Free-space optical technologies may be used as a means of ground, airborne and space-based digital communications and surveillance because of the cost-effectiveness, high-bandwidth and/or enhanced security associated with these technologies. Typically, commercially available optical communications systems may use near-infrared (NIR) lasers to provide a direct point-to-point, high-speed link through the atmosphere, over several kilometers, with a large available bandwidth. Optical communications based on non-linear and quantum optics have been proposed and developed to provide virtually unbreakable security during data transfer. Among these techniques are interferometric communications and retro-modulation communications. These techniques typically rely on photon flux modulation and/or second-order coherent phase modulation to transfer information.
With quantum cryptography or quantum key distribution, any attempt to intercept a data link would fails, thus ensuring a high degree of security. The challenge of this cryptographic technique, however, is that any practical implementation may require coincidence-count, post-processing with a separated and secondary data transmission through conventional channels, such as through physical wiring or another wireless network. While this technique may be suitable for fiber-optic communications, it may not be preferred in free-space communications, particularly communications in military systems.
Another state-of-the-art technique may use near-simultaneously generated quasi-entangled photons for communications to significantly increase immunity to background noise. Although this technique may use entangled coincident photon pairs over an identical optical path, it still requires two independent single photon detectors and a temporal coincidence gate to extract transmitted data.