  Generalized Dicke models can be implemented in hybrid quantum systems built
from ensembles of nitrogen-vacancy (NV) centers in diamond coupled to
superconducting microwave cavities. By engineering cavity assisted Raman
transitions between two spin states of the NV defect, a fully tunable model for
collective light-matter interactions in the ultra-strong coupling limit can be
obtained. Our analysis of the resulting non-equilibrium phases for a single
cavity and for coupled cavity arrays shows that different superradiant phase
transitions can be observed using existing experimental technologies, even in
the presence of large inhomogeneous broadening of the spin ensemble. The phase
diagram of the Dicke lattice model displays distinct features induced by
dissipation, which can serve as a genuine experimental signature for phase
transitions in driven open quantum systems.
