order to optimize the interferometer's phase sensitivity? This question is the practical question for high-sensitivity interferometry. We answer the question by considering the quantum Cramer-Rao bound for such a setup. The answer is squeezed vacuum, if there are no photon losses in the interferometer. For a lossy interferometer, the squeezed vacuum is the best choice for the practical case where the laser power is much bigger than the power put into the squeezing. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W35.00005: Optimal phase estimation using photon number counting in the presence of dephasing noise Kaushik Seshadreesan, Bhaskar RoyBardhan, Hwang Lee, Jonathan Dowling We study interferometric phase estimation in the presence of dephasing noise. We show that photon number counting and photon number parity measurement achieve the quantum Cramer-Rao bound of the optimal cosine state. Furthermore, we show that, when operated using a Bayesian update protocol, photon counting saturates the bound independently of the actual value of the unknown phase, thereby allowing for globally optimal phase estimation. We also show that both photon counting and parity measurement achieve the quantum Cramer-Rao bound of all path-symmetric probe states (a class which includes the optimal cosine state) in the presence of dephasing noise. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W35.00006: Quantum process tomography of near-unitary maps Amir Kalev, Charles Baldwin, Ivan Deutsch We study the problem of quantum process tomography given the prior information that the implemented map is near to a unitary map on a $d$-dimensional Hilbert space. In particular, we show that a perfect unitary map is completely characterized by a minimum of $d^2 + d$ measurement outcomes. This contrasts with the $d^4$ measurement outcomes required in general. To achieve this lower bound, one must probe the system with a particular set of $d$ states in a particular order. This order exploits unitarity but does not assume any other structure of the map. We further consider the more general case of noisy quantum maps, with a low level of noise. Our study indicates that transforming to the interaction picture, where the noiseless map is represented by a diagonal operator, can provide a useful tool to identify the noise structure. This, in turn, can lead to a substantial reduction in the numerical resources needed to estimate the noisy map. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W35.00007: Quantum process estimation via compressed sensing with convex optimization Charles Baldwin, Amir Kalev, Ivan Deutsch Quantum process tomography is the standard method for diagnosing an unknown process. However, it is extremely resource intensive requiring $O(d^4)$ measurement outcomes for a $d$ dimensional Hilbert space. In previous work, researchers have applied compressed sensing techniques allowing us to make use of previous knowledge of the system in order to reduce the resources required. We study different procedures for reconstructing a process matrix from compressed sensing in the form of convex-optimization. We show that different estimation applied to the same data are sensitive to different types of noise. The estimators could, therefore, be used as indicators of particular error models. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W35.00008: Compressed Sensing for Reconstructing Sparse Quantum States Kenneth Rudinger, Robert Joynt Compressed sensing techniques have been successfully applied to quantum state tomography, enabling the efficient determination of states that are nearly pure, i.e, of low rank. We show how compressed sensing may be used even when the states to be reconstructed are full rank. Instead, the necessary requirement is that the states be sparse in some known basis (e.g. the Pauli basis). Physical systems at high temperatures in thermal equilibrium are important examples of such states. Using this method, we are able to demonstrate that, like for classical signals, compressed sensing for quantum states exhibits the Donoho-Tanner phase transition. This method will be useful for the determination of the Hamiltonians of artificially constructed quantum systems whose purpose is to simulate condensed-matter models, as it requires many fewer measurements than demanded by standard tomographic procedures. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W35.00009: High-Confidence Quantum Gate Tomography Blake Johnson, Marcus da Silva, Colm Ryan, Shelby Kimmel, Brian Donovan, Thomas Ohki Debugging and verification of high-fidelity quantum gates requires the development of new tools and protocols to unwrap the performance of the gate from the rest of the sequence. Randomized benchmarking tomography\footnote{Kimmel et al. arXiv:1306.2348 [quant-ph] (2013)} allows one to extract full information of the unital portion of the gate with high confidence. We report experimental confirmation of the technique's applicability to quantum gate tomography. We show that the method is robust to common experimental imperfections such as imperfect single-shot readout and state preparation. We also demonstrate the ability to characterize non-Clifford gates. To assist in the experimental implementation we introduce two techniques. ``Atomic Cliffords'' use phase ramping and frame tracking to allow single-pulse implementation of the full group of single-qubit Clifford gates. Domain specific pulse sequencers allow rapid implementation of the many thousands of sequences needed. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W35.00010: Robust Characterization of Quantum Processes Invited Speaker: Shelby Kimmel Accurate characterization of the errors that occur in quantum systems will help to improve the performance of quantum computers. However, many characterization procedures suffer from systematic errors because they assume state preparation, measurement, and other controlling gates are error free. In this talk I will describe a method that can provide estimates of almost all parameters of a quantum map, yet is robust to many types of errors.\\[4pt] In collaboration with Marcus P. da Silva, Colm A. Ryan, Blake R. Johnson, and Thomas Ohki, Raytheon BBN Technologies. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W35.00011: Tomography via Correlation of Noisy Measurement Records Colm Ryan, Blake Johnson, Jay Gambetta, Jerry Chow, Marcus Silva, Oliver Dial, Thomas Ohki We present methods and results of shot-by-shot correlation of noisy measurements to extract entangled state and process tomography in a superconducting qubit architecture \footnote{Ryan et al. arXiv:1310.6448 (2013)}. We show that averaging continuous values, rather than counting discrete thresholded values, is a valid tomographic strategy and is in fact the better choice in the low signal-to-noise regime. We show that the effort to measure $N$-body correlations from individual measurements scales exponentially with $N$, but with sufficient signal-to-noise the approach remains viable for few-body correlations. We provide a new protocol to optimally account for the transient behavior of pulsed measurements. Despite single-shot measurement fidelity that is less than perfect, we demonstrate appropriate processing to extract and verify entangled states and processes. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W35.00012: Simpler, faster, better: robust randomized benchmarking tests for non-unitality and non-Markovianity in quantum devices Joel Wallman, Steve Flammia, Marie Barnhill, Joseph Emerson Characterizing and suppressing noise in quantum systems is the major obstacle to developing a universal quantum computer. It is impossible to completely characterize the noise acting on $n$ qubits efficiently, since a complete characterization would require an exponential number of parameters. However, we can efficiently obtain parameters of interest, such as the average gate fidelity (which gives the average error rate introduced by using a noisy gate instead of an ideal gate) using protocols such as randomized benchmarking, which will be reliable under certain strong assumptions (such as time-independent and gate-independent noise). In this talk, we will present recent results that allow randomized benchmarking to be used in a wider array of scenarios (in particular, to characterize time-dependent noise), with simpler operations (e.g., using only single-qubit operators, which makes the assumption of gate-independent noise more reasonable) and to obtain additional information about the noisy processes (e.g., whether they are unital and/or Markovian). These results allow a more efficient and complete characterization of noisy quantum systems. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W35.00013: Determining individual gate error models from the output of quantum error detection circuits Austin Fowler, John Martinis There are many different ways of determining an error model for a quantum gate, including process tomography and randomized benchmarking. These techniques focus on individual gates. A frequently discussed concern is whether any given error model accurately reflects how errors will compose and propagate when multiple gates are applied in a practical circuit. We approach the error model problem in a new manner, starting with the experimental output of a complete quantum error detection circuit, and determine error models for all gates from this single source of data. We argue that these error models are the most appropriate for predicting the error suppression performance of larger quantum circuits. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700