Probing the two-photon phase coherence of parametrically down-converted photons by a local oscillator

A degenerate beam of down-converted photon pairs from a nonlinear crystal pumped by pulsed light was mixed with coherent local oscillator pulses by a beam splitter, and the coincidence rate for detections of photons at the two output beams was measured. The coincidence rate showed interference fringes as the phase difference between the pump light and the local oscillator was varied. It showed a negative correlation in some range of the phase difference.