  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 quantum
computing, an ensemble quantum computing system proposed by Ohlsson et. al.
[Opt. Comm. 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.
