  This work 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 cavity dissipation is the key to
alter spin-cavity coherence, which then makes it possible to extract photons
from, and later return photons to the vaccum through the cavities. The
interplay between photon loss and emitter coherence hence sustains a
counter-intuitive steady current of photons between cavities without an
external pumping field.
