Spectroscopy of an ultracold Rydberg gas and signatures of Rydberg–Rydberg interactions

We report on experiments on Rydberg–Rydberg interaction-induced effects in a gas of 87 Rb Rydberg atoms. A compact setup for two-photon continuous-wave excitation of high-lying Rydberg states out of an ultracold atomic gas is presented. The performance of the apparatus is characterized by high-resolution spectroscopy of Rydberg states. Signatures of interaction-induced effects are identified by qualitatively analysing the dependence of Rydberg excitation spectra on the intensity and the duration of the second-step laser excitation.