Quantum interference arising from the superposition of states is striking evidence of the validity of\ud quantum mechanics, confirmed in many experiments and also exploited in applications. However, as for\ud any scientific theory, quantum mechanics is valid within the limits in which it has been experimentally\ud verified. In order to extend such limits, it is necessary to observe quantum interference in unexplored\ud conditions such as moving terminals at large distances in space. Here, we experimentally demonstrate\ud single photon interference at a ground station due to the coherent superposition of two temporal modes\ud reflected by a rapidly moving satellite a thousand kilometers away. The relative speed of the satellite\ud induces a varying modulation in the interference pattern. The measurement of the satellite distance in real\ud time by laser ranging allows us to precisely predict the instantaneous value of the interference phase. We\ud then observed the interference patterns with a visibility up to 67% with three different satellites and with a\ud path length up to 5000 km. Our results attest to the viability of photon temporal modes for fundamental\ud tests of physics and quantum communication in space

Satellite-based quantum communication is an invaluable resource for the realization of a quantum network at the global scale. In this regard, the use of satellites well beyond the low Earth orbits gives the advantage of long communication time with a ground station. However, high-orbit satellites pose a great technological challenge due to the high diffraction losses of the optical channel, and the experimental investigation of such quantum channels is still lacking. Here, we report on the first experimental exchange of single photons from Global Navigation Satellite System at a slant distance of 20000 kilometers, by exploiting the retroreflector array mounted on GLONASS satellites. We also observed the predicted temporal spread of the reflected pulses due to the geometrical shape of array. Finally, we estimated the requirements needed for an active source on a satellite, aiming towards quantum communication from GNSS with state-of-the-art technology.

Widespread adoption of Quantum Key Distribution (QKD) in current telecommunication networks will require the development of simple, low cost and stable systems. Current QKD implementations usually include separate sub-systems to implement auxiliary tasks such as temporal synchronization and polarization basis tracking. Here we present a QKD system with polarization encoding that performs synchronization, polarization compensation and QKD with the same optical setup without requiring any changes or any additional hardware. Polarization encoding is performed by a self-compensating Sagnac loop modulator which exhibits high stability and the lowest intrinsic QBER ever reported by an active polarization source fully implemented using only commercial offthe-shelf components. We tested our QKD system over a fiber-optic channel, tolerating up to 43 dB of total losses and representing an important step towards technologically mature QKD systems. * These authors contributed equally to this work.

The quantum wave-particle duality of light is confirmed at the unprecedented distance of low-Earth-orbit satellites.