An experiment has been carried out in which two pairs of light beams produced by down conversion in two nonlinear crystals driven by a common pump were mixed by two beam splitters, and the coincidence rate for simultaneous detections of mixed signal and idler photons was measured. It is found that the down-converted light carries information about the phase of the pump field through the entanglement of the down-converted photons with the vacuum.

• Received 14 August 1989

DOI:https://doi.org/10.1103/PhysRevA.41.566