Abstract

In recent experiments¹ the phenomenon of two-photon interference has been studied, and apparent nonlocal interference effects have been convincingly demonstrated.² In all these experiments interference effects are not manifest in the singles count rates but appear in the coincidence rate or rate of detection of pair events and are a result of entanglement of the pair photon wavefunction. Modulation of the coincidence rate provides the basis of various covert communications schemes. We have previously demonstrated a free-space propagation communication system using a classical time shift coding out to ranges of 200 m using light levels 1/1000th of ambient daylight levels. This has been extended to a fiber (200-m) based system that exploits nonlocal interference using a scheme first proposed by Franson? With high visibility interference effects single photons can be used to code individual bits of information leading to extremely secure communication.

© 1991 Optical Society of America