Nested Open Quantum Systems Approach to Photonic Bose$-$Einstein Condensation

The photonic Bose$-$Einstein condensate is a recently observed collective ground state of a coupled light-matter system. We describe this quantum state based on macroscopic quantum electrodynamics in dispersing and absorbing environments. To model the coupled photon$-$dye dynamics, we derive a master equation using a nested open quantum systems approach yielding all parameters essential to describe the condensation process. This approach allows us to describe photon condensates of arbitrary shapes because all geometry-dependent decay constants can be expressed in terms of the Green's tensor. In particular, we obtain the cavity mode absorption and emission rates of the dye molecules.