Two-photon dynamics in coherent Rydberg atomic ensemble
  We study the interaction of two photons in a Rydberg atomic ensemble under the condition of electromagnetically induced transparency, combining a semi-classical approach for pulse propagation and a complete quantum treatment for quantum state evolution. We find that the blockade regime is not suitable for implementing photon-photon cross-phase modulation due to pulse absorption and dispersion. However, approximately ideal cross-phase modulation can be realized based on relatively weak interactions, with counter-propagating and transversely separated pulses. 
Bing He, A. V. Sharypov, Jiteng Sheng, Christoph Simon, and Min Xiao
quant-ph