Robust quantum gates for systems subject to decoherence via optimal control: Markovian vs non-Markovian dynamics

Abstract

We study the implementation of one-, two-, and three-qubit quantum gates for interacting qubits using optimal control. Different Markovian and non-Markovian environments are compared and efficient optimisation algorithms utilising analytic gradient expressions and quasi-Newton updates are given for both cases. The performance of the algorithms is analysed for a large set of problems in terms of the fidelities attained and the observed convergence behaviour. New notions of success rate and success speed are introduced and density plots are utilised to study the effect of key parameters, such as gate operation times, and random variables, such as the initial fields required to start the iterative algorithm. Core characteristics of the optimal fields are statistically analysed. Substantial differences between Markovian and non-Markovian environments in terms of the possibilities for control and the control mechanisms are uncovered. In particular, in the Markovian case it is found that the optimal fields obtained without considering the environment cannot be improved substantially by taking the environment into account and the fidelities attained are determined mostly by the gate operation time as well as the overall strength of the environmental effects. Computation time is saved if the fields are pre-optimised neglecting decoherence. In the non-Markovian case, on the other hand, substantial improvements in the fidelities are observed when the details of the system-bath coupling are taken into account. In that case, field leakage is shown to be a significant issue which can make high gate fidelities impossible to obtain unless both the system and noise qubits are fully controlled.