Quantum Theory of the Electromagnetic Field

Abstract

The development of improved and new measuring techniques (e.g. coincidence techniques) created new possibilities of detecting single photons in the visible region, thus allowing the measurement of, for example, the statistics of photons. This was in contrast to earlier investigations where the detecting devices were able to respond only to intense currents of photons. Thus the particle character of the electromagnetic field may now be studied directly. Under such circumstances it is obviously advantageous to describe the coherence phenomena in the field by the methods of quantum electrodynamics, where a field operator corresponds to the classical field and the quanta of the field correspond to the classical wave field. As is well-known this correspondence is realized by the second quantization of the field.