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Entanglement-based secure quantum cryptography over 1,120 kilometres

Abstract

Quantum key distribution (QKD)1,2,3 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 long4,5,6,7. In the field, point-to-point QKD has been achieved from a satellite to a ground station up to 1,200 kilometres away8,9,10. However, real-world QKD-based cryptography targets physically separated users on the Earth, for which the maximum distance has been about 100 kilometres11,12. The use of trusted relays can extend these distances from across a typical metropolitan area13,14,15,16 to intercity17 and even intercontinental distances18. However, relays pose security risks, which can be avoided by using entanglement-based QKD, which has inherent source-independent security19,20. Long-distance entanglement distribution can be realized using quantum repeaters21, but the related technology is still immature for practical implementations22. 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 efficient23 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 channels24,25. Our method not only increases the secure distance on the ground tenfold but also increases the practical security of QKD to an unprecedented level.

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Fig. 1: Overview of the experimental set-up of entanglement based quantum key distribution.
Fig. 2: Distances and attenuations from satellite to Nanshan (Delingha).
Fig. 3: Monitoring and filtering against side channels.

Data availability

The data that support the findings of this study are available from the corresponding authors on reasonable request.

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Acknowledgements

We acknowledge discussions with X. Ma and C. Jiang. We thank colleagues at the National Space Science Center, China Xi’an Satellite Control Center, National Astronomical Observatories, Xinjiang Astronomical Observatory, Purple Mountain Observatory, and Qinghai Station for their management and coordination. We thank G.-B. Li, L.-L. Ma, Z. Wang, Y. Jiang, H.-B. Li, S.-J. Xu, Y.-Y. Yin, W.-C. Sun and Y. Wang for their long-term assistance in observation. This work was supported by the National Key R&D Program of China (grant number 2017YFA0303900), the Shanghai Municipal Science and Technology Major Project (grant number 2019SHZDZX01), the Anhui Initiative in Quantum Information Technologies, Science and Technological Fund of Anhui Province for Outstanding Youth (grant number 1808085J18) and the National Natural Science Foundation of China (grant numbers U1738201, 61625503, 11822409, 11674309, 11654005 and 61771443).

Author information

Authors and Affiliations

Authors

Contributions

C.-Z.P., A.K.E. and J.-W.P. conceived the research. J.Y., C.-Z.P. and J.-W.P. designed the experiments. J.Y., Y.-H.L., S.-K.L., M.Y., Y.C., J.-G.R., S.-L.L., C.-Z.P. and J.-W.P. developed the follow-up optics and monitoring circuit. J.Y., Y.-M.H., C.-Z.P. and J.-W.P. developed the efficiency telescopes. J.Y., S.-K.L., Y.C., L.Z., W.-Q.C., R.S., L.D., J.-Y.W., C.-Z.P. and J.-W.P. designed and developed the satellite and payloads. J.Y., L.Z., W.-Q.C., W.-Y.L. and C.-Z.P. developed the software. F.X., X.-B.W., A.K.E. and J.-W.P. performed the security proof and analysis. L.L., Q.Z., N.-L.L., Y.-A.C., X.-B.W., F.X., C.-Z.P., A.K.E. and J.-W.P. contributed to the theoretical study and implementation against device imperfections. F.X., C.-Y.L., C.-Z.P. and J.-W.P. analysed the data and wrote the manuscript, with input from J.Y., Y.-H.L., M.Y., Y.C. and A.K.E. All authors contributed to the data collection, discussed the results and reviewed the manuscript. J.-W.P. supervised the whole project.

Corresponding authors

Correspondence to Cheng-Zhi Peng or Jian-Wei Pan.

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Extended data figures and tables

Extended Data Fig. 1 Satellite-to-Delingha link efficiencies under different weather conditions.

a, The data in previous work23 was taken in different orbits during the period of 7 December 2016 to 22 December 2016. b, The data in current work was taken in different orbits during the period of 6 September 2018 to 22 October 2018. Here the change of link efficiencies on different days was caused by the weather conditions.

Extended Data Fig. 2 Multiple orbits of satellite-to-Delingha link efficiencies under good weather conditions.

Stable and high collection efficiencies were observed during the period of October 2018 to April 2019.

Extended Data Fig. 3 The comparison of satellite-to-Delingha link efficiency under the best-orbit condition.

a, After improving the link efficiency with high-efficiency telescopes and follow-up optics, on average, the current work shows a 3-dB enhancement in the collection efficiency over that of ref. 23. The lines are linear fits to the data. b, Some representative values.

Extended Data Fig. 4 The finite-key secret key rate R versus the QBER.

For the 3,100 s of data collected in our experiment, a QBER of below about 6.0% is required to produce a positive key. The previous work23 demonstrated a QBER of 8.1%, which is not sufficient to generate a secret key. In this work, a QBER of 4.5% and a secret key rate of 0.12 bits per second are demonstrated over 1,120 km. If one ignores the important finite-key effect, the QBER in ref. 23 is slightly lower than the well known asymptotic limit of 11% (ref. 43).

Extended Data Fig. 5 Schematics of the detection and blinding-attack monitoring circuit.

The biased voltage (HV) is applied to an avalanche photodiode through a passive quenching resistance (Rq = 500 kΩ) and a sampling resistance (Rs = 10 kΩ). The avalanche signals are read out as click or no-click events through a signal-discrimination circuit. The blinding signal monitor is shown in the dot-dash diagram. A resistor-capacitor filter and a voltage follower are used to smooth and minimize the impact on the signals. The outputs of an analogue to digital converter (ADC), at a sampling rate of 250 kHz, are registered by computer data acquisition (PC-DAQ). R1, resistor; C1, capacitor; OA, operational amplifier.

Extended Data Fig. 6

The transmission of the beam splitter within the selected bandwidth of wavelength.

Extended Data Table 1 Parameters of the system detection efficiencies
Extended Data Table 2 Comparison of the results between this work and the earlier experiment23
Extended Data Table 3 Typical quantum attacks and our countermeasures
Extended Data Table 4 Measured correlation coefficients required for the CHSH inequality

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Yin, J., Li, YH., Liao, SK. et al. Entanglement-based secure quantum cryptography over 1,120 kilometres. Nature 582, 501–505 (2020). https://doi.org/10.1038/s41586-020-2401-y

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