Collective two-photon processes in the presence of a thermal electromagnetic field

A study is made of a lumped system of emitters (nuclei, atoms, molecules) with dipole-forbidden transitions between the first two energy levels in interaction with a thermal electromagnetic field. Elimination of the boson operators leads to an equation for the statistical operator of the system of emitters describing two-photon interaction with the electromagnetic field. A Fokker–Planck equation is also obtained and solved in the stationary case, and the result is used to investigate the equilibrium fluctuations in the populations of the atomic levels. The kinetics of the system and the statistical properties of the electromagnetic field are considered.