  This paper considers the application of integral Linear Quadratic Gaussian
(LQG) optimal control theory to a problem of cavity locking in quantum optics.
The cavity locking problem involves controlling the error between the laser
frequency and the resonant frequency of the cavity. A model for the cavity
system, which comprises a piezo-electric actuator and an optical cavity is
experimentally determined using a subspace identification method. An LQG
controller which includes integral action is synthesized to stabilize the
frequency of the cavity to the laser frequency and to reject low frequency
noise. The controller is successfully implemented in the laboratory using a
dSpace DSP board.
