  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.
