  The dipole blockade of Rydberg excitations is a hallmark of the strong
interactions between atoms in these high-lying quantum states. One of the
consequences of the dipole blockade is the suppression of fluctuations in the
counting statistics of Rydberg excitations, of which some evidence has been
found in previous experiments. Here we present experimental results on the
dynamics and the counting statistics of Rydberg excitations of ultra-cold
Rubidium atoms both on and off resonance, which exhibit sub- and
super-Poissonian counting statistics, respectively. We compare our results with
numerical simulations using a novel theoretical model based on Dicke states of
Rydberg atoms including dipole-dipole interactions, finding good agreement
between experiment and theory.
