Coherent-Classical Estimation versus Purely-Classical Estimation for   Linear Quantum Systems
  We consider a coherent-classical estimation scheme for a class of linear quantum systems. It comprises an estimator that is a mixed quantum-classical system without involving coherent feedback. The estimator yields a classical estimate of a variable for the quantum plant. We demonstrate that for a passive plant that can be characterized by annihilation operators only, such coherent-classical estimation provides no improvement over purely-classical estimation. An example is also given which shows that if the plant is not assumed to be an annihilation operator only quantum system, it is possible to get better estimates with such coherent-classical estimation compared with purely-classical estimation. 
Shibdas Roy, Ian R. Petersen and Elanor H. Huntington
quant-ph cs.SY math.OC