We report the direct measurement of the van der Waals interaction between two isolated, single Rydberg atoms separated by a controlled distance of a few micrometers. Working in a regime where the single-atom Rabi frequency for excitation to the Rydberg state is comparable to the interaction, we observe partial Rydberg blockade, whereby the time-dependent populations of the various two-atom states exhibit coherent oscillations with several frequencies. Quantitative comparison of the data with a simple model based on the optical Bloch equations allows us to extract the van der Waals energy, and observe its characteristic $C_6/R^6$ dependence. The measured $C_6$ coefficients agree well with ab initio calculations, and we observe their dramatic increase with the principal quantum number $n$ of the Rydberg state.

• Received 22 March 2013

DOI:https://doi.org/10.1103/PhysRevLett.110.263201