Spontaneous superradiant photon current

This paper reports the spontaneous emergence of a photon current in a class of spin-cavity systems, where an assemble of quantum emitters interact with distinct photon modes confined in tunneling-coupled cavities. Specifically, with necessary symmetry breaking, photons in a superradiant phase afforded by coherent photon-emitter interaction spontaneously flow from a cavity with a lower resonance frequency to a different cavity with a higher resonance frequency. Theoretical analysis reveals that counterrotating terms in the system Hamiltonian and cavity dissipation are the key to alter spin-cavity coherence, which then makes it possible to extract photons from and later return photons to the surroundings through the cavities. The interplay between photon loss and emitter coherence hence sustains a counterintuitive steady current of photons between cavities without an external pumping field directly exciting the cavity modes.