CHARACTERIZATION OF QUANTUM-NONDEMOLITION MEASUREMENTS IN OPTICS

We present a review of the criteria developped to characterize the efficiency of quantum non-demolition measurement devices for travelling electromagnetic waves. These criteria describe respectively the quality of the measurement, the non-destruction of the signal, and the quantum correlations between the meter read-out and the signal output. Quantitative limits can be defined with respect to these criteria, delimiting classical and quantum domains of operation. Moreover, some questions related to the gain on the signal channel, and to repeated QND measurements, will be also discussed.