Density Dependence of Thermalization Among Nearly Harmonic Ladders

We cool and trap Rubidium-85 atoms in a static electric field and excite them to a Rydberg state near the center of a Stark manifold. Atoms can exchange energy through dipole-dipole interactions, which redistributes the population among the manifold states. Using microwave spectroscopy, we observe how population spreads at various densities. Based on notions of dynamical typicality, we expect the population to spread uniformly across the manifold as the system approaches equilibrium. However, at low densities, we observed a failure to thermalize.