Robust Filtering for Adaptive Homodyne Estimation of Continuously   Varying Optical Phase
  Recently, it has been demonstrated experimentally that adaptive estimation of a continuously varying optical phase provides superior accuracy in the phase estimate compared to static estimation. Here, we show that the mean-square error in the adaptive phase estimate may be further reduced for the stochastic noise process considered by using an optimal Kalman filter in the feedback loop. Further, the estimation process can be made robust to fluctuations in the underlying parameters of the noise process modulating the system phase to be estimated. This has been done using a guaranteed cost robust filter. 
Shibdas Roy, Ian R. Petersen, Elanor H. Huntington
quant-ph cs.SY math.OC