We propose an effective method to optimize the working parameters (WPs) of
microwave-driven quantum logical gates implemented with multi-level physical
qubits. We show that by treating transitions between each pair of levels
independently, intrinsic gate errors due primarily to population leakage to
undesired states can be estimated accurately from spectroscopic properties of
the qubits and minimized by choosing appropriate WPs. The validity and
efficiency of the approach are demonstrated by applying it to optimize the WPs
of two coupled rf SQUID flux qubits for controlled-NOT (CNOT) operation. The
result of this independent transition approximation (ITA) is in good agreement
with that of dynamic method (DM). Furthermore, the ratio of the speed of ITA to
that of DM scales exponentially as 2^n when the number of qubits n increases.Comment: 4pages, 3 figure