27,616 research outputs found
Generation of maximally entangled states of qudits using twin photons
We report an experiment to generate maximally entangled states of
D-dimensional quantum systems, qudits, by using transverse spatial correlations
of two parametric down-converted photons. Apertures with D-slits in the arms of
the twin fotons define the qudit space. By manipulating the pump beam correctly
the twin photons will pass only by symmetrically opposite slits, generating
entangled states between these differents paths. Experimental results for
qudits with D=4 and D=8 are shown. We demonstrate that the generated states are
entangled states.Comment: 04 pages, 04 figure
Quantized fields and gravitational particle creation in f(R) expanding universes
The problem of cosmological particle creation for a spatially flat,
homogeneous and isotropic Universes is discussed in the context of f(R)
theories of gravity. Different from cosmological models based on general
relativity theory, it is found that a conformal invariant metric does not
forbid the creation of massless particles during the early stages (radiation
era) of the Universe.Comment: 14 pages, 2 figure
Born-Infeld magnetars: larger than classical toroidal magnetic fields and implications for gravitational-wave astronomy
Magnetars are neutron stars presenting bursts and outbursts of X- and
soft-gamma rays that can be understood with the presence of very large magnetic
fields. Thus, nonlinear electrodynamics should be taken into account for a more
accurate description of such compact systems. We study that in the context of
ideal magnetohydrodynamics and make a realization of our analysis to the case
of the well-known Born-Infeld (BI) electromagnetism in order to come up with
some of its astrophysical consequences. We focus here on toroidal magnetic
fields as motivated by already known magnetars with low dipolar magnetic fields
and their expected relevance in highly magnetized stars. We show that BI
electrodynamics leads to larger toroidal magnetic fields when compared to
Maxwell's electrodynamics. Hence, one should expect higher production of
gravitational waves (GWs) and even more energetic giant flares from nonlinear
stars. Given current constraints on BI's scale field, giant flare energetics
and magnetic fields in magnetars, we also find that the maximum magnitude of
magnetar ellipticities should be . Besides, BI electrodynamics
may lead to a maximum increase of order of the GW energy radiated
from a magnetar when compared to Maxwell's, while much larger percentages may
arise for other physically motivated scenarios. Thus, nonlinear theories of the
electromagnetism might also be probed in the near future with the improvement
of GW detectors.Comment: 8 pages, no figures, accepted for publication in The European
Physical Journal C (EPJC
Parameterized Complexity of Equitable Coloring
A graph on vertices is equitably -colorable if it is -colorable and
every color is used either or times.
Such a problem appears to be considerably harder than vertex coloring, being
even for cographs and interval graphs.
In this work, we prove that it is for block
graphs and for disjoint union of split graphs when parameterized by the number
of colors; and for -free interval graphs
when parameterized by treewidth, number of colors and maximum degree,
generalizing a result by Fellows et al. (2014) through a much simpler
reduction.
Using a previous result due to Dominique de Werra (1985), we establish a
dichotomy for the complexity of equitable coloring of chordal graphs based on
the size of the largest induced star.
Finally, we show that \textsc{equitable coloring} is when
parameterized by the treewidth of the complement graph
From de Sitter to de Sitter: decaying vacuum models as a possible solution to the main cosmological problems
Decaying vacuum cosmological models evolving smoothly between two extreme
(very early and late time) de Sitter phases are capable to solve or at least to
alleviate some cosmological puzzles, among them: (i) the singularity, (ii)
horizon, (iii) graceful-exit from inflation, and (iv) the baryogenesis problem.
Our basic aim here is to discuss how the coincidence problem based on a large
class of running vacuum cosmologies evolving from de Sitter to de Sitter can
also be mollified. It is also argued that even the cosmological constant
problem become less severe provided that the characteristic scales of the two
limiting de Sitter manifolds are predicted from first principles.Comment: 7 pages, 2 figures, title changed, typos corrected, text and new
references adde
Counterrotation in magnetocentrifugally driven jets and other winds
Rotation measurement in jets from T Tauri stars is a rather difficult task.
Some jets seem to be rotating in a direction opposite to that of the underlying
disk, although it is not yet clear if this affects the totality or part of the
outflows. On the other hand, Ulysses data also suggest that the solar wind may
rotate in two opposite ways between the northern and southern hemispheres. We
show that this result is not as surprising as it may seem and that it emerges
naturally from the ideal MHD equations. Specifically, counterrotating jets
neither contradict the magnetocentrifugal driving of the flow nor prevent
extraction of angular momentum from the disk. The demonstration of this result
is shown by combining the ideal MHD equations for steady axisymmetric flows.
Provided that the jet is decelerated below some given threshold beyond the
Alfven surface, the flow will change its direction of rotation locally or
globally. Counterrotation is also possible for only some layers of the outflow
at specific altitudes along the jet axis. We conclude that the counterrotation
of winds or jets with respect to the source, star or disk, is not in
contradiction with the magnetocentrifugal driving paradigm. This phenomenon may
affect part of the outflow, either in one hemisphere, or only in some of the
outflow layers. From a time-dependent simulation, we illustrate this effect and
show that it may not be permanent.Comment: To appear in ApJ
Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures
The occurrence of inhomogeneous spin-density distribution in multilayered
ferromagnetic diluted magnetic semiconductor nanostructures leads to strong
dependence of the spin-polarized transport properties on these systems. The
spin-dependent mobility, conductivity and resistivity in
(Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a
function of temperature, scaled by the average magnetization of the diluted
magnetic semiconductor layers. An increase of the resistivity near the
transition temperature is obtained. We observed that the spin-polarized
transport properties changes strongly among the three materials.Comment: 3 pages, 4 figure
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