Nonlinear quantum optics mediated by Rydberg interactions
  By mapping the strong interaction between Rydberg excitations in ultra-cold atomic ensembles onto single photons via electromagnetically induced transparency, it is now possible to realize a nonlinear optical medium which exhibits a strong optical nonlinearity at the level of individual photons. We review the theoretical concepts and the experimental state-of-the-art of this exciting new field, and discuss first applications in the field of all-optical quantum information processing. 
Ofer Firstenberg, Charles S. Adams, Sebastian Hofferberth
quant-ph