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Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. However, the distance over which QKD is achievable has been limited to a few hundred kilometres, owing to the channel loss that occurs when using optical fibres or terrestrial free space that exponentially reduces the photon transmission rate. Satellite-based QKD has the potential to help to establish a global-scale quantum network, owing to the negligible photon loss and decoherence experienced in empty space. Here we report the development and launch of a low-Earth-orbit satellite for implementing decoy-state QKDa form of QKD that uses weak coherent pulses at high channel loss and is secure because photon-number-splitting eavesdropping can be detected. We achieve a kilohertz key rate from the satellite to the ground over a distance of up to 1,200 kilometres. This key rate is around 20 orders of magnitudes greater than that expected using an optical fibre of the same length. The establishment of a reliable and efficient space-to-ground link for quantum-state transmission paves the way to global-scale quantum networks., Author(s): Sheng-Kai Liao [1, 2]; Wen-Qi Cai [1, 2]; Wei-Yue Liu [1, 2]; Liang Zhang [2, 3]; Yang Li [1, 2]; Ji-Gang Ren [1, 2]; Juan Yin [1, 2]; Qi [...]

Transferring an unknown quantum state over arbitrary distances is essential for large-scale quantum communication and distributed quantum networks. It can be achieved with the help of long-distance quantum teleportation (1,2) [...]

Quantum key distribution (QKD) 1-3 is a theoretically secure way of sharing secret keys between remote users. It has been demonstrated in a laboratory over a coiled optical fibre up to 404 kilometres long 4-7 . In the field, point-to-point QKD has been achieved from a satellite to a ground station up to 1,200 kilometres away 8-10 . However, real-world QKD-based cryptography targets physically separated users on the Earth, for which the maximum distance has been about 100 kilometres 11,12 . The use of trusted relays can extend these distances from across a typical metropolitan area 13-16 to intercity 17 and even intercontinental distances 18 . However, relays pose security risks, which can be avoided by using entanglement-based QKD, which has inherent source-independent security 19,20 . Long-distance entanglement distribution can be realized using quantum repeaters 21 , but the related technology is still immature for practical implementations 22 . The obvious alternative for extending the range of quantum communication without compromising its security is satellite-based QKD, but so far satellite-based entanglement distribution has not been efficient 23 enough to support QKD. Here we demonstrate entanglement-based QKD between two ground stations separated by 1,120 kilometres at a finite secret-key rate of 0.12 bits per second, without the need for trusted relays. Entangled photon pairs were distributed via two bidirectional downlinks from the Micius satellite to two ground observatories in Delingha and Nanshan in China. The development of a high-efficiency telescope and follow-up optics crucially improved the link efficiency. The generated keys are secure for realistic devices, because our ground receivers were carefully designed to guarantee fair sampling and immunity to all known side channels 24,25 . Our method not only increases the secure distance on the ground tenfold but also increases the practical security of QKD to an unprecedented level.