Coherence in Spontaneous Radiation Processes

TLDR

The study treats a radiating gas as a single quantum‑mechanical system, showing that correlated energy levels and transitions between them produce coherent radiation and can excite super‑radiant states. The analysis focuses on a gas whose dimensions are smaller than the emitted wavelength. Calculated spontaneous radiation rates, natural line widths, recoil‑induced coherence effects in extended gases, and angular correlations of successive photons from a thermally equilibrated gas.

Abstract

By considering a radiating gas as a single quantum-mechanical system, energy levels corresponding to certain correlations between individual molecules are described. Spontaneous emission of radiation in a transition between two such levels leads to the emission of coherent radiation. The discussion is limited first to a gas of dimension small compared with a wavelength. Spontaneous radiation rates and natural line breadths are calculated. For a gas of large extent the effect of photon recoil momentum on coherence is calculated. The effect of a radiation pulse in exciting "super-radiant" states is discussed. The angular correlation between successive photons spontaneously emitted by a gas initially in thermal equilibrium is calculated.