Photon Devil's staircase: photon long-range repulsive interaction in   lattices of coupled resonators with Rydberg atoms
  The realization of strong coherent interactions between individual photons is a long-standing goal in science and engineering. In this report, based on recent experimental setups, we derive a strong photon long-range repulsive interaction, by controlling the van der Waals repulsive force between Cesium Rydberg atoms located inside different cavities in extended Jaynes-Cummings-Hubbard Lattices. We also find novel quantum phases induced by this photon long-range repulsive interaction. For example, without photon hopping, a photon Devil's staircase, induced by the breaking of long-range translation symmetry, can emerge. If photon hopping occurs, we predict a photon-floating solid phase, due to the motion of particle-and hole-like defects. More importantly, for a large chemical potential in the resonant case, the photon hopping can be frozen even if the hopping term exists. We call this new phase the photon-frozen solid phase. In experiments, these predicted phases could be detedted by measuring the number of polaritons via resonance fluorescence. 
Yuanwei Zhang, Jingtao Fan, J.-Q. Liang, Jie Ma, Gang Chen, Suotang   Jia and Franco Nori
quant-ph cond-mat.quant-gas