  Recent rapid growth in the number of satellite-constellation programs for
remote sensing and communications, thanks to the availability of small-size and
low-cost satellites, provides impetus for high capacity laser communication
(lasercom) in space. Quantum communication can enhance the overall performance
of lasercom, and also enables intrinsically hack-proof secure communication
known as Quantum Key Distribution (QKD). Here, we report a quantum
communication experiment between a micro-satellite (48 kg and 50 cm cube) in a
low earth orbit and a ground station with single-photon counters.
Non-orthogonal polarization states were transmitted from the satellite at a
10-MHz repetition rate. On the ground, by post-processing the received quantum
states at an average of 0.14 photons/pulse, clock data recovery and
polarization reference-frame synchronization were successfully done even under
remarkable Doppler shifts. A quantum bit error rate below 5% was measured,
demonstrating the feasibility of quantum communication in a real scenario from
space.
