Observation of strong electric dipole-dipole interactions between two atoms

We have prepared pairs of atoms at densities exceeding 10¹⁶/cm³ corresponding to n³>10 and studied how they interact with light. In this regime, the coherent part of the electric dipole-dipole interaction is larger than the dissipative part, and a single photon emission can transfer large momenta (larger than ten times the single photon recoil) to the atoms. For off-resonant laser light, the atoms are dressed with an electric dipole moment of ≈ 1 Debye and interact coherently. This light-induced dipole-dipole interaction has been previously observed only in the far-field mean-field regime. For two atoms, the resonance is shifted by the two-body Lamb shift which is dominated by the near-field part of the dipolar interaction. Dipolar resonance shifts have been observed before only for many atoms interacting with the far-field part of the dipolar interaction. Our work sheds new light on photoassociation processes and paves the way for studying driven quantum systems with strong electric dipole-dipole interactions.