Mesoscopic Rydberg-blockaded ensembles in the superatom regime and beyond

We report on the controlled creation and characterization of an isolated mesoscopic superatom by means of accurate density engineering and excitation to Rydberg P-states. \footnote{T.M.~Weber, M.~H\"oning, T.~Niederpr\"um, T.~Manthey, O. Thomas, M. Fleischhauer, G. Barontini and H.~Ott, \textbf{Nature Physics} Mesoscopic Rydberg-blockaded ensembles in the superatom regime and beyond, doi:10.1038/nphys3214} By monitoring continuous laser-induced ionization we are able to determine the $g^2(\tau)$ correlation function and observe anti bunching for resonant excitation, as well as bunching for off resonant coupling. We further investigate the transition from a strongly confined effective two-level to an extended many body system by manipulation of the spatial dimension of the atomic sample with the help of a focused electron beam. The observed amplitudes and timescales can be described with an effective rate-equation model.