Imaging Spatial Correlations of Rydberg Excitations in Cold Atom Clouds

We use direct spatial imaging of cold Rb85 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)PLRAAN1050-294710.1103/PhysRevA.72.063403]. 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.