Atoms and Photons: Kinetic Equations with Delay

In his recent works, the author drew attention to the fact that the extraction of a part of a closed Hamiltonian system turns the original well-known Liouville differential equation into an integro-differential equation with a delayed time argument, which describes the dynamics of the selected subsystem as an open system. It was shown that the integral operator can be represented in the form of a fractional differential operator of distributed order. In this paper, we show how the kinetic theory of the system “atoms+photons” is transformed for the subsystem consisting of excited atoms. We present the derivation of the telegraph equation with delay, derive the Biberman–Holstein equation in the fractional differential form (with the fractional Laplace operator), and discuss boundary effects in the nonlocal transport model. The final section is devoted to laser technologies, which include free-electron lasers and laser cooling of atoms. © 2022, Springer Science+Business Media, LLC, part of Springer Nature.