Long-distance free-space quantum key distribution in daylight towards inter-satellite communication Sheng-Kai Liao 1,2† , Hai-Lin Yong 1,2† , Chang Liu 1,2† , Guo-Liang Shentu 1,2† , Dong-Dong Li 1,2 , Jin Lin 1,2 , Hui Dai 1,2 , Shuang-Qiang Zhao 3 , Bo Li 1,2 , Jian-Yu Guan 1,2 , Wei Chen 1,2 , Yun-Hong Gong 1,2 , Yang Li 1,2 , Ze-Hong Lin 3 , Ge-Sheng Pan 1,2 , Jason S. Pelc 4 , M. M. Fejer 4 , Wen-Zhuo Zhang 1,2 , Wei-Yue Liu 3 , Juan Yin 1,2 , Ji-Gang Ren 1,2 , Hong Wang 2,5 , Qiang Zhang 1,2,5 * , Cheng-Zhi Peng 1,2 * and Jian-Wei Pan 1,2 * In the past, long-distance free-space quantum communication experiments could only be implemented at night. During the daytime, the bright background sunlight prohibits quantum communication in transmission under conditions of high channel loss over long distances. Here, by choosing a working wavelength of 1,550 nm and developing free-space single- mode fibre-coupling technology and ultralow-noise upconversion single-photon detectors, we have overcome the noise due to sunlight and demonstrate free-space quantum key distribution over 53 km during the day. The total channel loss is ∼48 dB, which is greater than the 40 dB channel loss between the satellite and ground and between low-Earth-orbit satellites. Our system thus demonstrates the feasibility of satellite-based quantum communication in daylight. Moreover, given that our working wavelength is located in the optical telecom band, our system is naturally compatible with ground fibre networks and thus represents an essential step towards a satellite-constellation-based global quantum network. S atellite-based quantum communication has proven to be a feas- ible way to achieve a global-scale quantum communication network 1–9 . Very recently, a low-Earth-orbit (LEO) satellite was launched 10 for this purpose. However, with a single satellite, an inefficient 3 day period 11 is required to provide worldwide con- nectivity. On the other hand, similar to how the Iridium system 12 functions in classical communication, a satellite constellation (SC) composed of many quantum satellites could provide global real-time quantum communication. Such an SC is expected to operate with LEO satellites or high-Earth-orbit satellites such as geosynchronous orbit (GEO) satellites. The probability of a satellite being in the Earth shadow zone decreases rapidly with increasing orbit height (Fig. 1). A LEO satellite system has a probability of ∼70% of being in the sunlight zone, whereas for a GEO satellite this rises to ∼99% (ref. 13). Meanwhile, the total channel loss between a LEO satellite and the Earth and between LEO satellites is typically ∼40–45 dB (refs 14,15). Therefore, to test the feasibility of an SC-based quantum network, quantum communication through a channel with at least ≥40 dB loss in daylight is essential. There have been several pioneering experiments on daylight quantum communication before our work 16–22 . Although the exper- iments were novel, the maximum loss calculated from them was only ∼20 dB. The main cause of the unsatisfactory performance was the strong background noise from the scattered sunlight, which was typically five orders of magnitude greater than the back- ground noise during the night 23 . We can reduce this noise in three ways: working wavelength selection, spectrum filtering and spatial filtering. Working wavelength selection We first switched the working wavelength to 1,550.14 nm from the 700–900 nm used in all previous experiments. The 1,550 nm wave- length is known to be an atmospheric window. In fact, the trans- mission efficiency is slightly higher at 1,550 nm than at 800 nm, as shown in Fig. 1a, and from the solar spectrum in Fig. 1b we can see that the sunlight intensity at 1,550 nm is around five times weaker than it is at 800 nm. Furthermore, the main type of scattering of solar noise for links between a satellite and Earth or between two satellites is Rayleigh scattering, the intensity of which is proportional to 1/λ 4 . Therefore, Rayleigh scattering at 1,550 nm is only 7% of its value at 800 nm. In total, the background noise with 1,550 nm light can be reduced to 3% of the background noise of 800 nm light. We measured the noise count rate of 1,550 nm light in the daylight case by pointing a telescope at the sky to simulate satellite-to-Earth communication. The result was smaller by a factor of 22.5 than for 850 nm light. Note that all existing free-space experiments without a satellite, including this work, have been implemented on Earth, and the direction of the free-space communication is parallel to the Earth rather than pointing at the sky. In this situation, Mie scat- tering, which does not follow the 1/λ 4 relation, will be the main noise source instead of Rayleigh scattering. Moreover, 1,550 nm is the telecom-band wavelength and is widely used for fibre-optical com- munication. Using the same wavelength for both free-space and fibre-optical communication is an optimal choice. Upconversion detectors and spectral filtering Despite the advantages of operating at 1,550 nm, researchers have been reluctant to use this wavelength due to a lack of good 1 Shanghai Branch, National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Shanghai 201315, China. 2 Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Shanghai 201315, China. 3 School of Information Science and Engineering, Ningbo University, Ningbo 315211, China. 4 Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA. 5 Jinan Institute of Quantum Technology, Shandong Academy of Information and Communication Technology, Jinan 250101, China. † These authors contributed equally to this work. *e-mail: qiangzh@ustc.edu.cn; pcz@ustc.edu.cn; pan@ustc.edu.cn ARTICLES PUBLISHED ONLINE: 24 JULY 2017 | DOI: 10.1038/NPHOTON.2017.116 NATURE PHOTONICS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturephotonics 1 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.