Quantum theory has been successfully validated in numerous laboratory experiments. But would such a theory, which effectively describes the behavior of microscopic physical systems and its predicted phenomena such as quantum entanglement, still be applicable on large length scales? From a practical perspective, how can quantum key distribution (where the security of establishing secret keys between distant parties is ensured by the laws of quantum mechanics) be made technologically useful on a global scale? Owing to photon loss in optical fibers and terrestrial free space, the achievable distance using direct transmission of single photons has been limited to a few hundred kilometers. A promising route to testing quantum physics over long distances and in the relativistic regimes, and thus realizing flexible global-scale quantum networks, is via the use of satellites and space-based technologies, where a significant advantage is that the photon loss and turbulence predominantly occurs in the lower $\sim 10\mathrm{km}$ of the atmosphere, and most of the photons’ transmission path in space is virtually in vacuum, with almost zero absorption and decoherence. Progress in free-space quantum experiments, with a focus on the fast-developing Micius satellite–based quantum communications, is reviewed. The perspective of space-ground integrated quantum networks and fundamental quantum optics experiments in space conceivable with satellites are discussed.

中文翻译：

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墨子号太空量子实验

量子理论已在众多实验室实验中成功得到验证。但是，这种有效描述微观物理系统行为及其预测现象（例如量子纠缠）的理论是否仍然适用于大尺度？从实践的角度来看，如何才能使量子密钥分发（通过量子力学定律确保远方之间建立密钥的安全性）在全球范围内发挥技术作用？由于光纤和地面自由空间中的光子损耗，单光子直接传输可达到的距离仅限于几百公里。在长距离和相对论范围内测试量子物理，从而实现灵活的全球规模量子网络的一个有前途的途径是通过使用卫星和基于空间的技术，其中一个显着的优势是光子损失和湍流主要是发生在较低的$～10\mathrm{公里}$大气中的大多数光子在太空中的传输路径实际上是在真空中，吸收和退相干几乎为零。回顾了自由空间量子实验的进展，重点是快速发展的墨子号卫星量子通信。讨论了天地一体化量子网络的前景以及卫星可想象的太空基础量子光学实验。