  Feedback control (based on the quantum continuous measurement) of quantum
systems inevitably suffers from estimation delays. In this paper we give a
delay-dependent stability criterion for a wide class of nonlinear stochastic
systems including quantum spin systems. We utilize a semi-algebraic problem
approach to incorporate the structure of density matrices. To show the
effectiveness of the result, we derive a globally stabilizing control law for a
quantum spin-1/2 systems in the face of feedback delays.
