Blockade and antiblockade of Rydberg excitations in ultracold gases

Experimental studies of the impact of strong interatomic interactions on the Rydberg excitation dynamics in ultracold gases have recently led to a verification of the interaction-induced suppression of excitation compared to a non-interacting reference system. This blockade effect has been demonstrated directly via the fraction of atoms that are excited by a laser pulse, and indirectly via the sub-Poissonian counting statistics of the number of excited atoms. For a theoretical treatment, we have developed a microscopic approach based on a description of the single-atom dynamics within a rate equation approximation and a Monte Carlo treatment of the many-particle system. This method is particularly well suited for a study of the statistical properties of the excitation process. Moreover, we show that under certain circumstances a reversal of the blockade effect is possible, leading to an enhancement of excitation rather than a suppression.