Detecting quantum phase transitions of photons through a defect cavity

The quantum phase transition of photons between the Mott-insulating and superfluid states can take place in a periodic array of nanocavities, where the photons are strongly coupled to a two-level system inside each cavity. Here we consider a defect cavity inside the array structure and examine the dynamics of the corresponding photons using the non-equilibrium formalism. The global signatures of the phase transition, such as the compressibility and order parameter, are revealed in the defect-cavity photon number. Furthermore, an open electron transport device (a transport qubit) can be embedded in the defect cavity for the detection of the quantum phase transition.