Excitation dynamics and full counting statistics for resonant and off-resonant excitation of a strongly correlated cold Rydberg gas

Atoms in high-lying Rydberg states strongly interact with each other via the dipole-dipole or van der-Waals potential thus permitting the exploration of a wide range of many-body phenomena in strongly interacting systems. The strong interactions between Rydberg atoms under resonant laser driving become manifest either as spatial correlations compatible with a radius of blockade around an excited atom or through a reduction of fluctuations leading to sub-Poissonian statistics [1]. On the other hand, away from resonance, the detuning can compensate for the energy shift induced by the Rydberg-Rydberg interaction, giving rise to resonant interaction processes [2, 3]. In such an off-resonant excitation scheme, two atoms can undergo a pair excitation if the atomic interaction matches the laser energy defect/excess. As a consequence an already excited Rydberg atom pair (or a Rydberg atom, off-resonantly excited) can shift other atoms into resonance [4], in a domino effect, leading to an increasing overall number of Rydberg excitations. This resonant condition is the opposite of the blockade effect, where the interactions suppresses excitations, allowing at most one single excitation within a blockade radius. I will present experimental observations for both the resonant and the off-resonant excitation scheme, with the excitation laser having a finite detuning from the 87Rb 70S state. I will illustrate the off-resonant excitation dynamics and full counting statistics in experiments in which the growth of excitations is controlled by using an initial Rydberg excitation as a seed. The information extracted from the full counting distribution makes possible a direct comparison with theoretical predictions that is far more sensitive than, i.e., the mean and standard deviation alone.