Reducing spectator errors in cross resonance gates

Abstract

Cross resonance two-qubit 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 cross-resonance 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 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 multi-qubit 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.