Dipole Blockade and Quantum Information Processing in Mesoscopic Atomic   Ensembles
  We describe a technique for manipulating quantum information stored in collective states of mesoscopic ensembles. Quantum processing is accomplished by optical excitation into states with strong dipole-dipole interactions. The resulting ``dipole blockade'' can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as non-classical photonic states and for scalable quantum logic gates. An example involving a cold Rydberg gas is analyzed. 
M. D. Lukin, M. Fleischhauer, R.Cote, L. M. Duan, D. Jaksch, J. I.   Cirac and P. Zoller