Observation of Rydberg blockade between two atoms
  We demonstrate experimentally that a single Rb atom excited to the $79d_{5/2}$ level blocks the subsequent excitation of a second atom located more than $10 \mu\rm m$ away. The observed probability of double excitation of $\sim 30%$ is consistent with a theoretical model based on calculations of the long range dipole-dipole interaction between atoms. 
E. Urban, T. A. Johnson, T. Henage, L. Isenhower, D. D. Yavuz, T. G.   Walker, and M. Saffman