Decoupling spectator qubits from entangling gates

Two-qubit entangling gates tend to be the most error-prone operations in NISQ-era superconducting quantum computers. To maximize the fidelity of quantum algorithms on these devices, suppressing these errors becomes a necessity. In addition to errors within the two-qubit subspace, these gates also induce coherent errors on adjacent idle “spectator” qubits.

In this work, we investigate spectator qubit errors from the echoed cross-resonance (ECR) gate, which is the entangling gate on the “Eagle” generation of 127-qubit superconducting IBM devices. We characterize these errors using numerical simulations of the ECR gate in the expanded subspace that includes spectator qubits. We demonstrate, both theoretically and experimentally, that dynamical decoupling techniques can either enhance or suppress these errors depending on the timing of the decoupling gates. Based on these observations, we develop a decoupling scheme that suppresses coherent spectator qubit errors in addition to single-qubit and two-qubit idling errors, and evaluate its ability to improve execution fidelity on a suite of quantum algorithms.