Hi In cQED and the Jaynes-Cummings model, three parameters are usually introduced which describe the atom-cavity system: The radiative lifetime γ, the cavity linewidth κ and the pure dephasing rate 1/T2. However I am not that familiar with the latter parameter. Basically I guess it describes how fast the off-diagonal elements of the density matrix decay. But how do the coherences physically enter in the interaction between an atom and light? What is it that they do? Best, Niles.