Prospects for photon blockade in four level systems in the N   configuration with more than one atom
  We show that for appropriate choices of parameters it is possible to achieve photon blockade in idealised one, two and three atom systems. We also include realistic parameter ranges for rubidium as the atomic species. Our results circumvent the doubts cast by recent discussion in the literature (Grangier et al Phys. Rev Lett. 81, 2833 (1998), Imamoglu et al Phys. Rev. Lett. 81, 2836 (1998)) on the possibility of photon blockade in multi-atom systems. 
Andrew D. Greentree, John A. Vaccaro, Sebastian R. de Echaniz, Alan V.   Durrant, Jon P. Marangos