https://www.sciencedaily.com/releases/2020/01/200124124523.htm

'Entanglement' describes a very particular type of quantum state which is not attributed to a single particle alone, but which is shared between two different particles. It irrevocably links their subsequent fates together -- no matter how far apart they are -- which famously led Albert Einstein to call the phenomenon as "spooky action at a distance." Entanglement has become a cornerstone of new technologies based on effects at the quantum level and is distribution over long distances a central goal in quantum communication. Now LMU researchers led by physicist Harald Weinfurter, in collaboration with a team at the University of the Saarland in Saarbrücken, have shown that the entangled state of an atom and a photon can be transmitted via an optic fiber (like those used in telecommunications networks) over a distance of up to 20 km. The previous record was 700 meters. "The experiment represents a milestone, insofar as the distance covered confirms that quantum information can be distributed on a large scale with little loss," says Weinfurter. "Our work therefore constitutes a crucial step toward the future realization of quantum networks."

Quantum networks essentially consist of quantum memories (made up of one or more atoms, for example) that act as nodes, and communication channels in which photons (light quanta) can propagate to link the nodes together. In their experiment, the researchers entangled a rubidium atom with a photon, and were able to detect the entangled state -- which now shares the quantum properties of both particles -- after its passage through a 20-km coil of optic fiber.