Giovanni Ferioli
We observe the establishement of a steady-state superradiant phase in a dense, elongated cloud of laser cooled atoms in free space, optically excited near resonance along its main axis.
We demonstrate experimentally that this system is described by the Driven Dicke model, an elementary example of a driven-dissipative model exhibiting a non-equilibrium phase transition. We show that this model, which assumes a sub-wavelength sample volume, reproduces very well the data for our extended cloud by simply using an effective atom number.
By measuring both the atomic and photonic degrees of freedom, i.e., the excited state population and the light emitted in the superradiant mode, we characterize the dynamics of the system and map its phase diagram as function of the driving parameters. In particular, we observe the characteristic scaling N^2 of the photon emission rate in the superradiant phase with the atom number N. Finally, we measure the statistic of the light emitted into the superradiant mode, and observe that it disagrees with predictions assuming and uncorrelated atomic system.