Dipole-Dipole Interactions in a Cold Rydberg Gas

We investigate the strength and effects of dipole-dipole interactions in a cold Rydberg atom system. Our experiments explore the absence of collective decay due to dipole-dipole interactions, characterize dipole-dipole interactions and their dependence on Rydberg atom density and atomic motion, and explore possible methods to control atom separations using dipole-dipole interactions.

We study the decay of Rydberg atoms in a magneto-optical trap. The absence of collective decay or superradiant decay is attributed to variations in transition energies within the atom sample. These variations are dominated by inhomogeneities due to dipole-dipole exchange interactions for initial s states and by a combination of dipole-dipole and electric field inhomogeneities for initial p states.

We characterize the strength of the dipole-dipole resonant energy transfer reaction 32p32p to 32s33s by measuring population transfer to the 32s33s pair state as a function of electronic energy difference between the initial and final atom-pair states. We obtain resonance line shapes and widths that agree with a randomly distributed, nearest-neighbor model. We also use the line shapes to estimate the electric field inhomogeneity.

We explore two possible methods of controlling atom separation using dipole-dipole interactions. The first uses a chirped-frequency control laser to push closely separated atoms apart in a fully formed magneto-optical trap. The second uses a fixed-frequency control laser to prevent atoms from approaching past a minimum separation as the trap is formed. While atomic separation control was not observed in these measurements, we present several improvements for future experiments.