  Communicating information at the few photon level typically requires some
complexity in the transmitter or receiver in order to operate in the presence
of noise. This in turn incurs expense in the necessary spatial volume and power
consumption of the system. In this work we present a self-synchronised
free-space optical communications system based on simple, compact and low power
consumption semiconductor devices. A temporal encoding method, implemented
using a gallium nitride micro-LED source and a silicon single photon avalanche
photo-detector (SPAD) demonstrates data transmission at rates up to 100 kb/s
for 8.25 pW received power, corresponding to 27 photons per bit. Furthermore,
the signals can be decoded in the presence of both constant and modulated
background noise at levels significantly exceeding the signal power. The
systems low power consumption and modest electronics requirements are
demonstrated employing it as a communications channel between two
nano-satellite simulator systems.

Optics