Mesoscopic Rydberg ensembles: Beyond the pairwise-interaction approximation

We examine state mixing in a dense, cold Rydberg gas. Cold Rb atom clouds in an optical dipole trap are excited into nD(5/2) Rydberg levels using narrow-bandwidth laser pulses. For n=43, two atoms excited to 43D(5/2) states can be converted to 41F(7/2) and 45P(3/2) product states via a Forster 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 Forster resonances, we use an exact, nonperturbative many-atom Hamiltonian in which product states are treated on an equal basis with the laser-excited nD(5/2) Rydberg level. Simplified many-body interaction models based on sums over pairwise atomic potentials cannot reproduce the measured mixing fractions.