  In quantum engineering, faults may occur in a quantum control system, which
will cause the quantum control system unstable or deteriorate other relevant
performance of the system. This note presents an estimator-based fault-tolerant
control design approach for a class of linear quantum stochastic systems
subject to fault signals. In this approach, the fault signals and some
commutative components of the quantum system observables are estimated, and a
fault-tolerant controller is designed to compensate the effect of the fault
signals. Numerical procedures are developed for controller design and an
example is presented to demonstrate the proposed design approach.
