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. 2017 Jul 24;8:15971. doi: 10.1038/ncomms15971

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Copyright © 2017, The Author(s)

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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(a) Average single-photon (blue line) and two-photon (red line) detection rate (100 ms integration time) after 1.2-km-long free-space transmission. The short-term signal fluctuated due to atmospheric turbulence, whereas the time-averaged rate of ∼20 kcps remained almost constant over several hours. (b) Relative clock drift between Alice and Bob. The inset depicts the normalized histogram of two-photon detection events in 80 ps time bins centred around the flight-time offset of ∼3.94 μs. All data acquired for night-time operation on 25–26 April 2016.