Quantum communications between Earth and Space

In this society people are always connected, and everyday they manage a lot of personal data also risking to be eavesdropped. Quantum science is one the most promising field of the next years, from quantum computing to quantum communications and above all quantum cryptography. Quantum cryptography is the first commercial application of quantum physics and moreover it results one of the most reliable solution for security problem. Using Quantum Physic law's it is possible to establish secure communications between two users, guaranteeing unconditionally security in the transmission of data. Unfortunately due to the intrinsic losses inside optical fibers, it is not possible to establish a quantum link over $300$ km until quantum repeater will be achievable. The natural extension of terrestrial quantum links are space communications, where however the problems due to environment, temperature and pressure are totally new for quantum devices. The study investigated the possibility of sending quantum signals through atmosphere, in particular trying to realize quantum communications between Earth and Space. In this perspective we used Laser Ranging corner-cubes mounted into satellites to recreate a space quantum link. It was possible to prove that even with high losses, variable attenuation, and high backgorund a quantum key distribution system works, and an unconditionally secure key, needful for encryption, can be generated. With this experiments we demonstrate that not only free-space quantum key distribution is a ready technology, but also that quantum satellite communications is nowadays possible and realizable. Moreover these results open the way to look towards a global space quantum network, where optical station (OGS) could talk with satellite and vice-versa. This work was supported by the Strategic Project QUANTUMFUTURE of University of Padova, by ESAGNSS program and realized in Luxor laboratories in Padova. The principal tests were made at Telespazio (Matera) using the Matera Laser Ranging Observatory and into Thales Alenia Space (Torino)