Strong electric dipole-dipole interaction between two atoms*

Electric dipole-dipole interaction is one of the fundamental forms of interaction between charge-neutral objects and underpins numerous phenomena and applications in atomic, molecular, and condensed matter physics. For atoms, the interaction arising from their atomic dipoles becomes significant only when the interatomic spacing is smaller than λ/2π (with λ being the transition wavelength). As a result, the dipole-dipole interaction between two atoms is usually difficult to observe, and its effects have only been detected when the contributions from many atoms are summed up. In this work, we investigate the strong electric dipole-dipole interaction between two atoms by tightly confining them in a three-dimensional optical lattice. The atoms are confined to a root-mean-square (rms) size of approximately 20nm, achieving a peak density exceeding 10¹⁶cm^-3. By preparing the atom pair in the symmetric Dicke state, we demonstrate that the electric dipole-dipole interaction can transfer an average momentum of around 20ℏk, despite the emission of only a single photon. When the atomic dipole is induced by a driving laser, the resulting dipole moment also gives rise to a strong electric dipole-dipole interaction and substantial momentum transfer. In this case, we show that the scattered light remains coherent if the light pulse is short enough to prevent the photons from entangling with the atomic motion, whereas longer pulses lead to decoherence. Additionally, fluorescence spectroscopy reveals an asymmetry in the line shape that depends on the aspect ratio of the atomic wavefunction, highlighting the anisotropic nature of the interaction. Finally, we demonstrate that our system offers an ideal platform for photoassociation studies, with the elastic-to-inelastic ratio of photon scattering being tunable via the detuning.