Observation of Rydberg blockade between two atoms

We demonstrate experimentally that a single Rb atom excited to the $79d_{5\mathrm{/}2}$ level blocks the subsequent excitation of a second atom located more than $10\mu \mathrm{m}$ 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.