  This paper reconsiders the method of adaptive measurement for qubit state
preparation developed by Jacobs and shows an alternative scheme that works even
under unknown unitary evolution of the state. 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 at
while that eigenstate converges to the target. The most significant feature of
this scheme is that the measurement strength can be taken constant unlike
Jacobs' one, which eventually provides fine robustness property of the
controlled state against the uncertainty of the unitary evolution.

Quantum Physics