Targeting qubit states using open-loop control

We present an open-loop (bang-bang) scheme which drives an open two-level quantum system to any target state, while maintaining quantum coherence throughout the process. The control is illustrated by a realistic simulation for both adiabatic and thermal decoherence. In the thermal decoherence regime, the control achieved by the proposed scheme is qualitatively similar, at the ensemble level, to the control realized by the quantum feedback scheme of Wang, Wiseman and Milburn (2001 Phys. Rev. A 64 063810) for the spontaneous emission of a two-level atom. The performance of the open-loop scheme compares favourably against the quantum feedback scheme with respect to robustness, target fidelity and transition times.