  The measurement of quantum signals that traveled through long distances is of
fundamental and technological interest. We present quantum-limited coherent
measurements of optical signals, sent from a satellite in geostationary Earth
orbit to an optical ground station. We bound the excess noise that the quantum
states could have acquired after having propagated 38600 km through Earth's
gravitational potential as well as its turbulent atmosphere. Our results
indicate that quantum communication is feasible in principle in such a
scenario, highlighting the possibility of a global quantum key distribution
network for secure communication.
