Density measurement using dipole-dipole dynamics among ultracold Rydberg atoms*

For experiments involving interactions among ultracold atoms, the density of Rydberg states can play a significant role. We can leverage this spatial dependence into a technique that can accurately measure the Rydberg atom density. Using rubidium-85 atoms in a MOT, we examine the time-dependence of the N-body dipole-dipole resonance N x 33p_{3/2, |mj|=1/2} → 33s_1/2 + 34s_1/2 + (N-2) x 33p_{3/2, |mj|=3/2} for N = 2 and 3, which scale differently with the spacing between atoms. We then calibrate the density of Rydberg states by comparing the measured dynamics to simulation.