Cavity-Controlled Collective Scattering at the Recoil Limit

We study collective scattering with Bose-Einstein condensates interacting with a high-finesse ring cavity. The condensate scatters the light of a transverse pump beam superradiantly into modes which, in contrast to previous experiments, are not determined by the geometrical shape of the condensate, but specified by a resonant cavity mode. Moreover, since the recoil-shifted frequency of the scattered light depends on the initial momentum of the scattered fraction of the condensate, we show that it is possible to employ the good resolution of the cavity as a filter selecting particular quantized momentum states.Comment: 4 pages, 4 figure

Observation of a Cooperative Radiation Force in the Presence of Disorder

Cooperative scattering of light by an extended object such as an atomic ensemble or a dielectric sphere is fundamentally different from scattering from many point-like scatterers such as single atoms. Homogeneous distributions tend to scatter cooperatively, whereas fluctuations of the density distribution increase the disorder and suppress cooperativity. In an atomic cloud, the amount of disorder can be tuned via the optical thickness, and its role can be studied via the radiation force exerted by the light on the atomic cloud. Monitoring cold $^{87}\text{Rb}$ atoms released from a magneto-optical trap, we present the first experimental signatures of radiation force reduction due to cooperative scattering. The results are in agreement with an analytical expression interpolating between the disorder and the cooperativity-dominated regimes

Cavity-enhanced superradiant Rayleigh scattering with ultra-cold and Bose-Einstein condensed atoms

We report on the observation of collective atomic recoil lasing and superradiant Rayleigh scattering with ultracold and Bose-Einstein condensed atoms in an optical ring cavity. Both phenomena are based on instabilities evoked by the collective interaction of light with cold atomic gases. This publication clarifies the link between the two effects. The observation of superradiant behavior with thermal clouds as hot as several tens of $\mu\textrm{K}$ proves that the phenomena are driven by the cooperative dynamics of the atoms, which is strongly enhanced by the presence of the ring cavity.Comment: 10 pages, 10 figure

Cavity-Controlled Collective Scattering at the Recoil Limit

We study collective scattering with Bose-Einstein condensates interacting with a high-finesse ring cavity. The condensate scatters the light of a transverse pump beam superradiantly into modes which, in contrast to previous experiments, are not determined by the geometrical shape of the condensate, but specified by a resonant cavity mode. Moreover, since the recoil-shifted frequency of the scattered light depends on the initial momentum of the scattered fraction of the condensate, we show that it is possible to employ the good resolution of the cavity as a filter selecting particular quantized momentum states.Comment: 4 pages, 4 figure

Cooperative Scattering by Cold Atoms

We have studied the interplay between disorder and cooperative scattering for single scattering limit in the presence of a driving laser. Analytical results have been derived and we have observed cooperative scattering effects in a variety of experiments, ranging from thermal atoms in an optical dipole trap, atoms released from a dark MOT and atoms in a BEC, consistent with our theoretical predictions.Comment: submitted for special issue of PQE 201

Dimension of the Torelli group for Out(F_n)

Let T_n be the kernel of the natural map from Out(F_n) to GL(n,Z). We use combinatorial Morse theory to prove that T_n has an Eilenberg-MacLane space which is (2n-4)-dimensional and that H_{2n-4}(T_n,Z) is not finitely generated (n at least 3). In particular, this recovers the result of Krstic-McCool that T_3 is not finitely presented. We also give a new proof of the fact, due to Magnus, that T_n is finitely generated.Comment: 27 pages, 9 figure

Evaluation of seedling resistance and marker assisted selection for leaf rust (Puccinia triticina) resistance in Pakistani wheat landraces, cultivars and advanced lines

Leaf rust is amongst major biotic constraints of wheat (Triticum aestivum L.) having ability to cause substantial yield reductions worldwide. A continuous exploration for novel sources of resistance is pre-requisite for its management. Objectives of study were to conduct resistance evaluation of 112 Pakistani landraces and 48 advanced lines/ cultivars at seedling stage with total 10 virulent pathotypes of leaf rust, 3 from Pakistan and 7 from U.S.A and to detect closely linked markers for Lr10, 16, 34 and 67 genes through marker-assisted selection (MAS). Findings revealed most of Pakistani landraces showed lack of resistance at seedling stage. Only 7 accessions of landraces and 11 advanced lines were found highly resistant against all pathotypes of Pakistan. Similarly, 10 advanced lines exhibited high resistance while variability in resistance was recorded for landraces against all pathotypes tested from USA. Marker-assisted selection revealed Lr genes i.e. Lr10, Lr16, Lr34 and Lr67 were present at various frequencies. Highest frequency was observed for Lr34 followed by Lr16 & Lr67 while lowest was recorded for Lr10. These genetic resources and lines identified effective against Pakistan and USA pathotypes are potential sources for improvement of leaf rust (LR) resistance and can be utilized as valuable material for breeding resistant wheat cultivars.Campus Ic

Modification of radiation pressure due to cooperative scattering of light

Cooperative spontaneous emission of a single photon from a cloud of N atoms modifies substantially the radiation pressure exerted by a far-detuned laser beam exciting the atoms. On one hand, the force induced by photon absorption depends on the collective decay rate of the excited atomic state. On the other hand, directional spontaneous emission counteracts the recoil induced by the absorption. We derive an analytical expression for the radiation pressure in steady-state. For a smooth extended atomic distribution we show that the radiation pressure depends on the atom number via cooperative scattering and that, for certain atom numbers, it can be suppressed or enhanced.Comment: 8 pages, 2 Figure