Photon-photon gate via the interaction between two collective Rydberg   excitations
  We propose a scheme for a deterministic controlled-phase gate between two photons based on the strong interaction between two stationary collective Rydberg excitations in an atomic ensemble. The distance-dependent character of the interaction causes both a momentum displacement of the collective excitations and unwanted entanglement between them. We show that these effects can be overcome by swapping the collective excitations in space and by optimizing the geometry, resulting in a photon-photon gate with high fidelity and efficiency. 
Mohammadsadegh Khazali, Khabat Heshami, Christoph Simon
quant-ph