Robust adaptive measurement scheme for qubit-state preparation

In this paper, the method of adaptive measurement for qubit-state preparation developed by Jacobs is reconsidered, and an alternative scheme that works even under unknown unitary evolution of the state is shown. The key idea is that the measurement is adaptively changed so that one of the eigenstates of the measured observable is always set between the current and the target states while that eigenstate converges to the target. The most significant feature of this scheme is that the measurement strength can be taken constant, unlike in Jacobs' scheme, which eventually provides fine robustness of the controlled state against the uncertainty of the unitary evolution.