Imaging Spatial Correlations of Rydberg Excitations in Cold Atom Clouds

We use direct spatial imaging of cold 85Rb Rydberg atom clouds to measure the Rydberg-Rydberg correlation function. The results are in qualitative agreement with theoretical predictions F. Robicheaux and J. V. Hernández, Phys. Rev. A 72, 063403 (2005). We determine the blockade radius for states 44D(5/2), 60D(5/2), and 70D(5/2) and investigate the dependence of the correlation behavior on excitation conditions and detection delay. Experimental data hint at the existence of long-range order.