Many-Body Effects in a Frozen Rydberg Gas

We studied the properties of a cold ( $\sim 100\mu \mathrm{K}$) and dense ( $\sim {10}^{8–10}{\mathrm{cm}}^{-3\mathrm{}}$) atomic Rydberg Cs gas, and found that the observed widths and shapes of resonances in population transfers cannot be explained in the framework of a usual gas model. We propose a “frozen Rydberg gas” model, where the interplay between two-body and many-body phenomena affects in an unexpected way the width and the shape of spectral lines.