Experimental single-photon exchange along a space link of 7000 km

Extending the single-photon transmission distance is a basic requirement for the implementation of quantum communication on a global scale. In this work we report the single-photon exchange from a medium Earth orbit satellite (MEO) at more than $7000\phantom{\rule{0.28em}{0ex}}\text{km}$ of slant distance to the ground station at the Matera Laser Ranging Observatory. The single-photon transmitter was realized by exploiting the corner cube retroreflectors mounted on the LAGEOS-2 satellite. Long duration of data collection is possible with such altitude, up to 43 min in a single passage. The mean number of photons per pulse (${\ensuremath{\mu}}_{\mathrm{sat}}$) has been limited to 1 for 200 s, resulting in an average detection rate of $3.0\phantom{\rule{0.28em}{0ex}}\text{counts}/\text{s}$ and a signal-to-noise ratio of 1.5. The feasibility of single-photon exchange from MEO satellites paves the way to tests of quantum mechanics in moving frames and to global quantum Information.