  Quantum theory has been successfully validated in numerous laboratory
experiments. But would such a theory, which excellently describes the behavior
of microscopic physical systems, and its predicted phenomena such as quantum
entanglement, be still applicable on very 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? Due 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 ~ 10 km of the atmosphere, and
most of the photons' transmission path in the space is virtually in vacuum with
almost zero absorption and decoherence. In this Article, we review the progress
in free-space quantum experiments, with a focus on the fast-developing Micius
satellite-based quantum communications. The perspective of space-ground
integrated quantum networks and fundamental quantum optics experiments in space
conceivable with satellites are discussed.
