1,049 research outputs found
Physical interpretation of the Wigner rotations and its implications for relativistic quantum information
We present a new treatment for the spin of a massive relativistic particle in
the context of quantum information based on a physical interpretation of the
Wigner rotations, obtaining different results in relation to the previous
works. We are lead to the conclusions that it is not possible to define a
reduced density matrix for the particle spin and that the Pauli-Lubanski (or
similar) spin operators are not suitable to describe measurements where spin
couples to an electromagnetic field in the measuring apparatus. These
conclusions contradict the assumptions made by most of the previous papers on
the subject. We also propose an experimental test of our formulation.Comment: 10 pages, 2 figures. Several changes were made on the text. One extra
example was include
Interferometric sensing of the tilt angle of a Gaussian beam
We investigate interferometric techniques to estimate the deflection angle of
an optical beam and compare them to the direct detection of the beam
deflection. We show that quantum metrology methods lead to a unifying treatment
for both single photons and classical fields. Using the Fisher information to
assess the precision limits of the interferometric schemes, we show that the
precision can be increased by exploiting the initial transverse displacement of
the beam. This gain, which is present for both Sagnac and Mach-Zehnder-like
configurations, can be considerable when compared to non-interferometric
methods. In addition to the fundamental increase in precision, the
interferometric schemes have the technical advantage that (i) the precision
limits can be saturated by a sole polarization measurement on the field, and
that (ii) the detection system can be placed at any longitudinal position along
the beam. We also consider position-dependent polarization measurements, and
show that in this case the precision increases with the propagation distance,
as well as the initial transverse displacement.Comment: Comments are welcom
General Pattern Search Applied to the Optimization of the Shell and Tube Heat Exchanger
The literature has different implementations and results for the mono-objective and multiobjective optimization of the shell and tube heat exchanger (STHE), most of them using evolutionary computation. However, there is a gap to find the optimal solution of this problem through direct search methods (numerical optimization). So, this paper uses the Pattern Search algorithm of MATLAB toolbox applied to this case study
O conceito de etologia, com especial referência ao comportamento dos primatas: (comentário)
a evolução do estudo do comportamento animal ou etologia é interpretada como consequência da combinação de duas áreasde investigação: os estudos biológicos e naturalísticos e a pesquisa sobr aprendizagem animal. A etologia, como disciplina autônoma, fundamenta-se mais na sua abordagem e em seus métodos do que nos problemas que suscitam o comportamento animal
Energy and momentum entanglement in parametric downconversion
We present a simple treatment for the phenomenon of parametric downconversion
considering the coherent scattering of one pump photon into a photon pair by a
nonlinear crystal. The energy and momentum entanglement of the quantum state of
the generated twin photons are seen as a consequence of the fundamental
indistinguishability of the time and the position in which the photon pair is
created inside the crystal. We also discuss some consequences of the system
entanglement.Comment: 6 pages, 2 figures. v3: Minor changes on the text. Some references
were include
The Asymptotics of Wilkinson's Iteration: Loss of Cubic Convergence
One of the most widely used methods for eigenvalue computation is the
iteration with Wilkinson's shift: here the shift is the eigenvalue of the
bottom principal minor closest to the corner entry. It has been a
long-standing conjecture that the rate of convergence of the algorithm is
cubic. In contrast, we show that there exist matrices for which the rate of
convergence is strictly quadratic. More precisely, let be the matrix having only two nonzero entries and let
be the set of real, symmetric tridiagonal matrices with the same spectrum
as . There exists a neighborhood of which is
invariant under Wilkinson's shift strategy with the following properties. For
, the sequence of iterates exhibits either strictly
quadratic or strictly cubic convergence to zero of the entry . In
fact, quadratic convergence occurs exactly when . Let be
the union of such quadratically convergent sequences : the set has
Hausdorff dimension 1 and is a union of disjoint arcs meeting at
, where ranges over a Cantor set.Comment: 20 pages, 8 figures. Some passages rewritten for clarit
Geometrically induced singular behavior of entanglement
We show that the geometry of the set of quantum states plays a crucial role
in the behavior of entanglement in different physical systems. More
specifically it is shown that singular points at the border of the set of
unentangled states appear as singularities in the dynamics of entanglement of
smoothly varying quantum states. We illustrate this result by implementing a
photonic parametric down conversion experiment. Moreover, this effect is
connected to recently discovered singularities in condensed matter models.Comment: v2: 4 pags, 4 figs. A discussion before the proof of Proposition 1
and tomographic results were included, Propostion 2 was removed and the
references were fixe
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