Photodetection

In his historic paper entitled "The Quantum Theory of Optical Coherence," Roy J. Glauber set a solid foundation for the quantum electronics/quantum optics enterprise. The experimental development of the optical maser and later laser at that time had made the classical concept of optical coherence inadequate. Glauber started from the quantum theory of light detection by considering the process of photoionization in which a photodetector is triggered by an ionizing absorption of a photon. In the quantum theory of radiation, the electric field operator in the Coulomb gauge may be written as the sum of positive and negative frequency parts