Implementation of a Direct Coupling Coherent Quantum Observer including   Observer Measurements
  This paper considers the problem of constructing a direct coupling quantum observer for a quantum harmonic oscillator system. The proposed observer is shown to be able to estimate one but not both of the plant variables and produces a measureable output using homodyne detection. 
Ian R. Petersen and Elanor H. Huntington
quant-ph cs.SY math.OC