  We observe a density-dependent collective suppression of optical pumping
between the hyperfine ground states in an array of submicrometer-sized clouds
of cold rubidium atoms. The suppressed Raman transition rate can be explained
by strong resonant dipole-dipole interactions that are enhanced by increasing
atom density. The observations are consistent with stochastic electrodynamics
simulations that incorporate the effects of the nonlinear population transfer
via internal atomic levels embedded in a coupled-dipole model.
