An integrated space-to-ground quantum communication network over 4,600 kilometres

Quantum key distribution (QKD)^1,2 has the potential to enable secure communication and information transfer³. In the laboratory, the feasibility of point-to-point QKD is evident from the early proof-of-concept demonstration in the laboratory over 32 centimetres⁴; this distance was later extended to the 100-kilometre scale^5,6 with decoy-state QKD and more recently to the 500-kilometre scale^7-10 with measurement-device-independent QKD. Several small-scale QKD networks have also been tested outside the laboratory^11-14. However, a global QKD network requires a practically (not just theoretically) secure and reliable QKD network that can be used by a large number of users distributed over a wide area¹⁵. Quantum repeaters^16,17 could in principle provide a viable option for such a global network, but they cannot be deployed using current technology¹⁸. Here we demonstrate an integrated space-to-ground quantum communication network that combines a large-scale fibre network of more than 700 fibre QKD links and two high-speed satellite-to-ground free-space QKD links. Using a trusted relay structure, the fibre network on the ground covers more than 2,000 kilometres, provides practical security against the imperfections of realistic devices, and maintains long-term reliability and stability. The satellite-to-ground QKD achieves an average secret-key rate of 47.8 kilobits per second for a typical satellite pass-more than 40 times higher than achieved previously. Moreover, its channel loss is comparable to that between a geostationary satellite and the ground, making the construction of more versatile and ultralong quantum links via geosynchronous satellites feasible. Finally, by integrating the fibre and free-space QKD links, the QKD network is extended to a remote node more than 2,600 kilometres away, enabling any user in the network to communicate with any other, up to a total distance of 4,600 kilometres.