| When Rydberg states are excited in a dense atomic gas the mean number of excited atoms reaches a stationary value after an initial transient period. We shed light on the origin of this steady state that emerges from a purely coherent evolution of a closed system. We derive an equation of motion which governs the system's evolution in excitation number space. Beyond a certain interaction strength this equation possesses a thermal state as steady state solution. We investigate the evolution of the system into this thermal state and discuss how to determine the corresponding temperature. |
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