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.

中文翻译：

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朝着星际通信的远距离自由空间量子密钥分配

过去，长距离自由空间量子通信实验只能在晚上进行。在白天，在远距离高信道损耗的情况下，明亮的背景太阳光会阻止传输中的量子通信。在这里，通过选择1,550 nm的工作波长并开发自由空间单模光纤耦合技术和超低噪声上转换单光子检测器，我们克服了阳光造成的噪声，并展示了超过53个自由空间量子密钥分布白天公里。总的信道损耗约为48 dB，大于卫星与地面之间以及低地球轨道卫星之间的40 dB信道损耗。我们的系统因此证明了在白天进行基于卫星的量子通信的可行性。而且，