510 research outputs found
Electromagnetically induced transparency and four-wave mixing in a cold atomic ensemble with large optical depth
We report on the delay of optical pulses using electromagnetically induced
transparency in an ensemble of cold atoms with an optical depth exceeding 500.
To identify the regimes in which four-wave mixing impacts on EIT behaviour, we
conduct the experiment in both rubidium 85 and rubidium 87. Comparison with
theory shows excellent agreement in both isotopes. In rubidium 87, negligible
four-wave mixing was observed and we obtained one pulse-width of delay with 50%
efficiency. In rubidium 85, four-wave-mixing contributes to the output. In this
regime we achieve a delay-bandwidth product of 3.7 at 50% efficiency, allowing
temporally multimode delay, which we demonstrate by compressing two pulses into
the memory medium.Comment: 8 pages, 6 figure
Experimental Demonstration of Post-Selection based Continuous Variable Quantum Key Distribution in the Presence of Gaussian Noise
In realistic continuous variable quantum key distribution protocols, an
eavesdropper may exploit the additional Gaussian noise generated during
transmission to mask her presence. We present a theoretical framework for a
post-selection based protocol which explicitly takes into account excess
Gaussian noise. We derive a quantitative expression of the secret key rates
based on the Levitin and Holevo bounds. We experimentally demonstrate that the
post-selection based scheme is still secure against both individual and
collective Gaussian attacks in the presence of this excess noise.Comment: 4 pages, 4 figure
Inflationary Models Driven by Adiabatic Matter Creation
The flat inflationary dust universe with matter creation proposed by
Prigogine and coworkers is generalized and its dynamical properties are
reexamined. It is shown that the starting point of these models depends
critically on a dimensionless parameter , closely related to the matter
creation rate . For bigger or smaller than unity flat universes
can emerge, respectively, either like a Big-Bang FRW singularity or as a
Minkowski space-time at . The case corresponds to a de
Sitter-type solution, a fixed point in the phase diagram of the system,
supported by the matter creation process. The curvature effects have also been
investigated. The inflating de Sitter is a universal attractor for all
expanding solutions regardless of the initial conditions as well as of the
curvature parameter.Comment: 25 pages, 2 figures(available from the authors), uses LATE
Effect of MoO3 in the cathode buffer layer on the behaviour of layered organic solar cells
The behaviour of small-molecule organic solar cells based on coper-phthalocyanine/fullerene with different cathode buffer layer is investigated as a function of air exposure duration. We present the study of the effect of MoO3 on the properties of photovoltaic solar cells (OPVCs) when it is introduced in the cathode buffer layer (CBL). Photovoltaic performances were measured as a function of time of air exposure. During the first days of air exposure the efficiency of the OPVCs with MoO3 in their CBL increases significantly, while it decreases immediately after air exposure in the case of reference OPVCs, i.e. without MoO3 in the CBL. Nevertheless, the lifetime of the OPVCs with MoO3 in their CBL is around 60 days, while it is only 10 days in the case of reference OPVCs. The initial increase of the OPVC with MoO3 in their CBL is attributed to the slow decrease of the work function of MoO3 due to progressive contamination. Then, the progressive degradation of the OPVCs efficiency is due water vapour and oxygen contamination of the organic layers. The use of a double CBL, Alq3/MoO3, allows to interrupt the growth of pinholes, defects and increases the path of permeating gas. Also it can prevent the contamination of the organic layer by Al. All this results in significant increase of the lifetime of the OPVCs
Emissões evitadas de GEE na expansão da soja no Brasil de 2010 a 2016.
Resumo - A Fixação Biológica de Nitrogênio (FBN) é um processo natural e existe independentemente da inoculação. A aplicação de inoculantes pode introduzir ou ampliar a capacidade da FBN, se usado para a substituição ou redução da aplicação de adubos nitrogenados. A soja foi a cultura que mais obteve sucesso com o processo da FBN, desta forma, a proposta deste trabalho é analisar as correlações das emissões de gases de efeito estufa evitadas na expansão de soja no Brasil de 2010 a 2016, período contido no Plano ABC. Para o cálculo de emissões foram utilizadas as variáveis ?rendimento? e ?área colhida?, da pesquisa Produção Agrícola Municipal (PAM), do Instituto Brasileiro de Geografia e Estatística (IBGE).Editores técnicos: Carla Geovana do Nascimento Macário, Carla Cristiane Osawa, Flávia Bussaglia Fiorini, Maria Fernanda Moura, Poliana Fernanda Giachetto
Cálculo de tamanho de amostra para análise de acurácia em mapeamentos temáticos.
Resumo - A análise da acurácia entre mapeamentos de fontes diversas é a metodologia mais difundida para estimar taxas de acerto e erro em produtos cartográficos. Essa metodologia consiste na comparação de um determinado elemento mapeado em uma classe em relação com um resultado ideal. O número e a alocação de pixels para a aferição do produto gerado com os dados de referência são definidos por técnicas estatísticas. A proposta neste trabalho é calcular a quantidade de pontos amostrais necessários para realizar a acurácia e verificar a sua confiabilidade em relação ao mapeamento.Editores técnicos: Carla Geovana do Nascimento Macário, Carla Cristiane Osawa, Flávia Bussaglia Fiorini, Maria Fernanda Moura, Poliana Fernanda Giachetto
Graviton Production in Elliptical and Hyperbolic Universes
The problem of cosmological graviton creation for homogeneous and isotropic
universes with elliptical (\vae =+1) and hyperbolical (\vae =-1) geometries
is addressed. The gravitational wave equation is established for a
self-gravitating fluid satisfying the barotropic equation of state , which is the source of the Einstein's equations plus a cosmological
-term. The time dependent part of this equation is exactly solved in
terms of hypergeometric functions for any value of and spatial
curvature \vae. An expression representing an adiabatic vacuum state is then
obtained in terms of associated Legendre functions whenever , where n is an integer. This includes most
cases of physical interest such as . The mechanism of
graviton creation is reviewed and the Bogoliubov coefficients related to
transitions between arbitrary cosmic eras are also explicitly evaluated.Comment: 25 pages, uses REVTE
Quantum cosmological perfect fluid models
Perfect fluid Friedmann-Robertson-Walker quantum cosmological models for an
arbitrary barotropic equation of state are constructed using
Schutz's variational formalism. In this approach the notion of time can be
recovered. By superposition of stationary states, finite-norm wave-packet
solutions to the Wheeler-DeWitt equation are found. The behaviour of the scale
factor is studied by applying the many-worlds and the ontological
interpretations of quantum mechanics. Singularity-free models are obtained for
.Comment: Latex file, 12 pages. New paragraphs in the Introduction and
Conclusion, and other minor corrections in the text and in some formulas.
Accepted for publication in General Relativity and Gravitatio
Self-tuning to the Hopf bifurcation in fluctuating systems
The problem of self-tuning a system to the Hopf bifurcation in the presence
of noise and periodic external forcing is discussed. We find that the response
of the system has a non-monotonic dependence on the noise-strength, and
displays an amplified response which is more pronounced for weaker signals. The
observed effect is to be distinguished from stochastic resonance. For the
feedback we have studied, the unforced self-tuned Hopf oscillator in the
presence of fluctuations exhibits sharp peaks in its spectrum. The implications
of our general results are briefly discussed in the context of sound detection
by the inner ear.Comment: 37 pages, 7 figures (8 figure files
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