Local reachability of stochastic quantum dynamics with application to feedback control of single-spin systems

A quantum state subject to a continuous measurement obeys a nonlinear stochastic differential equation called the stochastic master equation. In this paper, we utilize the technique of local reachability developed in control theory to characterize a class of the measurement and the controller in such a way that the local transition of the controlled quantum state is restricted to a low-dimensional manifold. We examine the proposed controller for single-spin systems with two typical measurements.