Nature | www.nature.com | 1 Article Entanglement-based secure quantum cryptography over 1,120 kilometres Juan Yin 1,2,3 , Yu-Huai Li 1,2,3 , Sheng-Kai Liao 1,2,3 , Meng Yang 1,2,3 , Yuan Cao 1,2,3 , Liang Zhang 2,3,4 , Ji-Gang Ren 1,2,3 , Wen-Qi Cai 1,2,3 , Wei-Yue Liu 1,2,3 , Shuang-Lin Li 1,2,3 , Rong Shu 2,3,4 , Yong-Mei Huang 5 , Lei Deng 6 , Li Li 1,2,3 , Qiang Zhang 1,2,3 , Nai-Le Liu 1,2,3 , Yu-Ao Chen 1,2,3 , Chao-Yang Lu 1,2,3 , Xiang-Bin Wang 2 , Feihu Xu 1,2,3 , Jian-Yu Wang 2,3,4 , Cheng-Zhi Peng 1,2,3✉ , Artur K. Ekert 7,8 & Jian-Wei Pan 1,2,3✉ 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. Our experimental arrangement is shown in Fig. 1. The two receiving ground stations are located at Delingha (37°22′ 44.43′′ N, 97°43′ 37.01′′ E; altitude 3,153 m) in Qinghai province, and Nanshan (43°28′ 31.66′′ N, 87°10′ 36.07′′ E; altitude 2,028 m) in Xinjiang province, China. The physi- cal distance between Delingha and Nanshan is 1,120 km. To optimize the receiving efficiencies, both the two ground telescopes are newly built with a diameter of 1.2 m, specifically designed for the entanglement distribution experiments. All the optical elements, such as mirrors, in the telescopes maintain polarization. The satellite is equipped with a compact spaceborne entangled pho- ton source with a weight of 23.8 kg. A periodically poled KTiOPO 4 crys- tal inside a Sagnac interferometer is pumped in both the clockwise and anticlockwise directions simultaneously by a continuous-wave laser with a wavelength centred at 405 nm and a linewidth of 160 MHz, and generates down-converted polarization-entangled photon pairs at 810 nm close to the form of Ψ H V V H | ⟩ = (| ⟩| ⟩ +| ⟩| ⟩ )/ 2 12 1 2 1 2 , where |H⟩ and |V⟩ denote the horizontal and vertical polarization states, respec- tively, and the subscripts 1 and 2 denote the two output spatial modes. The entangled photon pairs are then collected and guided by two single-mode fibres to two independent transmitters equipped in the satellite. Both transmitters have a near-diffraction-limited far-field divergence of about 10 μrad. Under a pump power of 30 mW, the source distributes up to 5.9 × 10 6 entangled photon pairs per second. The photons are collected by the telescopes on two optical ground stations. For each one, the follow-up optics is installed on one of the rotating arms and rotates along with the telescope. As shown in Fig. 1c, https://doi.org/10.1038/s41586-020-2401-y Received: 15 July 2019 Accepted: 13 May 2020 Published online: xx xx xxxx Check for updates 1 Hefei National Laboratory for Physical Sciences at the Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, China. 2 Shanghai Branch, CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Shanghai, China. 3 Shanghai Research Center for Quantum Science, Shanghai, China. 4 Key Laboratory of Space Active Opto-Electronic Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China. 5 The Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu, China. 6 Shanghai Engineering Center for Microsatellites, Shanghai, China. 7 Mathematical Institute, University of Oxford, Oxford, UK. 8 Centre for Quantum Technologies, National University of Singapore, Singapore, Singapore. ✉ e-mail: pcz@ustc.edu.cn; pan@ustc.edu.cn