Entanglement Assisted LPI and Covert Communications with SLM-based Beam Steering over Turbulent Optical Channels

In secure communications, the primary focus is on safeguarding the content of the message. In contrast, low probability of intercept (LPI) communications prioritize the privacy of the communication process itself. The most restrictive form of LPI communication, where an adversary attempts to disrupt the communication once its presence is detected, is known as covert communication. To hide the transmitted signal in solar radiation, we propose using the Gaussian quantum states generated by the degenerate parametric down-conversion process with signal photons being modulated with the phase-shift keying (PSK). To detect the transmitted signal, we propose transmitting the unmodulated idler photons (serving as the reference) together with modulated signal photons. To establish the link, we use the spatial light modulator (SLM)-based beam steering. We successfully demonstrate dual-pump-lasers-based degenerate entanglement-assisted (EA) LPI communication at 1 Gb/s over a 1.5 km terrestrial free-space optical (FSO) link. For the same link, the number of achievable EA covert bits for EA covert FSO communication is significantly higher compared to the corresponding classical covert communication scheme.