The State of the Art in Satellite Quantum Key Distribution: Protocols, Technologies, and Physical Limits

This review presents the state of the art in quantum key distribution (QKD) for satellite‐to‐ground free‐space optical links. It first surveys continuous‐variable, discrete‐variable, entanglement‐based, measurement‐device‐independent, and twin‐field protocols, and discusses their feasibility under the losses and time‐varying conditions of orbital passes. It also distinguishes protocol security from practical implementation security. The paper then reviews enabling technologies, including single‐photon sources and detectors, followed by atmospheric impairments, time‐evolving turbulence, aperture averaging, and adaptive optics. Finally, physical channel limits and the trust assumptions of satellite‐network architectures are discussed. Overall, the literature indicates that discrete‐variable decoy‐state BB84 in LEO downlink configurations is the most mature near‐term SatQKD option, including for daylight operation when appropriate filtering and pointing/turbulence mitigation are employed.