Light quanta are the fundamental units of radiant energy. When propagating freely they travel at the fastest attainable speed and live forever. These properties recommend them as the ideal messengers for communication of all sorts. Ordinary light sources generate quanta in such an overwhelming abundance however, and in such random states, that we ordinarily lose sight of their ultimate separability and individual behavior.

The realization of this separability is about 100 yers old. Its apparent contradiction of the well-established wave picture of light led to a succession of theoretical dilemmas that could only be resolved by the fully developed quantum field theory of electrodynamics, some years later. Those years have also seen the development of a succession of experimental techniques for generating individual light quanta or small numbers of them in controllable states and investigating their properties.