Mapping out the van der Waals interaction between two atoms. (a) In the experiment of Béguin et al. two atoms are trapped in the foci of two laser beams separated by a distance R. (b) Depending on R, the excitation laser field can couple the ground state |gg〉 of the atomic pair to states containing one atom in the Rydberg state (|gr〉 and |rg〉, respectively), or to a state with both atoms populating the Rydberg state |rr〉. The energy of the latter state is strongly shifted because of the van der Waals interaction UvdW between the atoms (see level diagram in the upper left), resulting in a distance-dependent coherent excitation dynamics of the atomic pair system. (c) By analyzing the time evolution of the atom-pair state, Béguin et al. deduce the van der Waals energy shift as a function of interatomic distance for different Rydberg states.