A Reduced Order Direct Coupling Coherent Quantum Observer for a Complex   Quantum Plant
  This paper extends previous results on constructing a direct coupling quantum observer for a quantum harmonic oscillator system. In this case, we consider a complex linear quantum system plant consisting of a network of quantum harmonic oscillators. Conditions are given for which there exists a direct coupling observer which estimates a collection of variables in the quantum plant. It is shown that the order of the observer can be the same as the number of variables to be estimated when this number is even and thus this is a reduced order observer. 
Ian R. Petersen and Elanor H. Huntington
quant-ph cs.SY math.OC