  A hybrid quantum-classical filtering problem, where a qubit system is
disturbed by a classical stochastic process, is investigated. The strategy is
to model the classical disturbance by using an optical cavity. Relations
between classical disturbances and the cavity analog system are analyzed. The
dynamics of the enlarged quantum network system, which includes a qubit system
and a cavity system, are derived. A stochastic master equation for the
qubit-cavity hybrid system is given, based on which estimates for the state of
the cavity system and the classical signal are obtained. The quantum extended
Kalman filter is employed to achieve efficient computation. Numerical results
are presented to illustrate the effectiveness of our methods.
