Superradiance in ultracold Rydberg gases
  Experiments in dense, ultracold gases of rubidium Rydberg atoms show a considerable decrease of the radiative excited state lifetimes compared to dilute gases. This accelerated decay is explained by collective and cooperative effects, leading to superradiance. A novel formalism to calculate effective decay times in a dense Rydberg gas shows that for these atoms the decay into nearby levels increases by up to three orders of magnitude. Excellent agreement between theory and experiment follows from this treatment of Rydberg decay behavior. 
Tun Wang, S. F. Yelin, R. Cote, E. E. Eyler, S. M. Farooqi, P. L.   Gould, M. Kostrun, D. Tong, D. Vrinceanu