Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B33: Quantum Characterization, Verification, and Validation IFocus Live

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Sponsoring Units: DQI Chair: Zijun Chen, Google Inc  Santa Barbara 
Monday, March 15, 2021 11:30AM  11:42AM Live 
B33.00001: Benchmarking and Mitigating Coherent Errors in ControlledPhase Gates due to Spectator Qubits Sebastian Krinner, Stefania Lazar, Ants Remm, Christian Kraglund Andersen, Nathan Lacroix, Graham J. Norris, Christoph Hellings, Mihai Gabureac, Christopher Eichler, Andreas Wallraff A major challenge in operating multiqubit quantum processors is to mitigate multiqubit coherent errors. For superconducting circuits, besides crosstalk originating from imperfect isolation of control lines, dispersive coupling between qubits is a major source of multiqubit coherent errors. We benchmark phase errors in a controlledphase gate due to dispersive coupling of either of the qubits involved in the gate to one or more spectator qubits [1]. We measure the associated gate infidelity using quantum process tomography. We point out that, due to coupling of the gate qubits to a noncomputational state during the gate, twoqubit conditional phase errors are enhanced. Finally, we show that we can mitigate the identified gate errors using dynamical decoupling techniques. Our work is important for understanding limits to the fidelity of twoqubit gates with finite on/off ratio in multiqubit. 
Monday, March 15, 2021 11:42AM  11:54AM Live 
B33.00002: Characterization and Benchmarking of Quantum Computers using Cycle Benchmarking Techniques Megan Lilly, Travis S Humble Characterization of noisy, intermediatescale quantum (NISQ) devices requires gathering detailed information about noise processes and their effects in quantum circuits. Cycle benchmarking is a protocol to estimate the process fidelity of a noisy quantum process. We pair this method with Pauli channel estimation to construct a description of noisy operations in a quantum circuit. We demonstrate cycle benchmarking and Pauli channel estimation in experiment on superconducting transmon qubit registers up to size 27. We use cycle benchmarking to test one and twoqubit quantum operations and report their process fidelity. We use Pauli channel estimation to identify the noise channels that arise in experiment and develop noise models. We use the noise models in numerical simulation and compare to experiment. Our results show that cycle benchmarking and Pauli channel estimation can be used to estimate quantum device behavior within at least 14% of experiment and are at least 10% more accurate than noiseless baseline estimates. 
Monday, March 15, 2021 11:54AM  12:06PM Live 
B33.00003: Simultaneous Gate Set Tomography Robin BlumeKohout, Susan M Clark, Akel Hashim, Craig Hogle, Daniel Lobser, Ravi K. Naik, Timothy Proctor, Kenneth Rudinger, David Ivan Santiago, Irfan Siddiqi, Kevin Young Crosstalk is a leading source of failure in multiqubit quantum information processors. It can arise from a wide range of disparate physical phenomena, and can introduce subtle correlations in the errors experienced by a device. Several hardware characterization protocols are able to detect the presence of crosstalk, but few provide sufficient information to distinguish various crosstalk errors from one another. In this talk we introduce simultaneous gate set tomography, a protocol for detailed characterization of crosstalk errors. We demonstrate our methods by performing tomographic reconstructions on a twoqubit trapped ion system and a threequbit superconducting qubit system. The results are consistent with expectations from simple physical models of these devices, but we also identify a number additional, unanticipated forms of crosstalk. SNL is managed and operated by NTESS under DOE NNSA contract DENA0003525. 
Monday, March 15, 2021 12:06PM  12:18PM Live 
B33.00004: Increasing Quantum Volume on a superconducting quantum computing system Petar Jurcevic, Ali Javadi, Lev S Bishop, Isaac Lauer Lauer, Daniela F Bogorin, Markus Brink, Lauren Capelluto, Oktay Gunluk, Toshinari Itoko, Naoki Kanazawa, Abhinav Kandala, George Keefe, Kevin Krsulich, William Landers, Eric Lewandowski, Douglas T McClure, Giacomo Nannicini, Adinath Narasgond, Hasan M Nayfeh, Emily Pritchett, Mary Beth Rothwel, Srikanth Srinivasan, Neereja Sundaresan, Cindy Wang Cindy Wang, Ken Wei We improve the quality of quantum circuits on superconducting quantum computing systems, as measured by the quantum volume, with a combination of dynamical decoupling, compiler optimizations, shorter twoqubit gates, and excited state promoted readout. This result shows that the path to larger quantum volume systems requires the simultaneous increase of coherence, control gate fidelities, measurement fidelities, and smarter software which takes into account hardware details, thereby demonstrating the need to continue to codesign the software and hardware stack for the foreseeable future. 
Monday, March 15, 2021 12:18PM  12:30PM Live 
B33.00005: Experimentally accrediting the outputs of noisy quantum computers Samuele Ferracin, Seth T Merkel, Animesh Datta The quantum computers currently available suffer levels of noise that cannot be neglected, therefore it is essential to develop methods to check the correctness of their outputs. To tackle this problem we provide and experimentally demonstrate an “accreditation protocol,” namely a protocol to upperbound the variation distance between the probability distribution of the outputs of a noisy quantum computer and its noiseless counterpart. The quantum circuits in our experiment range from depth 4 circuits on 10 qubits to depth 21 circuits on 4 qubits and are implemented on programmable superconducting hardware. Our protocol requires implementing the desired quantum circuit along with a set of random Clifford circuits and subsequently postprocessing the outputs of these Clifford circuits. Importantly, all the postprocessing can be done efficiently and our protocol is fully scalable for the long term. Moreover, by testing entire circuits rather than gates, our protocol can detect noise that may be missed by benchmarking individual gates. We thus demonstrate a scalable and reliable method to ascertain the correctness of the outputs of noisy quantum computers. 
Monday, March 15, 2021 12:30PM  12:42PM Live 
B33.00006: Scalable evaluation of quantumcircuit error loss using Clifford sampling Zhen Wang, Yanzhu Chen, Zixuan Song, Dayue Qin, Hekang Li, Qiujiang Guo, Haohua Wang, Chao Song, Ying Li A major challenge in developing quantum computing technologies is to accomplish high precision tasks by utilizing multiplex optimization approaches. Loss functions assessing the overall circuit performance provide the foundation for many optimization techniques. We use the quadratic error loss and the finalstate fidelity loss to characterize quantum circuits, which can be efficiently evaluated in a scalable way by sampling from Clifforddominated circuits. We demonstrate it by numerically simulating 10qubit quantum circuits with various error models and executing 4qubit circuits with up to 10 layers of 2qubit gates on a superconducting quantum processor. Our results pave the way towards the optimizationbased quantum device and algorithm design in the NISQ regime. 
Monday, March 15, 2021 12:42PM  12:54PM Live 
B33.00007: Scalable and targeted benchmarking of quantum computers Timothy Proctor, Kenneth Rudinger, Mohan Sarovar, Erik Nielsen, Kevin Young, Robin BlumeKohout A quantum processor’s performance is limited by the errors it experiences. But most current performance metrics do not accurately predict what programs a particular processor can successfully run, due to complex errors that only emerge at scale. In this talk we will present scalable, holistic, and flexible benchmarking techniques, built on “mirror circuits”, that can efficiently test the capabilities of any programmable quantum computer. We will show how to create mirror circuit benchmarks based on randomized, periodic, and algorithmic circuits; we will show how mirror circuit benchmarks can be used for scalable “full stack” benchmarking; and we will demonstrate these techniques with experiments. These experimental results show that current hardware suffers from structured errors whose effect cannot be predicted from standard error metrics inferred from randomized circuits. 
Monday, March 15, 2021 12:54PM  1:06PM Live 
B33.00008: An application benchmark for fermionic quantum simulations PierreLuc DallaireDemers, Michal Stechly, Jerome Gonthier, Ntwali Toussaint Bashige, Jhonathan Romero, Yudong Cao It is expected that the simulation of correlated fermions in chemistry and material science will be one of the first practical applications of quantum processors. Given the rapid evolution of quantum hardware, it is increasingly important to develop robust benchmarking techniques to gauge the capacity of quantum hardware specifically for the purpose of fermionic simulation. 
Monday, March 15, 2021 1:06PM  1:18PM Live 
B33.00009: Randomized analog verification for analog quantum simulators and gatebased quantum devices Ryan Shaffer, Hang Ren, Eli Megidish, Joseph Broz, WeiTing Chen, Hartmut Haeffner We introduce an experimentallymotivated technique for randomized analog verification (RAV) of quantum devices. The technique involves generating random sequences built from the primitive capabilities of the quantum device. For an analog quantum simulator, in which the system is designed to simulate the dynamics of a target Hamiltonian, these sequences are formed of small time steps of subsets of the target Hamiltonian. For gatebased quantum devices, these sequences are formed of gates that are native to the device architecture and that may be continuouslyparameterized. In both cases, sequences are approximately compiled such that the system returns nearly to a basis state, allowing for simple measurement of results. We report simulated results of RAV under various types of experimental error sources. We also demonstrate RAV experimentally on physical quantum systems of various architectures. 
Monday, March 15, 2021 1:18PM  1:30PM Live 
B33.00010: A general framework for randomized benchmarking Jonas Helsen, Ingo Roth, Emilio Onorati, Albert Werner, Jens Eisert Randomized benchmarking (RB) has over the past decade become the gold standard for characterizing quantum gates, and a great many variations have been devised. In this work, we develop a comprehensive framework of RB, general enough to encompass almost all known RB protocols. This framework allows us to rigorously guarantee, under realistic conditions, that the output of a RB experiment is welldescribed by a linear combination of matrix exponential decays. We complement this with a detailed discussion of the RB fitting problem: we discuss modern signal processing techniques and their guarantees, give analytical sample complexity bounds, and numerically evaluate their performance. To reduce the resource demands of this fitting problem, we also provide scalable postprocessing techniques to isolate exponential decays, improving the feasibility of a large set of RB protocols. These techniques generalize previously proposed methods such as character RB and linearcross entropy benchmarking. Finally we discuss the relation between RB decay rates and the average fidelity in full generality. On the technical side, our work significantly extends the recently developed Fouriertheoretic perspective on RB and combines it with advanced perturbation theory and ideas from signal processing. 
Monday, March 15, 2021 1:30PM  2:06PM Live 
B33.00011: Characterizing and mitigating errors in large quantum system Invited Speaker: Sarah Sheldon Accurate and reliable verification and validation methods are critical for marking progress on the path towards fault tolerance. But there are many other questions we want to answer such as is this device good enough to perform my algorithm and what is the nature of the noise in this quantum system. Gate error rates alone do not tell the whole story, and coarse metrics can be difficult to infer the likelihood of success for specific circuits. All of these tools exist on a continuum and give insight into device performance and capabilities. In this talk I will discuss demonstrations that mark progress in our understanding of noise and mitigation of errors. 
Monday, March 15, 2021 2:06PM  2:18PM Live 
B33.00012: Modelbased characterization on 10 qubits Erik Nielsen, Timothy Proctor, Kenneth Rudinger, Kevin Young, Robin BlumeKohout Predictive gatelevel models provide a valuable tool for characterizing the noise in quantum processors. Insight regarding the nature of the errors afflicting a system of qubits often identifies means of compensating for the errors and may suggest design choices for future hardware. Modelbased characterization, however, is known to scale poorly due to the large number of model parameters and difficulty of circuit simulation. In this talk, we discuss the necessary tradeoffs between the number of model parameters, the depth of test circuits, and the precision of the result. We demonstrate on a 10qubit system how model fitting and testing can be used to provide insight into underlying error mechanisms. 
Monday, March 15, 2021 2:18PM  2:30PM Live 
B33.00013: Wildcard error: Quantifying unmodeled errors in quantum processors Robin BlumeKohout, Kenneth Rudinger, Erik Nielsen, Timothy Proctor, Kevin Young Error models for quantum computing processors describe their deviation from ideal behavior and predict the consequences in applications. But experimental behavior is rarely consistent with error models, even in characterization experiments like randomized benchmarking (RB) or gate set tomography (GST). We show how to resolve these inconsistencies, and quantify the rate of unmodeled errors, by augmenting error models with a parameterized wildcard error model. Wildcard error relaxes predictions, and the amount of wildcard error needed quantifies the rate of unmodeled errors. We demonstrate the use of wildcard error to augment RB and GST, and to quantify leakage. 
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