title: Direct measurement of the van der Waals interaction between two Rydberg atoms.
abstract: 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 C6/R6 dependence. The measured C6 coefficients agree well with ab initio calculations, and we observe their dramatic increase with the principal quantum number n of the Rydberg state.
authors.affiliation: Laboratoire Charles Fabry, Institut d'Optique, CNRS, Univ Paris Sud, 2 avenue Augustin Fresnel, 91127 Palaiseau cedex, France.
authors.forename: L
authors.initials: L
authors.lastname: Béguin
authors.forename: A
authors.initials: A
authors.lastname: Vernier
authors.forename: R
authors.initials: R
authors.lastname: Chicireanu
authors.forename: T
authors.initials: T
authors.lastname: Lahaye
authors.forename: A
authors.initials: A
authors.lastname: Browaeys