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 QKD—a 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.
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We thank many colleagues at the National Space Science Center, especially B.-M. Xu, J. Li, J.-C. Gong, B. Chen and J. Liu for their management and coordination. We thank X.-F. Ma, C. Jiang, L. Li, X.-M. Zhang and Y.-W. Chen for discussions. This work was supported by the Strategic Priority Research Program on Space Science, Chinese Academy of Sciences, and the National Natural Science Foundation of China.
The authors declare no competing financial interests.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
The x axis is the shortest satellite-to-station distance, which occurs at the highest elevation angle and varies for different days. The y axis is the average sifted key rate that is obtained over the 273-s orbit. The inset shows the quantum bit error rate.
a, A full view of the Micius satellite before being assembled into the rocket. b, The experimental control box. c, The APT control box. d, The optical transmitter. e, Left side view of the optical transmitter optics head. f, Top side view of the optical transmitter optics head.
a, The two-axis gimbal telescope. b, Beacon laser and coarse camera. c, One of the two layers of the optical receiver box.
DM1: dichroic mirror transmitting 671-nm light and reflecting 850-nm light. DM2: transmitting 532-nm light; reflecting 671-nm light. DM3: transmitting 532-nm light; reflecting 850-nm light.
Extended Data Figure 5 A typical temporal distribution of 850-nm photons and the measured far-field pattern.
a, A typical temporal distribution of 850-nm photons after the time synchronization process. The data measurement time is 1 s. Each time bin is 100 ps. The counts are normalized and a variance of δ = 0.5 ns is obtained with Gaussian fitting. b, Far-field pattern measured from the thermal-vacuum test on the ground. The divergence angles (full angle at 1/e2 maximum) are 8 μrad for the X axis and 11 μrad for the Y axis. c, Far-field pattern measured from the satellite-to-ground scanning test. The divergence angles (full angle at 1/e2 maximum) are 9 μrad for the X axis and 11 μrad for the Y axis.
a, Instruction and data processes. b, Tracking and QKD processes during an orbit.
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Liao, S., Cai, W., Liu, W. et al. Satellite-to-ground quantum key distribution. Nature 549, 43–47 (2017). https://doi.org/10.1038/nature23655
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