Retrodiction as a tool for micromaser field measurements
  We use retrodictive quantum theory to describe cavity field measurements by successive atomic detections in the micromaser. We calculate the state of the micromaser cavity field prior to detection of sequences of atoms in either the excited or ground state, for atoms that are initially prepared in the excited state. This provides the POM elements, which describe such sequences of measurements. 
John Jeffers, Stephen M. Barnett and David T. Pegg