Whispering at the shot noise limit: communication using single photons through photon-starved channels

The ability to transmit information from one point to another using as low received power as possible has a fundamental impact in many contexts, including covert communication, energy-efficient networks, and communication over channels with extreme loss. An immediate application is deep-space exploration, in which there has been significant interest in moving from microwaves to optical signals due to considerably lower diffraction loss and larger bandwidth. At a given data rate and transmit power, systems with the highest photon information efficiency (PIE), defined as the number of information bits extracted per received photon, can bridge the largest loss. We report here an experimental demonstration, using large-order pulse-position modulation and superconducting single-photon detectors, of a record PIE of 14.5 (17.8) bits per incident (detected) photon after 87.5dB of channel loss. This corresponds to 8.84 zeptojoules, or 0.069 photons, per bit at 1550nm. The record result was achieved using a new low-complexity transmitter design that produces short optical pulses on-demand with an average extinction ratio exceeding 90dB per time slot. To our knowledge, this is the most sensitive optical detection system ever demonstrated for high-path-loss optical communication.