(Phys.org)—Quantum key distribution (QKD) may one day allow for nearly perfectly secure data communication on a large scale,but before this can happen, QKD networks must extend across distances that are large enough to connect cities and even span continents. Currently, or QKD links are limited to approximately 400 km due to optical fiber losses,and these links can be extended using quantum repeaters, which are, and however,challenging to implement at the moment.
Source: phys.org