5,899 research outputs found
On the propagation speed of evanescent modes
The group-velocity of evanescent waves (in undersized waveguides, for
instance) was theoretically predicted, and has been experimentally verified, to
be Superluminal (v_g > c). By contrast, it is known that the precursor speed in
vacuum cannot be larger than c. In this paper, by computer simulations based on
Maxwell equations only, we show the existence of both phenomena. In other
words, we verify the actual possibility of Superluminal group velocities,
without violating the so-called (naive) Einstein causality. (Subject classes:
General physics, Classical physics, Optics, Special Relativity; PACS nos.:
73.40Gk, 03.80+z, 03.65Bz; Keywords: evanescent waves; tunnelling photons;
Hartman effect; group velocity; Superluminal waves; precursors; transient
waves; front velocity; Maxwell equations; electromagnetic waves; computer
simulations; Special Relativity; Extended Relativity).Comment: plain LaTeX file (14 pages), plus 15 figures in .jp
Asymptotic freedom in a scalar field theory on the lattice
An alternative model to the trivial -theory of the standard model of
weak interactions is suggested, which embodies the Higgs-mechanism, but is free
of the conceptual problems of standard -theory. We propose a
N-component, O(N)-symmetric scalar field theory, which is originally defined on
the lattice. The model can be motivated from SU(2) gauge theory. Thereby the
scalar field arises as a gauge invariant degree of freedom. The scalar lattice
model is analytically solved in the large N limit. The continuum limit is
approached via an asymptotically free scaling. The renormalized theory evades
triviality, and furthermore gives rise to a dynamically formed mass of the
scalar particle.Comment: 10 pages, LaTeX, one figure and a motivation for the particular type
of action adde
Quantum-enhanced phase estimation using optical spin squeezing
Quantum metrology enables estimation of optical phase shifts with precision
beyond the shot-noise limit. One way to exceed this limit is to use squeezed
states, where the quantum noise of one observable is reduced at the expense of
increased quantum noise for its complementary partner. Because shot-noise
limits the phase sensitivity of all classical states, reduced noise in the
average value for the observable being measured allows for improved phase
sensitivity. However, additional phase sensitivity can be achieved using phase
estimation strategies that account for the full distribution of measurement
outcomes. Here we experimentally investigate the phase sensitivity of a
five-particle optical spin-squeezed state generated by photon subtraction from
a parametric downconversion photon source. The Fisher information for all
photon-number outcomes shows it is possible to obtain a quantum advantage of
1.58 compared to the shot-noise limit, even though due to experimental
imperfection, the average noise for the relevant spin-observable does not
achieve sub-shot-noise precision. Our demonstration implies improved
performance of spin squeezing for applications to quantum metrology.Comment: 8 pages, 5 figure
Alguns contributos da nanotecnologia para a sustentabilidade dos materiais de construção
Os avanços nanotecnológicos atingem inúmeras áreas da ciência, mas no âmbito da sustentabilidade dos materiais de construção os progressos embora importantes são escassos e acima de tudo objeto de reduzida divulgação. O presente artigo sintetiza uma avaliação do
estado da arte relativa a alguns dos contributos da nanotecnologia para a sustentabilidade dos materiais de construção. No mesmo se aborda a compreensão dos compostos gerados durante a hidratação do cimento Portland, o aumento da resistência e da durabilidade de argamassas e betões pela adição de nanopartículas e nanotubos e de que forma a adição de nanopartículas pode contribuir para a autolimpeza, a purificação do ar e a capacidade bactericida em materiais
construtivos por via do efeito fotocatalítico. O presente artigo aborda ainda os últimos
desenvolvimentos da nanotecnologia com vista à eficiência energética, nomeadamente pela produção de isolamentos térmicos de elevado desempenho, janelas com baixa condutibilidade térmica, vidros com transmitância variável e materiais de mudança de fase mais eficientes
Book Reviews
Book Reviews of: Jonathan Schell, The Fate of the Earth (Alfred A. Knopf) John Updike, Rabbit is Rich (Alfred A. Knopf) George Gilder, Wealth and Poverty (Basic Book, Inc.
Two independent photon pairs versus four-photon entangled states in parametric down conversion
We study the physics of four-photon states generated in spontaneous
parametric down-conversion with a pulsed pump field. In the limit where the
coherence time of the photons t_c is much shorter than the duration of the pump
pulse Delta t, the four photons can be described as two independent pairs. In
the opposite limit, the four photons are in a four-particle entangled state.
Any intermediate case can be characterized by a single parameter chi, which is
a function of t_c/Delta t. We present a direct measurement of chi through a
simple experimental setup. The full theoretical analysis is also provided.Comment: 10 pages, 3 figures, submitte
Botulinum toxin type A combined with neurodynamic mobilization for upper limb spasticity after stroke: a case report
Objective: The purpose of this study is to report a case in which combinatory therapy of botulinum toxin type A (BoNT-A) and neurodynamic mobilization (NM) was used as treatment for a patient with severe upper limb spasticity and pain after stroke. Clinical Features: A 76-year-old male patient had spastic muscles in the upper limb 10 months after an ischemic stroke. Intervention and Outcome: The patient underwent combined treatment with BoNT-A and NM of the upper limb in 6 monthly applications. Evaluation was performed pretreatment, 3 months after the first injection, 3 months after the second injection, and at a follow-up session 9 months after starting the treatment. The following outcomes were measured: pain by using a numeric rating scale, spasticity by the Modified Ashworth Scale for Grading Spasticity, acceptance and emotional reaction to the treatment by the Hospital Anxiety and Depression Scale, and functionality by ranges of motion. The patient improved in all outcomes after treatment, and results were maintained during the follow-up sessions. Conclusion: The combined NM and BoNT-A treatment appeared to decrease pain and improve joint ranges of motion during treatment for this patient. The patient showed decreased anxiety and depression during and after the treatment.Fil: Villafañe, Jorge Hugo. Universidad Rey Juan Carlos; EspañaFil: Silva, Guillermo Benjamin. Universidad Católica de Córdoba. Facultad de Ciencias Químicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; ArgentinaFil: Chiarotto, Alessandro. Universidad Rey Juan Carlos; EspañaFil: Ragusa, Orazio L. F.. No especifíca
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