  Deep space explorations require transferring huge amounts of data quickly
from very distant targets. Laser communication is a promising technology that
can offer a data rate of magnitude faster than conventional microwave
communication due to the fundamentally narrow divergence of light. This study
demonstrated a photon-sensitive receiver prototype with over Gigabit data rate,
immunity to strong background photon noise, and simultaneous tracking ability.
The advantages are inherited from a joint-optimized superconducting nanowire
single-photon detector (SNSPD) array, designed into a four-quadrant structure
with each quadrant capable of resolving six photons. Installed in a free-space
coupled and low-vibration cryostat, the system detection efficiency reached
72.7%, the detector efficiency was 97.5%, and the total photon counting rate
was 1.6 Gcps. Additionally, communication performance was tested for pulse
position modulation (PPM) format. A series of signal processing methods were
introduced to maximize the performance of the forward error correction (FEC)
code. Consequently, the receiver exhibits a faster data rate and better
sensitivity by about twofold (1.76 photons/bit at 800 Mbps and 3.40 photons/bit
at 1.2 Gbps) compared to previously reported results (3.18 photon/bit at 622
Mbps for the Lunar Laser Communication Demonstration). Furthermore,
communications in strong background noise and with simultaneous tracking
ability were demonstrated aimed at the challenges of daylight operation and
accurate tracking of dim beacon light in deep space scenarios.
