  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.
