Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C33: Noise Reduction and Error Mitigation in Quantum Computing IIFocus Live

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Sponsoring Units: DQI Chair: Daniel Egger, IBM Research  Zurich 
Monday, March 15, 2021 3:00PM  3:12PM Live 
C33.00001: Errordivisible twoqubit quantum gates David Rodriguez Perez, Tanay Roy, Ziqian Li, David I Schuster, Eliot Kapit We present theoretical results for a set of criteria and waveforms in performing errordivisible twoqubit gates, where the error for a fractional gate decreases proportionally to the quantum rotation desired. This is achieved by instantaneously cancelling unwanted terms over the entire duration of the quantum gate, instead of only as a net result at the end of the gate. This would provide a significant advantage for implementing noisy intermediatescale quantum (NISQ) algorithms, where an algorithm such as VQE or QAOA implemented with errordivisible gates could see error rates up to an order of magnitude lower than one using a standard gate set (e.g. CZ + single qubit rotations). The techniques presented in this work using an errordivisible implementation of a twoqubit gate achieve an eightfold reduction in final gate error for a CPHASE(π/4) operation compared to a stock gate set implementation using CZ gates. 
Monday, March 15, 2021 3:12PM  3:24PM Live 
C33.00002: Methods and benchmarks for error mitigation on noisy quantum computers Ryan LaRose, Tudor GiurgicaTiron, Yousef Hindy, Andrea Mari, Peter Karalekas, Nathan Shammah, William Zeng Quantum error mitigation refers to a set of techniques which improve computational performance (with respect to noise) with minimal overhead in quantum resources. This is generally achieved by mapping an input quantum circuit to a set of related quantum circuits, then sampling from these circuits and classically combining the results. In this presentation, we discuss contributions to the theory of error mitigation [1], show benchmark results on noisy quantum computers [12], and introduce our software package, Mitiq, for error mitigation [2]. In particular, we show how zeronoise extrapolation can be performed at the digital level and generalize inference techniques for extrapolation. We present benchmarks of multiple error mitigation techniques on IBM and Rigetti quantum processors, then finish with more details on our opensource package, Mitiq. 
Monday, March 15, 2021 3:24PM  3:36PM Live 
C33.00003: Diagnosing Errors in Qubit Gates Using Continuous Measurements (Theory) John Steinmetz, Debmalya Das, Gerwin Koolstra, Noah Stevenson, Karthik Siva, William Livingston, Ravi K. Naik, David Ivan Santiago, Irfan Siddiqi, Andrew N Jordan We propose a method of characterizing systematic, timedependent errors in single and twoqubit gates using continuous weak measurements. By tracking the qubit state throughout the gate operation, we can estimate the time dynamics of unknown parameters in the Hamiltonian. Measurement backaction affects the gate dynamics, but not the parameter estimation. We demonstrate this method on imperfect gates using simulated data, and show that we can reconstruct arbitrary coherent errors, as well as leakage to an unwanted third level. 
Monday, March 15, 2021 3:36PM  3:48PM Live 
C33.00004: Error Mitigation with Artificial Symmetries William Huggins, Sam McArdle, Thomas O'Brien, Joonho Lee, Nicholas Rubin, Birgitta K Whaley, Ryan Babbush, Jarrod McClean Incoherent noise arising from imperfect control and measurement presents a serious obstacle to efforts to apply noisy intermediatescale quantum (NISQ) computation to meaningful problems. We present an error mitigation technique that reduces the error in the estimation of expectation values by introducing artificial symmetries more amenable to NISQ devices than traditional quantum error correcting codes. As an example, we present some numerical data showing the effectivenes of our technique applied to the time evolution of a onedimensional Heisenberg chain. We show that our technique can provide more than an order of magnitude reduction in error over a wide range of noise strengths and system sizes and analytically characterize its expected performance in a few simple limits. 
Monday, March 15, 2021 3:48PM  4:00PM Live 
C33.00005: Gate error models for superconducting qubit architectures Devin Underwood, Jiri Stehlik, Timothy Phung, David Zajac, James J Raftery, Muir Kumph In current superconducting qubit devices the best gate fidelities are not high enough for practical error correction, and for gates that require error correction an often quoted experimental milestone is a gate error rate of 1e4. While decoherence is typically the leading order cause of gate error; noise that arises from hardware can also result in gate errors greater than 1e4. For this milestone to be achievable it is important to identify and minimize all sources of gate error above this threshold. There are many types of twoqubit gates used in superconducting qubits; however, with respect to control hardware these gates can be generalized to fall under two categories: narrow band RF controlled gates, and broadband flux tunable gates. We will present experimentally motivated gate error models that provide insight on how noise from system hardware gives rise to gate errors for these two control types. 
Monday, March 15, 2021 4:00PM  4:12PM Live 
C33.00006: NoiseAware Error Rate Reduction of SingleQubit Gates Thomas Maldonado, Alexey Galda, Johannes Flick, Stefan Krastanov, Prineha Narang In the current era of Noisy IntermediateScale Quantum technology, the practical use of quantum computers remains inhibited by computational errors arising from our inability to aptly decouple qubits from their environment. In this work, we introduce a protocol by which knowledge of the initial quantum state (e.g., after qubit initialization) and standard parameters describing the system’s noise can be leveraged to reduce the noise present during the execution of a singlequbit gate. We benchmark our protocol using cloudbased access to 2 of IBM’s 5qubit devices. On one, we demonstrate a reduction in singlequbit error rates by 38%, from 1.6e3 to 1.0e3, provided the initial quantum state is known. On the other, we demonstrate the protocol’s resilience to drifts and miscalibrations in coherence times by using T1 and T2 times offset from their true values by up to 2 orders of magnitude to nonetheless outperform default hardware gate implementations. The protocol can be used to improve the fidelity of both quantum state preparation and quantum circuits for which some knowledge of the qubits’ intermediate states can be inferred. This work presents a pathway to using information about noise levels to significantly reduce the error rates associated with single and twoqubit gates. 
Monday, March 15, 2021 4:12PM  4:24PM Live 
C33.00007: Designing singlequbit gates for a silicon threequbit device with alwayson exchange coupling Sidney Wolin, David Kanaar, Utkan Güngördü, Jason Kestner Lie algebra subspaces have already proven to be useful in designing robust pulses for coupled twoqubit systems, such as electron spin qubits in silicon double quantum dots. We apply similar techniques, after a SchreifferWolff transformation and rotating wave approximation, to decompose a coupled threequbit system (formed in a triple quantum dot) into independent subalgebras. We are then able to choose analytical composite pulses in the applied magnetic field to perform specific logical gates. We can choose the pulse parameters to construct singlequbit and twoqubit gates, including local X rotations of any qubit and nonlocal CZ gates, by making use of the Euler decomposition in su(2) subalgebras. The resulting pulses are simple and short, though they are not inherently robust to noise. To construct robust singlequbit gates, we can make use of existing techniques such as SUPCODE, with modifications to be more efficient for our specific system. The construction of robust twoqubit gates is the topic of the following talk. 
Monday, March 15, 2021 4:24PM  4:36PM Live 
C33.00008: Diagnosing Gate Errors in Superconducting Qubits Using Continuous Measurements (Experiment) Gerwin Koolstra, Noah Stevenson, Karthik Siva, William Livingston, Ravi K. Naik, John Steinmetz, Debmalya Das, Andrew N Jordan, David Ivan Santiago, Irfan Siddiqi Coherent gate errors form an obstacle for successful execution of quantum algorithms on superconducting quantum computers. Improving both single and twoqubit gate fidelities 
Monday, March 15, 2021 4:36PM  4:48PM Live 
C33.00009: Optimized Single Qubit Gates via Filter Function Design Yasuo Oda, Dennis Lucarelli, Kevin Schultz, David Clader, Greg Quiroz Error mitigation protocols represent a class of techniques that are meant to reduce gate error rates via specially designed control sequences. Here, we discuss a protocol that optimizes control sequences to combat temporally correlated noise, a class of noise that is known to be particularly detrimental to quantum error correction. We leverage the filter function formalism to transform the control problem into a filter design problem and show that the frequency response of a quantum system can be carefully tailored even in the presence of multiaxis noise. The control ansatz is specifically chosen to be a functional expansion of Slepians, a discrete time basis known to be optimally concentrated in time and frequency, and quite attractive when faced with experimental control hardware constraints. Using gradient ascent, we obtain optimized filter functions and investigate the relationship between filter function design, control bandwidth, and noise characteristics. We show that under certain noise conditions, it is possible to achieve high fidelity, arbitrary single qubit gate operations that simultaneously yield highpass or bandpass filter functions. 
Monday, March 15, 2021 4:48PM  5:00PM Live 
C33.00010: Nonlinear Signal Distortion Corrections Through Quantum Sensing. Kevin Chaves Having accurate gate generation is essential for precise control of a quantum system. The generated gate usually suffers from linear and nonlinear distortion. Previous works have demonstrated how to use a qubit to correct linear frequency distortions but have not commented on how to handle nonlinear distortions. This is an important issue as we show that nonlinear amplitude distortions from the RF electronics can affect Rabi pulses by as much as 10%. We present work that demonstrates how a transmon qubit can be used as a highly sensitive cryogenic detector to characterize these nonlinear amplitude distortions. We show that a correction can drive these errors down to <1\% over a 700 MHz range. This correction technique provides a method to minimize the effects of signal distortions and can be easily applied to broadband control pulses to produce higher fidelity arbitrary quantum gates. 
Monday, March 15, 2021 5:00PM  5:36PM Live 
C33.00011: Reducing spectator errors in cross resonance gates Invited Speaker: Emily Pritchett Cross resonance twoqubit gates are the cornerstone of many of today’s accessible superconducting quantum computers due to their reliability and stability. We show how to mitigate known unitary errors contributing to the the crossresonance gate — higher order effects of ZZ interaction and spectator entanglement — with the addition of resonant, target rotary pulses. Using specialized Hamiltonian error amplifying tomography, we confirm a reduction of these error terms with target rotary which directly translates to improved twoqubit gate fidelity. Beyond improvement in the controltarget subspace, the target rotary reduces entanglement between target and target spectators caused by residual quantum interactions. We further characterize multiqubit performance improvement enabled by target rotary pulsing using unitarity benchmarking and quantum volume measurements, achieving a measurable increase in Heavy Output Probability when target rotary pulses are applied. 
Monday, March 15, 2021 5:36PM  5:48PM Live 
C33.00012: Robust quantum gates using smooth pulses and physicsinformed neural networks Utkan Güngördü, Charles Tahan, Jason Kestner The presence of decoherence in quantum computers necessitates the suppression of noise. Dynamically corrected gates via specially designed control pulses offer a path forward, but hardwarespecific experimental constraints can cause complications. Here, we present a widely applicable method for obtaining robust smooth pulses which is not based on a sampling approach and does not need any assumptions with regards to the underlying statistics of the experimental noise for both quasistatic and broadband noise. We demonstrate the capability of our approach in the context of spin qubits and transmon by finding smooth shapes which suppress the effects of noise within the logical subspace as well as leakage out of that subspace. 
Monday, March 15, 2021 5:48PM  6:00PM On Demand 
C33.00013: Pulse optimization for errorrobust control on cloudbased superconducting hardware Andre Carvalho, Harrison Ball, Michael Biercuk, Michael Hush, Felix Thompsen We describe an experimental effort designing and deploying errorrobust singlequbit operations on IBM Quantum hardware. In realistic circuits, coherent errors dominate default DRAGpulse performance, manifesting as circuit errors an order of magnitude larger than suggested by randomized benchmarking. Using optimized pulses these errors are suppressed under both serial and parallel (including crosstalk) execution of singlequbit gates on multiqubit hardware. We design numericallyoptimized pulses that implement target operations and exhibit robustness to various error processes including dephasing noise, instabilities in control amplitudes, and crosstalk. Pulse optimization is performed using a flexible optimization package incorporating a device model and physicallyrelevant constraints (e.g. bandwidth limits on the transmission lines of the dilution refrigerator housing IBM quantum hardware). Calibration techniques to accurately deploy optimized pulses via cloudaccess controls, and maximize performance gains on ‘greybox’ hardware, are presented. Performance gains of ~10x are achieved across multiple metrics relevant to systemlevel performance, including achievable circuit depth, stability over time, errorrate variability across qubits, and resilience to crosstalk. 
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