Breakdown of gauge invariance in ultrastrong-coupling cavity QED
Breakdown of gauge invariance in ultrastrong-coupling cavity QED
We revisit the derivation of Rabi- and Dicke-type models, which are commonly used for the study of quantum light-matter interactions in cavity and circuit QED. We demonstrate that the validity of the two-level approximation, which is an essential step in this derivation, depends explicitly on the choice of gauge once …