2,371 research outputs found
Quantum Gowdy Model: Schrodinger Representation with Unitary Dynamics
The linearly polarized Gowdy model is paradigmatic for studying
technical and conceptual issues in the quest for a quantum theory of gravity
since, after a suitable and almost complete gauge fixing, it becomes an exactly
soluble midisuperspace model. Recently, a new quantization of the model,
possessing desired features such as a unitary implementation of the gauge group
and of the time evolution, has been put forward and proven to be essentially
unique. An appropriate setting for making contact with other approaches to
canonical quantum gravity is provided by the Schr\"odinger representation,
where states are functionals on the configuration space of the theory. Here we
construct this functional description, analyze the time evolution in this
context and show that it is also unitary when restricted to physical states,
i.e. states which are solutions to the remaining constraint of the theory.Comment: 21 pages, version accepted for publication in Physical Review
Multivoicedness as a tool for expanding school leaders’ understandings and practices for school-based professional development
This article presents findings from the implementation of a pilot study of a professional development model, Schools Inquiring and Learning with Peers (SILP) in Chile. SILP includes a network learning setting involving a cluster of three schools supported by university partners that conduct peer reviews and a school learning setting involving school teams in conversations to mobilise new leadership for learning practices. The notions of joint practice development and multivoicedness in professional conversations are the essence of these processes. Data produced through the review process, understood as voices, enabled participants to incorporate the perspectives, conceptual horizon, and intentions of administrators, students, and teachers who need to collaborate for teaching to produce learning. Across schools we observed common learning as well as important differences. These differences show that by taking an active role in making sense of their participation in a professional development programme, learners achieve outcomes that may be much more meaningful than what program designers prescribed. As facilitators we learned with participants about their specific contexts, expanding the affordances they and we envisioned for the use of new tools that became available through their participation in SILP
Laser Induced Breakdown Spectroscopy
This review describes the fundamentals, instrumentation, applications and future trends of an analytical technique that is in its early stages of consolidation and is establishing its definitive niches among modern spectrometric techniques. The technique has been named Laser Induced Breakdown Spectroscopy (LIBS) and its main characteristic stands in the use of short laser pulses as the energy source to vaporize samples and excite the emission of electromagnetic radiation from its elements and/or molecular fragments. The emitted radiation is analyzed by high resolution optics and the intensities are recorded, usually by fast triggered solid state detectors. Together, these devices allow producing and registering a wide ranging emission spectrum of the short-lived phenomenon induced by the laser pulse. The spectrum contains qualitative and quantitative information which can be correlated with sample identity or can be used to determine the amount of its constituents. This review is divided in four parts. First, the relevant historical and theoretical concepts associated with LIBS are presented; then the main practical aspects of the several experimental and instrumental approaches employed for implementation of the technique are critically described; the applications related in the literature, including those making use of chemometrics, are classified and exemplified with relevant and recently published work. Finally, an attempt to portray an overall evaluation and future perspectives of the technique are presented.Esta revisão descreve os aspectos fundamentais, a instrumentação, as aplicações e tendências futuras de uma técnica analítica que se encontra em seu estágio de consolidação e que está em vias de estabelecer o seu nicho entre as técnicas espectrofotométricas modernas. A técnica é denominada Espectroscopia de Emissão em Plasma Induzido por Laser (em inglês, Laser Induced Breakdown Spectroscopy, LIBS) e sua principal característica está no uso de pulsos de laser como fonte de energia para vaporizar a amostra e excitar a emissão de radiação eletromagnética, a partir de seus elementos e/ou fragmentos moleculares. A radiação emitida é analisada por meio de instrumentos ópticos de alta resolução e as suas intensidades são medidas, usualmente com detectores rápidos de estado sólido. Em conjunto, esses dispositivos permitem a geração e a medida de um espectro de emissão de faixa ampla do fenômeno induzido pelo pulso de laser. O espectro registrado contém informação qualitativa e quantitativa que pode ser correlacionada com a identidade da amostra ou empregada na determinação da quantidade de seus constituintes. Essa revisão é dividida em quatro partes. A primeira aborda aspectos históricos da técnica e os conceitos teóricos relevantes associados com LIBS; então, os aspectos práticos de diversas abordagens experimentais e instrumentais empregadas na implementação da técnica são revistos de forma crítica; as aplicações encontradas na literatura, incluindo aquelas que empregam quimiometria, são classificadas e exemplificadas por meio de trabalhos relevantes recentemente publicados. Finalmente, uma tentativa de estabelecer uma avaliação global e as perspectivas futuras para a técnica é apresentada.463512Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq
Uniqueness of the Fock quantization of fields with unitary dynamics in nonstationary spacetimes
The Fock quantization of fields propagating in cosmological spacetimes is not
uniquely determined because of several reasons. Apart from the ambiguity in the
choice of the quantum representation of the canonical commutation relations,
there also exists certain freedom in the choice of field: one can scale it
arbitrarily absorbing background functions, which are spatially homogeneous but
depend on time. Each nontrivial scaling turns out into a different dynamics
and, in general, into an inequivalent quantum field theory. In this work we
analyze this freedom at the quantum level for a scalar field in a
nonstationary, homogeneous spacetime whose spatial sections have
topology. A scaling of the configuration variable is introduced as part of a
linear, time dependent canonical transformation in phase space. In this
context, we prove in full detail a uniqueness result about the Fock
quantization requiring that the dynamics be unitary and the spatial symmetries
of the field equations have a natural unitary implementation. The main
conclusion is that, with those requirements, only one particular canonical
transformation is allowed, and thus only one choice of field-momentum pair (up
to irrelevant constant scalings). This complements another previous uniqueness
result for scalar fields with a time varying mass on , which selects a
specific equivalence class of Fock representations of the canonical commutation
relations under the conditions of a unitary evolution and the invariance of the
vacuum under the background symmetries. In total, the combination of these two
different statements of uniqueness picks up a unique Fock quantization for the
system. We also extend our proof of uniqueness to other compact topologies and
spacetime dimensions.Comment: 12 page
A multivariate approach for multi-step demand forecasting in assembly industries: Empirical evidence from an automotive supply chain
PreprintDemand forecasting works as a basis for operating, business and production planning decisions in many supply chain contexts. Yet, how to accurately predict the manufacturer's demand for components in the presence of end-customer demand uncertainty remains poorly understood. Assigning the proper order quantities of components to suppliers thus becomes a nontrivial task, with a significant impact on planning, capacity and inventory-related costs. This paper introduces a multivariate approach to predict manufacturer's demand for components throughout multiple forecast horizons using different leading indicators of demand shifts. We compare the autoregressive integrated moving average model with exogenous inputs (ARIMAX) with Machine Learning (ML) models. Using a real case study, we empirically evaluate the forecasting and supply chain performance of the multivariate regression models over the component's life-cycle. The experiments show that the proposed multivariate approach provides superior forecasting and inventory performance compared with traditional univariate benchmarks. Moreover, it reveals applicable throughout the component's life-cycle, not just to a single stage. Particularly, we found that demand signals at the beginning of the life-cycle are predicted better by the ARIMAX model, but it is outperformed by ML-based models in later life-cycle stages.INCT-EN - Instituto Nacional de Ciência e Tecnologia para Excitotoxicidade e Neuroproteção(UIDB/00319/2020
A study of atmospheric neutrinos with the IMB detector
A sample of 401 contained neutrino interactions collected in the 3300 metric ton fiducial mass IMB detector was used to study neutrino oscillations, geomagnetic modulation of the flux and to search for point sources. The majority of these events are attributed to neutrino interactions. For the most part, these neutrinos are believed to originate as tertiary products of cosmic ray interactions in the atmosphere. The neutrinos are a mixture of v sub e and v sub micron
Decoherence times of universal two-qubit gates in the presence of broad-band noise
The controlled generation of entangled states of two quantum bits is a
fundamental step toward the implementation of a quantum information processor.
In nano-devices this operation is counteracted by the solid-state environment,
characterized by a broadband and non-monotonic power spectrum, often 1/f at low
frequencies. For single-qubit gates, incoherent processes due to fluctuations
acting on different time scales result in peculiar short- and long-time
behavior. Markovian noise gives rise to exponential decay with relaxation and
decoherence times, T1 and T2, simply related to the symmetry of the
qubit-environment coupling Hamiltonian. Noise with the 1/f power spectrum at
low frequencies is instead responsible for defocusing processes and algebraic
short-time behavior. In this paper, we identify the relevant decoherence times
of an entangling operation due to the different decoherence channels
originating from solid-state noise. Entanglement is quantified by concurrence,
which we evaluate in an analytic form employing a multi-stage approach. The
'optimal' operating conditions of reduced sensitivity to noise sources are
identified. We apply this analysis to a superconducting \sqrt{i-SWAP} gate for
experimental noise spectra.Comment: 35 pages, 11 figure
Espécies de Frankliniella (Thysanoptera, Thripidae): novos registros em mangueira (Mangifera indica) no Brasil.
O objetivo do trabalho foi conhecer as espécies de tripes (Thysanoptera) que atacam pomares de mangueira no semiárido pernambucano. Foram realizadas coletas no período de um ano, em três plantios comerciais da cv. ?Tommy Atkins?. Avaliações semanais foram realizadas em 75 plantas, nos estádios vegetativo e reprodutivo, perfazendo um total de 10.704 amostras em folhas e 390 em panículas. Os espécimes coletados foram levados ao laboratório da Embrapa Semiárido, Petrolina, PE, para triagem dos adultos em microscópio estereoscópico e, posteriormente, enviados à Universidade Federal de Lavras, MG, para identificação. As espécies Frankliniella gardeniae Moulton e Frankliniella Brevicaulis Hood, pertencentes à família Thripidae, representaram, respectivamente, 77% e 23% do total dos tripes coletados. Este estudo relata a primeira ocorrência dessas duas espécies em associação à cultura da mangueira, no Brasil
Quantum Gowdy model: A unitary description
The quantization of the family of linearly polarized Gowdy spacetimes
is discussed in detail, starting with a canonical analysis in which the true
degrees of freedom are described by a scalar field that satisfies a
Klein-Gordon type equation in a fiducial time dependent background. A time
dependent canonical transformation, which amounts to a change of the basic
(scalar) field of the model, brings the system to a description in terms of a
Klein-Gordon equation on a background that is now static, although subject to a
time dependent potential. The system is quantized by means of a natural choice
of annihilation and creation operators. The quantum time evolution is
considered and shown to be unitary, allowing both the Schr\"odinger and
Heisenberg pictures to be consistently constructed. This has to be contrasted
with previous treatments for which time evolution failed to be implementable as
a unitary transformation. Possible implications for both canonical quantum
gravity and quantum field theory in curved spacetime are commented
Construyendo dominios de encuentro para problematizar acerca de las prácticas pedagógicas de profesores secundarios de Ciencias: Incorporando el modelo de Investigación-Acción como plan de formación continua
Frente a la actual crisis de la educación científica, la capacidad docente de transformar sus prácticas para mejorar el logro de aprendizajes parece ser fundamental. Los programas de formación docente continua tradicionales, generalmente prescriptivos y descontextualizados del aula, han tenido bajo impacto en los procesos de transformación de las prácticas docentes. El presente trabajo describe la primera fase de un programa de formación continua consistente en la construcción de dominios de encuentro entre el saber académico generado en la Universidad y el saber profesional de 16 profesores secundarios de Ciencias a través de un proceso de investigación-acción conjunta, basado en el trabajo colaborativo, la reflexión y la retroalimentación. Los resultados dan cuenta de la complejidad de generar significados comunes, los que una vez logrados, permiten fortalecer el razonamiento científico, la autoeficacia y conformación de una comunidad de aprendizaje que posibilita una actitud indagatoria hacia sus prácticas en Ciencias. / Considering the current scientific education crisis, teachers' capacity to transform their practice in order to improve learning achievement seems fundamental. The traditional programs of continuous professional development, generally prescriptive and uncontextualized from the classroom, have had a low impact on processes of teachers' practice transformation. This work describes the first stage of continuous development program, consisting on building encounter domains between the academic knowledge of Universities, and professional knowledge of 16 secondary science teachers, through a joint action-research process, based on collaborative work, reflection and feedback. The results show the complexity of building shared sense which, once achieved, allows strong scientific reasoning, self-efficacy and the construction of a learning community facilitating an inquiry-driven attitude towards their science teaching practice
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