Mesoscopic Rydberg-blockaded ensembles in the superatom regime and   beyond
  Controlling strongly interacting many-body systems enables the creation of tailored quantum matter, with properties transcending those based solely on single particle physics. Atomic ensembles which are optically driven to a Rydberg state provide many examples of this, such as atom-atom entanglement, many-body Rabi oscillations, strong photon-photon interaction and spatial pair correlations. In its most basic form, Rydberg quantum matter consists of an isolated ensemble of strongly interacting atoms spatially confined to the blockade volume - a so-called superatom. Here we demonstrate the controlled creation and characterization of an isolated mesoscopic superatom by means of accurate density engineering and excitation to Rydberg $p$-states. Its variable size allows to investigate the transition from effective two-level physics for strong confinement to many-body phenomena in extended systems. By monitoring continuous laser-induced ionization we observe a strongly anti-bunched ion emission under blockade conditions and extremely bunched ion emission under off-resonant excitation. Our experimental setup enables in vivo measurements of the superatom, yielding insight into both excitation statistics and dynamics. We anticipate straightforward applications in quantum optics and quantum information as well as future experiments on many-body physics. 
T. M. Weber, M. H\"oning, T. Niederpr\"um, T. Manthey, O. Thomas, V.   Guarrera, M. Fleischhauer, G. Barontini and H. Ott
physics.atom-ph cond-mat.quant-gas quant-ph