Coherence and Photons

Abstract

After the previous extensive exploration into the wave character of light, the present chapter focuses on its particle properties and on the statistical properties of photons. In Sect. 2.1 concepts such as ``quasi-monochromatic'' and ``partially coherent'' light will be defined and exemplified by simple models for a laser and a classical light source. We shall familiarize ourselves with the fundamental experiments, beginning with the famous ``Hanbury Brown-Twiss experiment''. In Sect. 2.2 we shall try to find a pragmatic approach to the quantum mechanical description of photon states -- giving an introduction for ``pedestrians'' so to say. Finally, we shall in Sect. 2.3 apply the new tools to the theory of absorption and emission of light -- this time with explicit consideration of the quantum nature of photons. This will allow us for the first time to derive the basic formulas for spontaneous emission -- as opposed to the previous, hand waving introduction of this inherently quantum mechanical phenomenon.