  We study the phenomenon of controllable localization-delocalization
transition in a quantum many-body system composed of nitrogen-vacancy centers
coupled to photonic crystal cavities, through tuning the different detunings
and the relative amplitudes of two optical fields that drive two nondegenerate
transitions of the $\Lambda $-type configuration. We not only characterize how
dissipation affects the phase boundary using the mean-field quantum master
equation, but also provide the possibility of observing this photonic quantum
phase transition (QPT) by employing several experimentally observable
quantities, such as mean intracavity photon number, density correlation
function and emitted spectrum, exhibiting distinct optical signatures in
different quantum phases. Such a spin-cavity system opens new perspectives in
quantum simulation of condensed-matter and many-body physics in a
well-controllable way.
