Robust $H_\infty$ Coherent-Classical Estimation of Linear Quantum   Systems
  We study robust $H_\infty$ coherent-classical estimation for a class of physically realizable linear quantum systems with parameter uncertainties. Such a robust coherent-classical estimator, with or without coherent feedback, can yield better disturbance-to-error performance than the corresponding robust purely-classical estimator for an uncertain plant. Moreover, coherent feedback allows for such a robust coherent-classical estimator to be more robust to uncertainty in comparison to the robust classical-only estimator. 
Shibdas Roy and Ian R. Petersen
cs.SY math.OC quant-ph