  Single-photon detectors (SPDs) play important roles in highly sensitive
detection applications, such as fluorescence spectroscopy, remote sensing and
ranging, deep space optical communications, elementary particle detection, and
quantum communications. However, the adverse conditions in space, such as the
increased radiation flux and thermal vacuum, severely limit their noise
performances, reliability, and lifetime. Herein, we present the first example
of spaceborne, low-noise, high reliability SPDs, based on commercial
off-the-shelf (COTS) silicon avalanche photodiodes (APD). Based on the high
noise-radiation sensitivity of silicon APD, we have developed special shielding
structures, multistage cooling technologies, and configurable driver
electronics that significantly improved the COTS APD reliability and mitigated
the SPD noise-radiation sensitivity. This led to a reduction of the expected
in-orbit radiation-induced dark count rate (DCR) from ~219 counts per second
(cps) per day to ~0.76 cps/day. During a continuous period of continuous
operations in orbit which spanned of 1029 days, the SPD DCR was maintained
below 1000 cps, i.e., the actual in-orbit radiation-induced DCR increment rate
was ~0.54 cps/day, i.e., two orders of magnitude lower than those evoked by
previous technologies, while its photon detection efficiency was > 45%. Our
spaceborne, low-noise SPDs established a feasible satellite-based up-link
quantum communication that was validated on the quantum experiment science
satellite platform. Moreover, our SPDs open new windows of opportunities for
space research and applications in deep-space optical communications,
single-photon laser ranging, as well as for testing the fundamental principles
of physics in space.
