Micius quantum experiments in space

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 similar to 10 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.

Automated summary

Quantum theory has been validated in laboratory experiments, but its applicability on large length scales is still uncertain. Using satellites and space-based technologies, quantum experiments can be conducted over long distances and in relativistic regimes, enabling the development of flexible global-scale quantum networks.