In a feat that demonstrates the feasibility of using satellites to transmit uncrackable quantum messages, scientists have measured the quantum properties of photons sent to space and back again.
Physicists beamed the blips of light up to a satellite that reflected them back to Earth. Upon the photons’ return, the team, led by Paolo Villoresi of the University of Padua in Italy, observed a property known as quantum interference.
Ref: Interference at the single photon level along satellite-ground channels. Physical Review Letters (24 May 2016)
Quantum interference arising from superposition of states is a striking evidence of the validity of Quantum Mechanics, confirmed in many experiments and also exploited in applications. However, as for any scientific theory, Quantum Mechanics is valid within the limits in which it has been experimentally verified. In order to extend such limits, it is necessary to observe quantum interference in unexplored conditions such as moving terminals at large distance in Space. Here we experimentally demonstrate single photon interference at a ground station due to the coherent superposition of two temporal modes reflected by a rapidly moving satellite thousand kilometers away. The relative speed of the satellite induces a varying modulation in the interference pattern. The measurement of the satellite distance in real time by laser ranging allowed us to precisely predict the instantaneous value of the interference phase. We then observed the interference patterns with visibility up to 67\% with three different satellites and with path length up to 5000 km. Our results attest the viability of photon temporal modes for fundamental tests of Physics and Quantum Communications in Space.