Abstract
We present an improvement to the cross resonance gate realized with the addition of resonant, target rotary pulses. These pulses, applied directly to the target qubit, are simultaneous to and in phase with the echoed cross resonance pulses. Using specialized Hamiltonian error amplifying tomography, we confirm a reduction of error terms with target rotary—directly translating to improved two-qubit gate fidelity. Beyond improvement in the control-target 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 new record quantum volume for a superconducting qubit system.Received 31 July 2020Accepted 21 October 2020DOI:https://doi.org/10.1103/PRXQuantum.1.020318Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum benchmarkingQuantum controlQuantum gatesQuantum information with solid state qubitsQuantum Information
Highlights
Improving the performance of near-term quantum systems is important to quantum information technology today
We introduce tailored tomographic methods—which we refer to as Hamiltonian error amplifying tomography (HEAT)—that amplify and measure errors in our system
As the target-target spectator dynamics are dominated by the target rotary pulse, we utilize the HEAT sequences [shown in Fig. 2(b)] to identify and track the dependence of dominant entangling Hamiltonian terms IYZ and IZZ with target rotary amplitude
Summary
Improving the performance of near-term quantum systems is important to quantum information technology today. Interactions with other qubits in the device—“spectators”—cause unwanted entanglement to accumulate across the system While these ZZ-induced errors can be corrected with more complicated pulse sequences, such as higher-order echoes to address spectator-induced error [24] or additional singlequbit rotations to correct unitary errors in the two-qubit subspace, we show here that a resonant drive of the target qubit reduces both types of error simultaneously without increasing the duration or the depth of the two-qubit gate sequence. This “target rotary” pulsing, presented schematically, is performed in parallel to the CR drive of the control qubit and switches sign in the standard two-pulse echo sequence. We find that the QV, a holistic measure of device performance affected by unitary and purity errors amongst others, increases to 32 with the addition of target rotary on many five-qubit subsystems of the 20-qubit device tested
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