Beamshaping beacon light for satellite quantum key distribution: spatial separation and point ahead angle

Quantum Key Distribution (QKD) uses quantum states in an optical channel to grow a key with guaranteed security. Satellite QKD (SatQKD) is a proposed topology for QKD which provides unlimited terrestrial range for links and allows mobile access. SatQKD (and generally satellite optical communications) Optical Ground Stations (OGSs) require an optical beacon, a beam from OGS to satellite to assist pointing the satellite's telescope(s). Beaconing is challenging for SatQKD due to the single-photon sensitivity of the receiver to back-reflected beacon light and the need to point the outgoing beacon ahead of the downlink channel (the Point-Ahead Angle (PAA)). Hence, beacon light is often propagated through separate telescopes, increasing complexity. We describe a spatially separated beacon which reduces back-reflection errors and how its PAA is achieved. The far field beams of this and other beacon architectures are compared. Beaconing in this way can give greater up-time (due to less maintenance) and a lower cost barrier to development. A related system could have similar benefits for laser-guide-star telescopes.