Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M16: Characterizing Quantum Computing Systems and Components IFocus
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Sponsoring Units: DQI Chair: Matthew Ware, BBN Technology - Massachusetts Room: 201 |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M16.00001: Algorithm-specific Performance Analysis of Transmon Qubit Devices Michael O'Keeffe, Morten Kjaergaard, Mollie Schwartz, Gabriel Samach, Amy Greene, Chris McNally, Danna Rosenberg, William Oliver, Andrew James Kerman, Kevin Obenland Many demonstrations of quantum algorithms exhibit a tradeoff between the accuracy of the algorithm and the fidelity of the circuit that implements that algorithm. In particular, for algorithms that rely on Trotter decomposition to approximate a target unitary, increasing the number of steps reduces the algorithm error. However, in current devices, uncontrolled interactions with the environment and suboptimal control limit qubit coherence and gate fidelity, which ultimately restrict circuit depth. We analyze algorithm performance on transmon qubit devices using simulated, model-based, and experimentally measured process maps for gates, and compare a number of characterization metrics. In the case of a Trotterized algorithm, we determine the optimal operating point and predict the expected performance in good agreement with experiment. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M16.00002: Oscillator noise spectroscopy via displaced Schrödinger-cat states Alistair Milne, Claire Edmunds, Cornelius Hempel, Harrison Ball, Michael Hush, André Carvalho, Michael Biercuk We present a technique for the characterization of noise in oscillator-mediated entangling operations applicable to a range of gate implementations in both trapped ion and superconducting circuit architectures. A major source of error in this class of entangling gates is residual coupling between the system of qubits and the intermediate oscillator modes, often caused by noise on the mode frequencies. In order to characterize this leading source of gate error, we use discrete phase modulation of the field driving the gate to construct sequences of oscillator-phase-space displacements with a band-limited sensitivity to oscillator frequency noise. By displacing the motion of a single trapped 171Yb+ ion, we characterize the sensitivity of these sequences to engineered noise, verifying analytic predictions in the filter function formalism for the observed signal strength. We also perform sensing of intrinsic noise, leveraging tools for spectrum reconstruction based on a singular value decomposition approach developed at Q-CTRL. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M16.00003: Detector Tomography on IBM Quantum Computers and Mitigation of Imperfect Measurement Yanzhu Chen, Maziar Farahzad, Shinjae Yoo, Tzu-Chieh Wei We use quantum detector tomography to characterize the qubit readout in terms of measurement POVMs on IBM Quantum Computers IBM Q 5 Tenerife and IBM Q 5 Yorktown. Our results suggest that the characterized detector model deviates noticeably from the ideal projectors. Further improvement on this characterization can be made by adopting two- or more-qubit detector models instead of independent single-qubit detectors for all the qubits in one device. We also find evidence indicating correlations in the detector behavior, i.e. the detector characterization is slightly altered when other qubits and their detectors are in operation. We also discuss how the characterized detectors' POVM can be used to estimate the ideal detection distribution. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M16.00004: Characterizing mid-circuit measurements with a new form of gate set tomography part 1: Theory Kenneth Rudinger, Timothy Proctor, Erik Nielsen, Guilhem Ribeill, Matthew Ware, Luke Govia, Thomas A Ohki, Kevin Young, Robin Blume-Kohout While quantum circuits end with measurements, they can also include measurements in the middle. Such mid-circuit measurements are required for real-time feedforward control applications, such as quantum error correction. Understanding error processes in these mid-circuit measurements will be critical for building next-generation quantum processors. To that end, we extend gate set tomography (GST), a highly accurate and self-consistent protocol for diagnosing quantum gate errors, to be able to also characterize mid-circuit measurements. We will describe this extension and demonstrate its success in simulations. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M16.00005: Characterizing mid-circuit measurements with a new form of gate set tomography part 2: Experiment Guilhem Ribeill, Matthew Ware, Luke Govia, Kenneth Rudinger, Timothy Proctor, Thomas A Ohki Quantum computers rely on classical electronics for qubit control and readout. An important capability for the implementation of complex algorithms such as quantum error correction is the ability to operate the classical hardware in a feedback loop with the quantum processor. Mid-circuit measurements are the key operation in this type of control scheme, and their efficient and precise characterization will be critical to understanding the performance of algorithms on near-term quantum devices. To that end, we demonstrate the use of an extension to gate set tomography (GST), a highly accurate protocol for diagnosing quantum processes, to characterize intermediate measurements on a superconducting transmon qubit. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M16.00006: A proposed gold standard family of protocols for benchmarking and diagnosing elementary quantum gate operations Kristine Boone, Arnaud Carignan-Dugas, Joel Wallman, Ian Hincks, Dar Dahlen, Joseph V Emerson We propose a family of randomized benchmarking protocols as a gold standard for assessing and diagnosing error rates on elemntary one and two qubit gates. We discuss various advantages of our bespoke family of protocols relative to other standard approaches. These advantages include: reducing the number of single shot experiments and the number of distinct random sequences including; improving the accuracy and convergence of confidence intervals; accommodating arbitrary gates within SU(4); and reliably assessing the error rate and error type associated with individual gates from any universal gate set. Time-permitting, we will report results from implementing these protocols across a variety of leading platforms, including both superconducting qubits and trapped ions. |
Wednesday, March 4, 2020 12:27PM - 1:03PM |
M16.00007: Efficient learning of quantum noise Invited Speaker: Robin Harper Noise is the central obstacle to building large-scale quantum computers. Quantum systems with sufficiently uncorrelated and weak noise could be used to solve computational problems that are intractable with current digital computers. There has been substantial progress towards engineering such systems. However, continued progress depends on the ability to characterize quantum noise reliably and efficiently with high precision. Here I will discuss a newly introduced protocol that completely and efficiently characterizes the qubit error rates of quantum noise. The method returns an estimate of the effective noise with relative precision and detects all correlated errors. I will show how the reconstruction allows the easy visualization of these correlated errors, enabling both the discovery of long-range correlations in the device and the construction of scalable models that describe the noise in the device to arbitrary precision. These properties of the protocol make it exceptionally well suited for high-precision noise metrology in quantum information processors. Our results are the first implementation of a provably rigorous, diagnostic protocol capable of being run on state of the art devices and beyond. These results pave the way for noise metrology in next-generation quantum devices, calibration in the presence of crosstalk, bespoke quantum error-correcting codes, and customized fault-tolerance protocols that can greatly reduce the overhead in a quantum computation. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M16.00008: Long-Sequence Quantum Process Tomography Takanori Sugiyama It is indispensable for development of a quantum computer to further improve accuracy of elemental quantum operations. Details of errors during the operations are useful information for achieving the accuracy improvement. Quantum tomography has a potential to provide such information, but its standard protocols suffer from low reliability due to its high sensitivity against state preparation and measurement (SPAM) errors. Self-consistent quantum tomography such as GST and RSCQT overcomes the low reliability, but it requires too large costs of experiments and data-processing. Here, we propose a new tomographic method with an error amplification, named long-sequence quantum process tomography. We theoretically prove that the error amplification can suppress effects of SPAM errors in arbitrary finite dimensional system, and numerically show that its implementation costs can be practical for one- and two-qubit systems. We also explain how to use the method for characterizing leakage and crosstalk errors, and discuss its practicality beyond two-qubit systems. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M16.00009: Spectral Quantum Tomography Jonas Helsen, Francesco Battistel, Barbara Maria Terhal We introduce spectral quantum tomography, a simple method to extract the eigenvalues of a noisyfew-qubit gate, represented by a trace-preserving superoperator, in a SPAM-resistant fashion, usinglow resources in terms of gate sequence length. The eigenvalues provide detailed gate information,supplementary to known gate-quality measures such as the gate fidelity, and can be used as a gatediagnostic tool. We apply our method to one- and two-qubit gates on two different superconduct-ing systems available in the cloud, namely the QuTech Quantum Infinity and the IBM QuantumExperience. We discuss how cross-talk, leakage and non-Markovian errors affect the eigenvalue data. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M16.00010: Independent State and Measurement Characterization on Quantum Computers Junan Lin, Joel Wallman, Raymond Laflamme Correctly characterizing state preparation and measurement (SPAM) processes is a necessary step towards building reliable quantum processing units (QPUs). In this work, we approach this problem by assuming certain structure on SPAM and gate elements, and derive a simple experimental procedure to separately estimate the SPAM error strengths on a QPU. After discussing principles behind the experimental design, we present the protocol along with an asymptotic bound for the uncertainty in the estimated parameters in terms of quantities that can be estimated independently of SPAM processes. We test this protocol on a publicly available 5-qubit QPU and discuss the applicability of our protocol on near-term devices. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M16.00011: Extracting Coherence Information From Random Circuits Using “Speckle Purity Benchmarking” Julian Kelly, Sergio Boixo, Zijun Chen, John M Martinis, Hartmut Neven Budgeting the contributions of coherent and incoherent noise sources is an important component of benchmarking quantum gates. Typically, methods such as Cross Entropy Benchmarking (XEB) or Randomized Benchmarking are used to measure an error-per-gate that includes noise and control errors. These sequences can be extended to quantify the decay of a quantum state due to noise only by measuring the state purity with tomography as described in previous publications. Here, we introduce a method that allows us to extract the same information with exponentially fewer sequences from raw XEB data. We introduce “Speckle Purity Benchmarking” which quantifies the purity via the contrast (or “speckliness!”) of output bitstring probabilities. Pure quantum states generated by the XEB procedure will have high contrast, while incoherent mixtures will have low contrast. Compared to conventional XEB, this procedure can be done with zero information about the actual quantum process. Additionally, this can be scaled to a handful of qubits. |
Wednesday, March 4, 2020 1:51PM - 2:03PM |
M16.00012: Quantum noise spectroscopy for multiaxis noise models Leigh Norris, Gerardo A Paz Silva, Felix Beaudoin, Lorenza Viola Characterizing decoherence that arises from coupling to noisy environments is essential for optimized control and error correction strategies in realistic quantum information processors. Motivated by this challenge, quantum noise spectroscopy (QNS) seeks to estimate the spectral properties of noise affecting quantum systems. To date, QNS protocols have largely focused on platforms dominated by dephasing (T2) processes, rendering them inapplicable to systems in which longitudinal relaxation (T1) processes occur on a comparable timescale. To move beyond dephasing-dominated platforms, we extend frequency-comb based QNS to a multi-axis qubit noise model that takes into account both T1 and T2 processes from either classical or quantum environments. Targeted control of the qubit permits a complete spectral reconstruction, including arbitrary cross-axis correlations. Using a novel spherical representation for the noise operators, we show that three noise spectra characterize the reduced dynamics in a regime where the qubit energy splitting is large. This spherical representation enables a straightforward multi-axis extension to QNS protocols based on continuous driving, such as spin-locking. |
Wednesday, March 4, 2020 2:03PM - 2:15PM |
M16.00013: Cayley path and quantum computational supremacy: A proof of average-case #P-hardness of Random Circuit Sampling with quantified robustness Ramis Movassagh A one-parameter unitary-valued interpolation between any two unitary matrices is constructed based on the Cayley transformation, which extends our work. The entries of the interpolated unitaries are shown to be low-degree rational functions in the parameter, which we proved can be efficiently determined using an extension of the Berlekamp-Welch algorithm. We prove that this path provides scrambled unitaries with probability distributions arbitrarily close to the Haar measure. We then prove the simplest known average-case #P-hardness of random circuit sampling (RCS), which is the task of sampling from the output distribution of a quantum circuit whose local gates are random Haar unitaries, and is the lead candidate for demonstrating quantum supremacy in the NISQ era. We show that a previous work based on the truncations of the power series representation of the exponential function does not provide practical robustness. Explicit bounds on noise resilience are proved, which on a grid of sqrt(n)xsqrt(n) qubits with circuit depth sqrt(n) is 2^O(-n^3), and with constant-depth it is 2^O(-n^2). Improvements to O(2^(−n)/poly(n)) would prove the quantum supremacy conjecture, which may be false. |
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