Link budget and background noise for satellite quantum key distribution

Optical quantum communication exploiting satellites is the most promising field to enable global quantum communication. Bonato et al. (2009) discussed the feasibility of quantum key distribution (QKD), for vertical space to Earth and Earth to space links at different time conditions and transmitter and receiver specifications, outlining a set of needed technical requirements. In a real scenario a satellite is seen at any time at a different position from an Earth station: we analyze the link attenuation and the signal to noise ratio (SNR) for its visibility time. In particular, we study the atmosphere effects on the beam propagation for different altitudes and zenith angles, modelling the dynamic quantum link for an orbiting satellite. Our results show the feasibility of QKD from low-Earth orbit satellites to Earth during night time; QKD uplinks are much more difficult to achieve, and their implementation is feasible reducing the field of view of the receiving telescope. Feasibility of QKD with satellite at higher altitude is a technological challenge due to the narrow portion of sky that an Earth station can effectively use for communication.