A Direct Coupling Coherent Quantum Observer for a Single Qubit Finite   Level Quantum System
  This paper considers the problem of constructing a direct coupling quantum observer for a single qubit finite level quantum system plant. The proposed observer is a single mode linear quantum system which is shown to be able to estimate one of the plant variables in a time averaged sense. A numerical example and simulations are included to illustrate the properties of the observer. 
Ian R. Petersen
quant-ph cs.SY math.OC