  We consider classical estimators for a class of physically realizable linear
quantum systems. Optimal estimation using a complex Kalman filter for this
problem has been previously explored. Here, we study robust $H_\infty$
estimation for uncertain linear quantum systems. The estimation problem is
solved by converting it to a suitably scaled $H_\infty$ control problem. The
solution is obtained in the form of two algebraic Riccati equations. Relevant
examples involving dynamic squeezers are presented to illustrate the efficacy
of our method.
