Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X35: Characterization and Reduction of Noise in Quantum Computing Architectures IFocus

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Sponsoring Units: DQI Chair: Seth Merkel, HRL Laboratories Room: BCEC 205B 
Friday, March 8, 2019 8:00AM  8:36AM 
X35.00001: Software and hardware for improved quantum volume of transmon processors Invited Speaker: Lev Bishop The quantum volume (arXiv:1811.12926 [quantph]) is a singlenumber metric useful for pragmatic quantification and comparison of performance for NISQ devices. It is affected by the software toolchain as well as the selection and fidelity of available quantum gates. I will discuss improvements on both axes. In particular, we introduced a surprisingly effective circuit rewriting method for approximately decomposing arbitrary target twoqubit SU(4) gates while optimally trading off approximation error against the gate error. Combining this technique with updated hardware achieves the highest quantum volume measured to date. 
Friday, March 8, 2019 8:36AM  8:48AM 
X35.00002: Investigating Crosstalk and Correlated Errors with Randomized Benchmarking on Multiple Qubits Christian Kraglund Andersen, Stefania Balasiu, Johannes Heinsoo, Ants Remm, Sebastian Krinner, JeanClaude Besse, Simone Gasparinetti, Christopher Eichler, Andreas Wallraff Quantum computing with superconducting circuits has recently shown progress in implementing multiqubit quantum processors with promising performance. To ensure scalability of these systems for quantum applications, it is critical that any errors remain small, local, and uncorrelated when increasing the number of qubits. In particular, the threshold theorem for quantum error correction is typically derived under the assumption of uncorrelated errors and constant error rates for each qubit. In this talk, we will present characterizations of crosstalk during gate operations using an extended simultaneous randomized benchmarking (RB) protocol. We apply sequences of randomly chosen elements from the single qubit Clifford group simultaneously on up to four qubits and use singleshot readout to measure individual qubits and all σ_{z}correlators between the qubits. From these measurements, we estimate the multiqubit error as the average nqubit infidelity per application of n singlequbit Clifford gates. We find the correlated errors to be composed mainly by twoqubit correlations. We also extend this method to analyse the crosstalk and correlated errors induced during twoqubit RB. 
Friday, March 8, 2019 8:48AM  9:00AM 
X35.00003: Pairwise Perturbative Ansatz for Quantum Process Tomography Part 1: Theory Luke Govia, Guilhem Ribeill, Matthew Ware, Hari K Krovi As candidate quantum processors increase both in size and fidelity, so too does the need for robust verification and validation of their operation. Full characterization of these devices would be highly desirable; however, standard quantum process tomography scales exponentially with the number of qubits. Even for small scale systems, the experimental resource requirements make full tomography very challenging in practice. To circumvent this, we present an ansatz to describe an arbitrary quantum process on a multiqubit system that only requires characterization of twoqubit processes, such that the number of measurements scales only quadratically with the number of qubits. Our Pairwise Perturbative Ansatz (PAPA) builds a description of the multiqubit process from tomographic reconstruction of the reduced twoqubit processes on all pairs of qubits in the system. In part 1 of this talk we outline the PAPA approach to multiqubit process tomography, and show through theoretical simulation how it can be used for excellent characterization of multiqubit quantum processes. 
Friday, March 8, 2019 9:00AM  9:12AM 
X35.00004: Pairwise Perturbative Ansatz for Quantum Process Tomography Part 2: Experiment Matthew Ware, Guilhem Ribeill, Luke Govia, Hari K Krovi In part two of this talk we present a Pairwise Perturbative Ansatz (PAPA) reconstruction of a three qubit process. For this demonstration, we perform GST characterization on neighboring pairs of qubits in a fully connected three qubit system. The local GST reconstructions constrain the global process. As outlined in part one, this data is used to bootstrap a description of a three qubit process from multiple twoqubit reconstructions. Such an ansatz could lead to characterization techniques that scale favorably in the number of qubits while making few assumptions about the structure of system noise. 
Friday, March 8, 2019 9:12AM  9:24AM 
X35.00005: Twoqubit spectroscopy of spatiotemporally correlated noise in superconducting qubits. Part 1: theory Felix Beaudoin, Leigh Norris, Uwe von Lüpke, Youngkyu Sung, Morten Kjærgaard, Daniel Campbell, David K Kim, Jonilyn L Yoder, IoanMihai Pop, Simon Gustavsson, William D Oliver, Lorenza Viola Characterization of realistic noise environments is crucial to the design of optimally tailored control methods for error suppression, and to the validation of microscopic models of noise. In particular, spatial correlations between noise afflicting distinct qubits are important in the determination of thresholds for faulttolerant quantum computation. Recently, spinlocking relaxometry has been implemented to measure the singlequbit spectrum of quantum noise in superconducting circuits. In this talk, we present a generalization of this technique to twoqubit systems, enabling to simultaneously reconstruct the singlequbit and crosscorrelation spectra of quantum dephasing noise. We also present simulations showing how this technique can be experimentally benchmarked using two superconducting qubits exposed to engineered photon shot noise from a common microwave cavity mode. 
Friday, March 8, 2019 9:24AM  9:36AM 
X35.00006: Twoqubit spectroscopy of spatiotemporally correlated noise in superconducting qubits. Part 2: experiment Uwe Von Luepke, Felix Beaudoin, Leigh Norris, Youngkyu Sung, Morten Kjærgaard, Dan Campbell, David K Kim, Jonilyn L Yoder, IoanMihai Pop, Lorenza Viola, Simon Gustavsson, William D Oliver Faulttolerant quantum computing relies on quantum error correction techniques. Most of these correction schemes assume the noise causing errors to exhibit only weak, spatially decaying correlations between distinct qubits. To investigate these assumptions experimentally, there is a need to extend established noise spectroscopy techniques from single to multiqubits systems. In this work, we present experiments on correlated dephasing of two superconducting qubits due to photon shot noise in a shared microwave cavity. Utilizing both free and driven evolution protocols, we demonstrate a frequencyselective method to probe the twoqubit correlations, and show how quantum noise spectroscopy techniques can reconstruct both singlequbit and twoqubit crossspectra. 
Friday, March 8, 2019 9:36AM  9:48AM 
X35.00007: Suppressing error correlations in space and time using quantum control Claire Edmunds, Cornelius Hempel, Robert Harris, Harrison Ball, Virginia Frey, Thomas Stace, Michael Jordan Biercuk We perform measurements of spatiotemporal error correlations in linear arrays of trapped ions. Experiments demonstrate that slow drifts and systematic calibration errors arising from microwave amplifiers, quasistatic magnetic field gradients, and spatial microwave inhomogeneities contribute to operational errors at the 10^{4} level over chains up to seven ions. By replacing standard "primitive" microwave gate operations on the 12.6 GHz hyperfine qubit in ^{171}Yb^{+} we demonstrate an ability to reduce error correlations due to these native error sources. Measurements of randomized benchmarking on individual ions show that even when overall error rates are reduced only marginally, signatures of temporal error correlations are reduced through use of noisefiltering controls. Moreover, we demonstrate that the measured correlations between errors on neighboring qubits are reduced when appropriate control solutions are employed. These experiments demonstrate an important role for quantum control operations in the context of quantum error correction even when the added complexity of the errorsuppressing controls limits the absolute suppression of error rates in the system. 
Friday, March 8, 2019 9:48AM  10:00AM 
X35.00008: Randomized Benchmarking of Majorana Based Qubits Alan Tran, Bela Bauer, Parsa Bonderson, Steven Flammia We analyze randomized benchmarking (RB) in the context of Majorana zero mode (MZM) based qubits. In particular, we focus on the recently proposed MZM hexon architectures which allow for topologically protected measurementbased braiding gates at the expense of a nonminimal encoding of the qubit. This leads to a departure from standard RB in that the gates depend on nondeterministic measurement outcomes and are prone to nontrivial unitary effects due to the larger encoding. Furthermore, the effect of imperfect measurement bases and the resulting projections must be considered. In this work we adapt the framework of RB to MZM qubits with measurementbased gates and expand upon it by offering a protocol to directly characterize the fidelity of the underlying measurements themselves. 
Friday, March 8, 2019 10:00AM  10:12AM 
X35.00009: TimeResolved Tomography of Quantum Gates Kevin Young, Timothy Proctor, Kenneth Rudinger, Erik Nielsen, Robin BlumeKohout The performance of a quantum computer depends critically on a large number of highlytuned parameters. Time dependence (drift) in these parameters is generally unheralded, but can have a pernicious and possibly devastating impact on quantum gate fidelities. Identifying the source of this drift is a critical first step on the path to mitigating it, but techniques to do so have been cumbersome at best. In this talk, we discuss a suite of quantum circuit experiments and data analysis tools that is capable of identifying and characterizing Fouriersparse drift in quantum gates, measurements, and state preparation operations. By incorporating a model of the experiment that is aware of the controllable parameters, these tools can often help to establish the precise source of any unwanted time dependence. We illustrate our work with data from both simulation and experiment. 
Friday, March 8, 2019 10:12AM  10:24AM 
X35.00010: Robust Decorrelation of Errors in Quantum Gates by Random Gate Synthesis Anthony Polloreno Coherent errors in quantum operations are ubiquitous. Whether arising from spurious environmental couplings or errors in control fields, such errors can accumulate rapidly and degrade the performance of a quantum circuit significantly more than an average gate fidelity may indicate. Furthermore, coherent errors are considerably more difficult to model than stochastic errors, and understanding their impact on a generic quantum circuit or algorithm can be challenging. In this talk, we discuss using robust optimal control techniques to construct many different implementations of a target gate, each with a different coherent error. As Hastings and Campbell have recently shown, randomly sampling over that ensemble yields an effective quantum channel that well approximates the target, but with dramatically suppressed coherent error. Our results extend those of Hastings and Campbell to include robustness to drifting external control parameters. We have implemented these constructions using a superconducting qubit and will discuss randomized benchmarking results consistent with a marked reduction in coherent error. 
Friday, March 8, 2019 10:24AM  10:36AM 
X35.00011: 2019: A Hilbert Space Odyssey into the characterization of twoqubit gates using phase estimation Kyle Gulshen, Amy Brown, Alexa N Staley, Eric C Peterson, Joshua Combes Fast and accurate gate calibration is necessary for quantum computing. Process tomography is inaccurate, in part because of SPAM. Procedures like Gate Set tomography are accurate but too slow. In this talk we present protocols that allow you to robustly tune up arbitrary two qubit gates with high accuracy. We generalize iterative phase and Hamiltonian estimation procedures developed for single qubits to two qubit gates. The resulting protocol has Heisenberg limited scaling when estimating any of the gate parameters. We present results from an experimental demonstration of the protocol run through the full stack at Rigetti Computing. 
Friday, March 8, 2019 10:36AM  10:48AM 
X35.00012: Calibration for singlequbit gates using robust phase estimation William Kirby, Shelby Kimmel Constructing singlequbit unitary gates with high accuracy is necessary for most implementations of gatebased quantum computation. A crucial component of this task is gate calibration, which requires methods for measuring the discrepancies between actual, physical gate implementations and ideal gates. In this talk we describe improved practical methods for quantifying such discrepancies using robust phase estimation, which requires only weak assumptions about the initial accuracies of the gate implementations. 
Friday, March 8, 2019 10:48AM  11:00AM 
X35.00013: Calibrating twoqubit gates via robust phase estimation Kenneth Rudinger, Guilhem Ribeill, Luke Govia, Matthew Ware, Shelby Kimmel Robust phase estimation (RPE) is a Heisenberglimited characterization protocol for learning the phases of quantum gate operations. Unlike many other similar techniques, RPE requires neither ancillae qubits nor highly accurate state preparation and measurement. Additionally, the computational requirements of RPE are minimal, needing nothing more complex than inverse trigonometric functions. To date, RPE has been used for singlequbit gate characterization and calibration. We extend RPE for two qubits, showing that it may be used to learn the phase for any standard twoqubit gate (e.g., CNOT, CPHASE, MolmerSorensen, SWAP, crossresonance, etc.). We provide both numerical and experimental results. 
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