Optical communication channel based on coincident photon pairs.

Photons emitted in the process of spontaneous parametric downconversion of pump light in a nonlinear crystal are made the basis of a new optical communication scheme, which depends on the fact that two downconverted photons are always produced simultaneously. If the pump light is modulated and the downconverted photons are transmitted in two separate beams which fall on two separate detectors, registering their simultaneous arrival allows great discrimination against background and noise to be achieved. Communication is possible with relatively few photons and even when the signal is ordinarily lost in the noise, so that the communication channel can be made virtually interception proof. The feasibility of the scheme is demonstrated by a pilot experiment.