Mitigating off-resonant error in the cross-resonance gate

An off-resonant error for a driven quantum system refers to interactions due to the input drives having nonzero spectral overlap with unwanted system transitions. For the cross-resonance gate, this includes leakage as well as off-diagonal computational interactions that lead to bit-flip error on the control qubit. In this paper, we quantify an off-resonant error, with more focus on the less studied off-diagonal control interactions, for a direct controlled-not (cnot) gate implementation. Our results are based on a numerical simulation of the dynamics while we demonstrate the connection to time-dependent Schrieffer-Wolff and Magnus perturbation theories. We present two methods for suppressing such error terms. First, pulse parameters need to be optimized so off-resonant transition frequencies coincide with the local minima due to the pulse spectrum sidebands. Second, we show the advantage of a Y-DRAG pulse on the control qubit in mitigating the off-resonant error. Depending on qubit-qubit detuning, the proposed methods can improve the average off-resonant error from approximately 10-3 closer to the 10-4 level for a direct cnot calibration.