Micius quantum experiments in space
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Abstract
Quantum theory has been successfully validated in numerous laboratory\nexperiments. But would such a theory, which excellently describes the behavior\nof microscopic physical systems, and its predicted phenomena such as quantum\nentanglement, be still applicable on very large length scales? From a practical\nperspective, how can quantum key distribution -- where the security of\nestablishing secret keys between distant parties is ensured by the laws of\nquantum mechanics -- be made technologically useful on a global scale? Due to\nphoton loss in optical fibers and terrestrial free space, the achievable\ndistance using direct transmission of single photons has been limited to a few\nhundred kilometers. A promising route to testing quantum physics over long\ndistances and in the relativistic regimes, and thus realizing flexible\nglobal-scale quantum networks is via the use of satellites and space-based\ntechnologies, where a significant advantage is that the photon loss and\nturbulence predominantly occurs in the lower ~ 10 km of the atmosphere, and\nmost of the photons' transmission path in the space is virtually in vacuum with\nalmost zero absorption and decoherence. In this Article, we review the progress\nin free-space quantum experiments, with a focus on the fast-developing Micius\nsatellite-based quantum communications. The perspective of space-ground\nintegrated quantum networks and fundamental quantum optics experiments in space\nconceivable with satellites are discussed.\n