MEASUREMENT OF THE QUANTUM PHASE BY PHOTON-COUNTING

We address the problem of identifying the dynamical variables representing the phase of a quantum field by analyzing what is measured in two simple homodyne experiments. This leads us to identify operators C(M),S(M) corresponding to the measured cosine and sine of the phase difference. The predictions of the theory are tested in an experiment in which the input fields are derived from a highly stable He:Ne laser. Photon-counting measurements extending from mean photon numbers 30 down to 10(-2) show good agreement between theory and experiment. The implications for identifying the "correct" phase operator are briefly discussed.