title: Subradiance in a Large Cloud of Cold Atoms.
abstract: Since Dicke's seminal paper on coherence in spontaneous radiation by atomic ensembles, superradiance has been extensively studied. Subradiance, on the contrary, has remained elusive, mainly because subradiant states are weakly coupled to the environment and are very sensitive to nonradiative decoherence processes. Here, we report the experimental observation of subradiance in an extended and dilute cold-atom sample containing a large number of particles. We use a far detuned laser to avoid multiple scattering and observe the temporal decay after a sudden switch-off of the laser beam. After the fast decay of most of the fluorescence, we detect a very slow decay, with time constants as long as 100 times the natural lifetime of the excited state of individual atoms. This subradiant time constant scales linearly with the cooperativity parameter, corresponding to the on-resonance optical depth of the sample, and is independent of the laser detuning, as expected from a coupled-dipole model.
authors.affiliation: Institut Non Linéaire de Nice, CNRS and Université Nice Sophia Antipolis, 1361 route des Lucioles, 06560 Valbonne, France.
authors.forename: William
authors.initials: W
authors.lastname: Guerin
authors.affiliation: Institut Non Linéaire de Nice, CNRS and Université Nice Sophia Antipolis, 1361 route des Lucioles, 06560 Valbonne, France.|CAPES Foundation, Ministry of Education of Brazil, Brasília DF 70040-020, Brazil.
authors.forename: Michelle O
authors.initials: MO
authors.lastname: Araújo
authors.affiliation: Institut Non Linéaire de Nice, CNRS and Université Nice Sophia Antipolis, 1361 route des Lucioles, 06560 Valbonne, France.
authors.forename: Robin
authors.initials: R
authors.lastname: Kaiser