Feynman diagrams for ultrastrong cavity QED
When an atom is ultrastrongly coupled to a cavity, i.e., when the light-matter coupling strength is comparable to the atomic transition frequency or the resonance frequency of the cavity mode, the ground state of the system, and also the excited states, contain contributions from virtual excitations. In this work, we show how these contributions can be visualized using Feynman diagrams. We also show how stimulated emission, using an auxiliary atomic level, can convert the virtual excitations into real photons (and back).
Feynman-diagrams approach to the quantum Rabi model for ultrastrong cavity QED: stimulated emission and reabsorption of virtual particles dressing a physical excitation
Omar Di Stefano, Roberto Stassi, Luigi Garziano, Anton Frisk Kockum, Salvatore Savasta, and Franco Nori
arXiv:1603.04984