3,054 research outputs found
Fluorescence during Doppler cooling of a single trapped atom
We investigate the temporal dynamics of Doppler cooling of an initially hot
single trapped atom in the weak binding regime using a semiclassical approach.
We develop an analytical model for the simplest case of a single vibrational
mode for a harmonic trap, and show how this model allows us to estimate the
initial energy of the trapped particle by observing the fluorescence rate
during the cooling process. The experimental implementation of this temperature
measurement provides a way to measure atom heating rates by observing the
temperature rise in the absence of cooling. This method is technically
relatively simple compared to conventional sideband detection methods, and the
two methods are in reasonable agreement. We also discuss the effects of RF
micromotion, relevant for a trapped atomic ion, and the effect of coupling
between the vibrational modes on the cooling dynamics.Comment: 12 pages, 11 figures, Submitted to Phys. Rev.
A photonic basis for deriving nonlinear optical response
Nonlinear optics is generally first presented as an extension of conventional optics. Typically the subject is introduced with reference to a classical oscillatory electric polarization, accommodating correction terms that become significant at high intensities. The material parameters that quantify the extent of the nonlinear response are cast as coefficients in a power series - nonlinear optical susceptibilities signifying a propensity to generate optical harmonics, for example. Taking the subject to a deeper level requires a more detailed knowledge of the structure and properties of each nonlinear susceptibility tensor, the latter differing in form according to the process under investigation. Typically, the derivations involve intricate development based on time-dependent perturbation theory, assisted by recourse to a set of Feynman diagrams. This paper presents a more direct route to the required results, based on photonic rather than semiclassical principles, and offers a significantly clearer perspective on the photophysics underlying nonlinear optical response. The method, here illustrated by specific application to harmonic generation and down-conversion processes, is simple, intuitive and readily amenable for processes of arbitrary photonic order. © 2009 IOP Publishing Ltd
Raman cooling and heating of two trapped Ba+ ions
We study cooling of the collective vibrational motion of two 138Ba+ ions
confined in an electrodynamic trap and irradiated with laser light close to the
resonances S_1/2-P_1/2 (493 nm) and P_1/2-D_3/2 (650 nm). The motional state of
the ions is monitored by a spatially resolving photo multiplier. Depending on
detuning and intensity of the cooling lasers, macroscopically different
motional states corresponding to different ion temperatures are observed. We
also derive the ions' temperature from detailed analytical calculations of
laser cooling taking into account the Zeeman structure of the energy levels
involved. The observed motional states perfectly match the calculated
temperatures. Significant heating is observed in the vicinity of the dark
resonances of the Zeeman-split S_1/2-D_3/2 Raman transitions. Here two-photon
processes dominate the interaction between lasers and ions. Parameter regimes
of laser light are identified that imply most efficient laser cooling.Comment: 8 pages, 5 figure
Local and global Fokker-Planck neoclassical calculations showing flow and bootstrap current modification in a pedestal
In transport barriers, particularly H-mode edge pedestals, radial scale
lengths can become comparable to the ion orbit width, causing neoclassical
physics to become radially nonlocal. In this work, the resulting changes to
neoclassical flow and current are examined both analytically and numerically.
Steep density gradients are considered, with scale lengths comparable to the
poloidal ion gyroradius, together with strong radial electric fields sufficient
to electrostatically confine the ions. Attention is restricted to relatively
weak ion temperature gradients (but permitting arbitrary electron temperature
gradients), since in this limit a delta-f (small departures from a Maxwellian
distribution) rather than full-f approach is justified. This assumption is in
fact consistent with measured inter-ELM H-Mode edge pedestal density and ion
temperature profiles in many present experiments, and is expected to be
increasingly valid in future lower collisionality experiments. In the numerical
analysis, the distribution function and Rosenbluth potentials are solved for
simultaneously, allowing use of the exact field term in the linearized
Fokker-Planck collision operator. In the pedestal, the parallel and poloidal
flows are found to deviate strongly from the best available conventional
neoclassical prediction, with large poloidal variation of a different form than
in the local theory. These predicted effects may be observable experimentally.
In the local limit, the Sauter bootstrap current formulae appear accurate at
low collisionality, but they can overestimate the bootstrap current near the
plateau regime. In the pedestal ordering, ion contributions to the bootstrap
and Pfirsch-Schluter currents are also modified
Back-Reaction In Lightcone QED
We consider the back-reaction of quantum electrodynamics upon an electric
field E(x_+) = - A'_-(x_+) which is parallel to x^3 and depends only on the
lightcone coordinate x_+ = (x^0 + x^3)/\sqrt{2}. Novel features are that the
mode functions have simple expressions for arbitrary A_-(x_+), and that one
cannot ignore the usual lightcone ambiguity at zero + momentum. Each mode of
definite canonical momenta k_+ experiences pair creation at the instant when
its kinetic momentum p_+=k_+ - e A_-(x_+) vanishes, at which point operators
from the surface at x_- =-\infty play a crucial role. Our formalism permits a
more explicit and complete derivation of the rate of particle production than
is usually given. We show that the system can be understood as the infinite
boost limit of the analogous problem of an electric field which is homogeneous
on surfaces of constant x^0.Comment: 37 pages, 2 figures, LaTeX 2 epsilo
Convergence of simple adaptive Galerkin schemes based on h − h/2 error estimators
We discuss several adaptive mesh-refinement strategies based on (h − h/2)-error estimation. This class of adaptivemethods is particularly popular in practise since it is problem independent and requires virtually no implementational overhead. We prove that, under the saturation assumption, these adaptive algorithms are convergent. Our framework applies not only to finite element methods, but also yields a first convergence proof for adaptive boundary element schemes. For a finite element model problem, we extend the proposed adaptive scheme and prove convergence even if the saturation assumption fails to hold in general
Recognizing Emotions in a Foreign Language
Expressions of basic emotions (joy, sadness, anger, fear, disgust) can be recognized pan-culturally from the face and it is assumed that these emotions can be recognized from a speaker's voice, regardless of an individual's culture or linguistic ability. Here, we compared how monolingual speakers of Argentine Spanish recognize basic emotions from pseudo-utterances ("nonsense speech") produced in their native language and in three foreign languages (English, German, Arabic). Results indicated that vocal expressions of basic emotions could be decoded in each language condition at accuracy levels exceeding chance, although Spanish listeners performed significantly better overall in their native language ("in-group advantage"). Our findings argue that the ability to understand vocally-expressed emotions in speech is partly independent of linguistic ability and involves universal principles, although this ability is also shaped by linguistic and cultural variables
Produtividade de milho para ensilagem e feijão-miúdo em cultivo consorciado no noroeste do Rio Grande do Sul.
A bovinocultura de leite no noroeste do Rio Grande do Sul está em expansão e vem se mostrando uma alternativa rentável para pequenas propriedades. O feijão-miúdo é uma leguminosa tropical que vem se difundindo rapidamente, porém informações sobre seu cultivo e potencial de uso são escassos. O objetivo deste trabalho é avaliar a produtividade de milho para ensilagem e de feijão-miúdo em cultivo consorciado no noroeste do Rio Grande do Sul. Os tratamentos foram quatro variedades de feijão-miúdo cultivadas em consórcio com milho para ensilagem. Foram determinadas a estatura e a densidade real de plantas e as produções de massas verde e seca de milho e de feijão-miúdo na ocasião da colheita do milho. Não foram verificadas diferenças estatísticas nas variáveis analisadas, devido, provavelmente, aos altos coeficientes de variação obtidos. Foram encontrados valores expressivos de produções de massas verde e seca de milho, apesar de a densidade real de plantas de milho ter sido inferior à recomendada. As produções de massas verde e seca das variedades de feijão-miúdo ficaram aquém das obtidas em outras pesquisas regionais, sendo explicadas pela baixa densidade populacional no cultivo consorciado, época tardia de semeadura e avaliação de apenas um corte. A produção de forragem não foi afetada pelas variedades da leguminosa. O consórcio de milho e feijão-miúdo tem desempenho satisfatório e é uma alternativa na composição de sistemas de produção mais sustentáveis e racionais
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