  Satellite quantum communications have rapidly evolved in the past few years,
culminating in the proposal, development, and deployment of satellite missions
dedicated to quantum key distribution and the realization of fundamental tests
of quantum mechanics in space. However, in comparison with the more mature
technology based on fiber optics, several challenges are still open, such as
the capability of detecting, with high temporal accuracy, single photons coming
from orbiting terminals. Satellite laser ranging, commonly used to estimate
satellite distance, could also be exploited to overcome this challenge. For
example, high repetition rates and a low background noise can be obtained by
determining the time-of-flight of faint laser pulses that are retro-reflected
by geodynamics satellites and then detected on Earth at the single-photon
level. Here we report an experiment with regard to achieving a temporal
accuracy of approximately 230 ps in the detection of an optical signal of few
photons per pulse reflected by satellites in medium Earth orbit, at a distance
exceeding 7500 km, by using commercially available detectors. Lastly, the
performance of the Matera Laser Ranging Observatory is evaluated in terms of
the detection rate and the signal-to-noise ratio for satellite quantum
communications.
