Four wave mixing with self-phase matching due to collective atomic recoil

We describe a method for non-degenerate four-wave mixing in a cold sample of 4-level atoms. An integral part of the four-wave mixing process is a collective instability which spontaneously generates a periodic density modulation in the cold atomic sample with a period equal to half of the wavelength of the generated high-frequency optical field. Due to the generation of this density modulation, phase-matching between the pump and scattered fields is not a necessary initial condition for this wave-mixing process to occur, rather the density modulation acts to "self phase-match" the fields during the course of the wave-mixing process. We describe a one-dimensional model of this process, and suggest a proof-of-principle experiment which would involve pumping a sample of cold Cs atoms with three infra-red pump fields to produce blue light.Comment: to appear in Physical Review Letter

A quantum model for collective recoil lasing

Free Electron Laser (FEL) and Collective Atomic Recoil Laser (CARL) are described by the same model of classical equations for properly defined scaled variables. These equations are extended to the quantum domain describing the particle's motion by a Schr\"{o}dinger equation coupled to a self-consistent radiation field. The model depends on a single collective parameter $\bar \rho$ which represents the maximum number of photons emitted per particle. We demonstrate that the classical model is recovered in the limit $\bar \rho\gg 1$, in which the Wigner function associated to the Schr\"{o}dinger equation obeys to the classical Vlasov equation. On the contrary, for $\bar \rho\le 1$, a new quantum regime is obtained in which both FELs and CARLs behave as a two-state system coupled to the self-consistent radiation field and described by Maxwell-Bloch equations

The Semiclassical and Quantum Regimes of Superradiant Light Scattering from a Bose-Einstein Condensate

We show that many features of the recent experiments of Schneble et al. [D. Schneble, Y. Torii, M. Boyd, E.W. Streed, D.E. Pritchard and W. Ketterle, Science vol. 300, p. 475 (2003)], which demonstrate two different regimes of light scattering by a Bose-Einstein condensate, can be described using a one-dimensional mean-field quantum CARL model, where optical amplification occurs simultaneously with the production of a periodic density modulation in the atomic medium. The two regimes of light scattering observed in these experiments, originally described as ``Kapiza-Dirac scattering'' and ``Superradiant Rayleigh scattering'', can be interpreted as the semiclassical and quantum limits respectively of CARL lasing.Comment: 10 pages, 5 figures - to appear in Journal of Optics

Inducing strong density modulation with small energy dispersion in particle beams and the harmonic amplifier free electron laser

We present a possible method of inducing a periodic density modulation in a particle beam with little increase in the energy dispersion of the particles. The flow of particles in phase space does not obey Liouville's Theorem. The method relies upon the Kuramoto-like model of collective synchronism found in free electron generators of radiation, such as Cyclotron Resonance Masers and the Free Electron Laser. For the case of an FEL interaction, electrons initially begin to bunch and emit radiation energy with a correlated energy dispersion which is periodic with the FEL ponderomotive potential. The relative phase between potential and particles is then changed by approximately 180 degrees. The particles continue to bunch, however, there is now a correlated re-absorption of energy from the field. We show that, by repeating this relative phase change many times, a significant density modulation of the particles may be achieved with only relatively small energy dispersion. A similar method of repeated relative electron/radiation phase changes is used to demonstrate supression of the fundamental growth in a high gain FEL so that the FEL lases at the harmonic only

An investigation of the occurrence of antibiotic resistance genes among strains of <i>Staphylococcus aureus</i> isolated from animals and their relationship to human strains of <i>Staphylococcus aureus</i>

Data has emerged which indicates that antimicrobial use in animals has created a reservoir of resistant bacteria and resistance genes that have spread to humans. The aim of this study was to examine the influence of animal antimicrobial use on human strains of S. aureus. Phenotypic and genotypic methods assessed the genetic population structure, potential for host adaptation, frequency of antimicrobial resistance, presence and frequency of genes encoding tetracycline and macrolide resistance and structural variation in tetK genes in S. aureus from animals compared with human clinical strains. In addition, the transferability of tetK resistance plasmids from animal strains to S. aureus 8325-4 was investigated. DNA based typing exhibited 100% typeability, high levels of discrimination and a high degree of concordance between methods. Multi-locus sequence typing (MLST) identified six sequence types (CC5, CC15, CC22, CC25, CC30 and CC45) common to both isolate collections that represented four of the five major human methicillin resistant Staphylococcus aureus (MRSA) clonal lineages. The relatedness of these clones was further supported by the analysis of 20 different virulence determinants. Isolates of CC5 exhibited resistance to ciprofloxacin, clindamycin, erythromycin, penicillin, streptomycin, tobramycin and tylosin, CC 15 to penicillin and tetracycline, CC22 to penicillin and rifampicin and CC30 to penicillin. Isolates of CC45 were fully susceptible. Low level biocide resistance was detected but the significance of this was unclear. The tetK and ermC genes were the predominant tetracycline and macrolide resistance genes. tetK was harboured by animal isolates of CC5 and CC15 and ermC by animal isolates of CC5. Restriction fragment length polymorphism (RFLP) of tetK amplicons produced indistinguishable restriction patterns. High frequency transfer of a tetK plasmid from a chicken S. aureus (CC5) to S. aureus 8325-4 was observed. These data support the hypothesis that animals represent an important reservoir of antibiotic resistant S. aureus with the ability for strain and antibiotic resistance gene transfer to humans

A note on Dolby and Gull on radar time and the twin "paradox"

Recently a suggestion has been made that standard textbook representations of hypersurfaces of simultaneity for the travelling twin in the twin "paradox" are incorrect. This suggestion is false: the standard textbooks are in agreement with a proper understanding of the relativity of simultaneity.Comment: LaTeX, 3 pages, 2 figures. Update: added new section V and updated reference

Annual Report Readership: A Study of an Agricultural Supply Cooperative

Recent corporate collapses have focussed attention on the (un)reliability of financial information. However, although the agricultural sector, which is significant globally, is run primarily using the cooperative form, there is scant research on these users' perception of financial information. Therefore this paper examines members' readership and understanding of the annual reports of a large, fertiliser cooperative. The findings show that there is a lack of readership of the annual report, due to a lack of understanding and a lack of time. A minority of non-readers trust directors to "do a good job". Preparers of information should focus on making reports more user-friendly and evidence suggests that financial information could be released more strategically using other sources of communication, namely other print media and the internet.cooperatives, annual reports, readership, understanding, Agribusiness,

Willingness-to-Pay for Improved Air Quality in Hamilton-Wentworth: A Choice Experiment

Prepared for Hamilton-Wentworth Air Quality Initiative pursuant to a memorandum of understanding among McMaster University, the Ontario Ministry of Environment and Energy and the Regional Municipality of Hamilton-Wentworth, dated November 5, 1996.

Mode-locked Bloch oscillations in a ring cavity

We present a new technique for stabilizing and monitoring Bloch oscillations of ultracold atoms in an optical lattice under the action of a constant external force. In the proposed scheme, the atoms also interact with a unidirectionally pumped optical ring cavity whose one arm is collinear with the optical lattice. For weak collective coupling, Bloch oscillations dominate over the collective atomic recoil lasing instability and develop a synchronized regime in which the atoms periodically exchange momentum with the cavity field.Comment: 7 pages, 5 figure