Free-space optical communications for terrestrial and space-borne applications are alive and well thanks to steady advances in light sources and detection schemes that extend data rates and physical reach despite atmospheric challenges. Since the earliest demonstrations of free-space optical (FSO) communications for defense applications in the 1960s, steady improvements in light sources, detectors, and optical components continue to extend the reach for FSO networks, as well as increase the wish list for possible applications. For example, the same University of Padua (Padua, Italy) researchers who demonstrated free-space transmission of single photons over a distance of 1500 km in 2008 have, just two years later, proposed a space-based quantum-key-distribution (QKD) terminal for the International Space Station.
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