  Noise fundamentally limits the capacity and reach in all communication links.
In optical space communications, noise primarily originates from the detection
process and limits the signal fidelity. . Therefore, the receiver sensitivity
plays a key role, dictating the minimum power needed to recover the information
transmitted. The widely explored approach of using the pulse-position
modulation format trades-off sensitivity against receiver bandwidth and thus
data-rate. Here we report on a novel, spectrally efficient, approach based on a
coherent receiver with a near-noiseless phase-sensitive pre-amplifier operating
at room temperature and demonstrate a sensitivity of one photon-per-bit of
incident power at a data rate of 10 Gb/s. The results provide a path to future
high-capacity inter-satellite and deep space, and other free-space
communication links
