Robust quantum control against clock noises in multiqubit systems

High-precision manipulation of multiqubit quantum systems requires strictly clocked and synchronized multichannel control signals. However, practical arbitrary wave-form generators always suffer from random signal jitters and channel latencies that induce nonignorable state or gate operation errors. In this paper, we analyze the average gate error caused by clock noises, from which an estimation formula is derived for quantifying the control robustness against clock noises. This measure is then employed for finding robust controls via a homotopic optimization algorithm. We also introduce our recently proposed stochastic optimization algorithm, b-GRAPE, for training robust controls via randomly generated clock-noise samples. Numerical simulations on a two-qubit example demonstrate that both algorithms can greatly improve the control robustness against clock noises. The homotopic algorithm converges much faster than the b-GRAPE algorithm, but the latter can achieve higher control robustness.