Collective excitation of Rydberg-atom ensembles beyond the $\sqrt{N}$   enhancement
  In an ensemble of laser-driven atoms involving strongly interacting Rydberg states, the excitation probability is usually strongly suppressed. In contrast, here we identify a regime in which the steady-state Rydberg excited fraction is enhanced by the interaction. This effect is associated with the build-up of many-body coherences, induced by coherent multi-photon excitations between collective states. The excitation enhancement should be observable under currently-existing experimental conditions, and may serve as a direct probe for the presence of coherent multi-photon dynamics involving collective quantum states. 
Martin G\"arttner, Shannon Whitlock, David W. Sch\"onleber, J\"org   Evers
physics.atom-ph quant-ph