Robust H_∞ estimation of uncertain linear quantum systems

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_∞ estimation for uncertain linear quantum systems. The estimation problem is solved by converting it to a suitably scaled H_∞ 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.