Periodic Complexes and Group Actions

In this paper we show that the cohomology of a connected CW complex is periodic if and only if it is the base space of an orientable spherical fibration with total space that is homotopically finite dimensional. As applications we characterize those discrete groups that act freely and properly on a cartesian product of euclidean space and a sphere; we construct non-standard free actions of rank two simple groups on finite complexes Y homotopy equivalent to a product of two spheres and we prove that a finite p-group P acts freely on such a complex if and only if it does not contain a subgroup isomorphic to Z/p X Z/p X Z/p.Comment: Revised versio

Full observation of single-atom dynamics in cavity QED

We report the use of broadband heterodyne spectroscopy to perform continuous measurement of the interaction energy between one atom and a high-finesse optical cavity, during individual transit events of $\sim 250$ $\mu$s duration. Measurements over a wide range of atom-cavity detunings reveal the transition from resonant to dispersive coupling, via the transfer of atom-induced signals from the amplitude to the phase of light transmitted through the cavity. By suppressing all sources of excess technical noise, we approach a measurement regime in which the broadband photocurrent may be interpreted as a classical record of conditional quantum evolution in the sense of recently developed quantum trajectory theories.Comment: Submitted to Applied Physics B. Uses Revtex, 13 pages with 11 EPS figure

Short-lived Radio Bursts from the Crab Pulsar

Our high-time-resolution observations reveal that individual main pulses from the Crab pulsar contain one or more short-lived microbursts. Both the energy and duration of bursts measured above 1 GHz can vary dramatically in less than a millisecond. These fluctuations are too rapid to be caused by propagation through turbulence in the Crab Nebula or the interstellar medium; they must be intrinsic to the radio emission process in the pulsar. The mean duration of a burst varies with frequency as $\nu^{-2}$, significantly different from the broadening caused by interstellar scattering. We compare the properties of the bursts to some simple models of microstructure in the radio emission region.Comment: 34 pages, 10 figures; accepted for publication in Ap