Provably secure and practical quantum key distribution over 307 km of optical fibre

A discrete-variable quantum key distribution system that is capable of distributing a provably-secure cryptographic key over 307 kilometres is demonstrated at a telecom wavelength. Proposed in 1984, quantum key distribution (QKD) allows two users to exchange provably secure keys via a potentially insecure quantum channel 1 . Since then, QKD has attracted much attention and significant progress has been made both in theory and practice 2 , 3 . On the application front, however, the operating distance of practical fibre-based QKD systems is limited to about 150 km (ref. 4 ), mainly due to the high background noise of practical single-photon detectors 5 , 6 and inefficient finite-key security analysis 7 , 8 , 9 . Here, we present, for the first time, a compact and autonomous QKD system that is capable of distributing provably secure cryptographic keys over 307 km of optical fibre. This is achieved by using semiconductor single-photon detectors with record low background noise 10 and a novel finite-key security analysis, which is efficient even for short key lengths. This demonstrates the feasibility of practical long-distance QKD based on standard fibre-optic telecom components.