Mesoscopic Rydberg ensembles: Beyond the pairwise-interaction approximation

Abstract

We examine state mixing in a dense, cold Rydberg gas. Cold Rb atom clouds in an optical dipole trap are excited into $n{D}_{5/2}$ Rydberg levels using narrow-bandwidth laser pulses. For $n=43$, two atoms excited to $43{D}_{5/2}$ states can be converted to $41{F}_{7/2}$ and $45{P}_{3/2}$ product states via a F\"orster resonance. We find, unexpectedly, that up to 50% of the Rydberg atoms are detected in such product states after only 100 ns of interaction time. The experiment is modeled using many-body quantum simulations. To reproduce the mixing fractions measured near F\"orster resonances, we use an exact, nonperturbative many-atom Hamiltonian in which product states are treated on an equal basis with the laser-excited $n{D}_{5/2}$ Rydberg level. Simplified many-body interaction models based on sums over pairwise atomic potentials cannot reproduce the measured mixing fractions.