Abstract
Long-range interparticle interactions are revealed in extremely dilute thermal atomic ensembles using highly sensitive nonlinear femtosecond spectroscopy. Delocalized excitons are detected in the atomic systems at particle densities where the mean interatomic distance (>10 μm) is much greater than the laser wavelength and multi-particle coherences should destructively interfere over the ensemble average. With a combined experimental and theoretical analysis, we identify an effective interaction mechanism, presumably of dipolar nature, as the origin of the excitonic signals. Our study implies that even in highly-dilute thermal atom ensembles, significant transition dipole-dipole interaction networks may form that require advanced modeling beyond the nearest neighbor approximation to quantitatively capture the details of their many-body properties.
Highlights
Dipolar interactions are the driving forces of most manybody quantum phenomena
We show a strong indication for this statement by applying a highly sensitive nonlinear spectroscopy method that allows us to study the long-range behavior of transition dipole–dipole interactions at extreme dilute conditions, demonstrated for the D line excitations in a thermal atomic rubidium (Rb) ensemble
We find indications for a significant interaction among the atoms even at mean interparticle distances much greater than the wavelength of the coherent excitation field (B790 nm) and at densities of t107 cmÀ3, being five orders of magnitude smaller so far reported in atomic vapors[32] and about three orders of magnitude smaller than typical densities in ultracold atom clouds.[15]
Summary
Dipolar interactions are the driving forces of most manybody quantum phenomena. A intriguing and unique feature of the dipole potential is its long-range nature, scaling with 1/r3 (r denotes the interparticle distance), due to which it was suggested that cooperative effects may be present at all particle densities.[22] In the current work, we show a strong indication for this statement by applying a highly sensitive nonlinear spectroscopy method that allows us to study the long-range behavior of transition dipole–dipole interactions at extreme dilute conditions, demonstrated for the D line excitations in a thermal atomic rubidium (Rb) ensemble. Our results cannot be quantitatively reproduced by a two-body model, indicating, that the nearest-neighbor approximation is inappropriate in a non-ordered gas, even at the investigated ultra low densities
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