Designing robust gate implementations for quantum-information processing

Quantum-information processing systems are often operated through time-dependent controls; choosing these controls in a way that makes the resulting operation insensitive to variations in unknown or uncontrollable system parameters is an important prerequisite for obtaining high-fidelity gate operations. In this article we present a numerical method for constructing such robust control sequences for a quite general class of quantum-information processing systems. As an application of the method we have designed a robust implementation of a phase-shift operation central to rare-earth-metal quantum computing, an ensemble quantum computing system proposed by Ohlsson [Opt. Commun. 201, 71 (2002)] . In this case the method has been used to obtain a high degree of insensitivity with respect to differences between ensemble members, but it is equally well suited for quantum computing with a single physical system.