Space communication for deep-space missions,inter-satellite data transfer and Earth monitoring requires high-speed data connectivity.The reach is fundamentally dictated by the available transmission power,the aperture size,and the receiver sensitivity.A transition from radio-frequency links to optical links is now seriously being considered,as this greatly reduces the channel loss caused by diffraction.A widely studied approach uses power-efficient formats along with nanowire-based photon-counting receivers cooled to a few Kelvins operating at speeds below 1 Gb/s.However,to achieve the multi-Gb/s data rates that will be required in the future,systems relying on pre-amplified receivers together with advanced signal generation and processing techniques from fibre communications are also considered.The sensitivity of such systems is largely determined by the noise figure(NF)of the pre-amplifier,which is theoretically 3 dB for almost all amplifiers.Phase-sensitive optical amplifiers(PSAs)with their uniquely low NF of 0 dB promise to provide the best possible sensitivity for Gb/s-rate long-haul free-space links.Here,we demonstrate a novel approach using a PSA-based receiver in a free-space transmission experiment with an unprecedented bit-error-free,black-box sensitivity of 1 photon-per-information-bit(PPB)at an information rate of 10.5 Gb/s.The system adopts a simple modulation format(quadrature-phase-shift keying,QPSK),standard digital signal processing for signal recovery and forward-error correction and is straightforwardly scalable to higher data rates.
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