15,348 research outputs found
Strongly coupled large-angle stimulated Raman scattering of short laser pulses in plasma-filled capillaries
Strongly coupled large-angle stimulated Raman scattering (LA SRS) of a short
intense laser pulse proceeds in a plane plasma-filled capillary differently
than in a plasma with open boundaries. Oblique mirror reflections off capillary
walls partly suppress the lateral convection of scattered radiation and
increase the growth rate of the instability: the convective gain of the LA SRS
falls with an angle much slower than in an unbounded plasma and even for the
near-forward SRS can be close to that of the direct backscatter. The long-term
evolution of LA SRS in the interior of the capillary is dominated by
quasi-one-dimensional leaky modes, whose damping is related to the transmission
of electromagnetic waves through capillary walls.Comment: 11 pages, 6 figures; to be submitted to Physics of Plasma
Searching for signatures of planet formation in stars with circumstellar debris discs
(Abridged) Tentative correlations between the presence of dusty debris discs
and low-mass planets have been presented. In parallel, detailed chemical
abundance studies have reported different trends between samples of planet and
non-planet hosts. We determine in a homogeneous way the metallicity, and
abundances of a sample of 251 stars including stars with known debris discs,
with debris discs and planets, and only with planets. Stars with debris discs
and planets have the same [Fe/H] behaviour as stars hosting planets, and they
also show a similar -Tc trend. Different behaviour in the -Tc
trend is found between the samples of stars without planets and the samples of
planet hosts. In particular, when considering only refractory elements,
negative slopes are shown in cool giant planet hosts, whilst positive ones are
shown in stars hosting low-mass planets. Stars hosting exclusively close-in
giant planets show higher metallicities and positive -Tc slope. A
search for correlations between the -Tc slopes and the stellar
properties reveals a moderate but significant correlation with the stellar
radius and as well as a weak correlation with the stellar age. The fact that
stars with debris discs and stars with low-mass planets do not show neither
metal enhancement nor a different -Tc trend might indicate a
correlation between the presence of debris discs and the presence of low-mass
planets. We extend results from previous works which reported differences in
the -Tc trends between planet hosts and non hosts. However, these
differences tend to be present only when the star hosts a cool distant planet
and not in stars hosting exclusively low-mass planets.Comment: Accepted for publication in Astronomy and Astrophysic
Enabling science with Gaia observations of naked-eye stars
ESA's Gaia space astrometry mission is performing an all-sky survey of
stellar objects. At the beginning of the nominal mission in July 2014, an
operation scheme was adopted that enabled Gaia to routinely acquire
observations of all stars brighter than the original limit of G~6, i.e. the
naked-eye stars. Here, we describe the current status and extent of those
observations and their on-ground processing. We present an overview of the data
products generated for G<6 stars and the potential scientific applications.
Finally, we discuss how the Gaia survey could be enhanced by further exploiting
the techniques we developed.Comment: 16 pages, 8 figures. Submitted for the proceedings of the 2016 SPIE
Astronomical Instrumentation and Telescopes conference (SPIE 9904
Feeding the imaginary
The imaginary that has dominated the fashion system since the mid-twentieth
century seems, in recent years, to have been challenged by empirical
phenomena.
\u2018Imaginary\u2019 is a complex notion that can be addressed from many
perspectives. Here, we refer to the stock of images, values, practices and rules
that dominate the western fashion system and that its participants take for
granted in their relationship with fashion. Of course, different participants
base their understanding of fashion on different imaginaries, and different
imaginaries may be shared by different communities, but a hegemonic imaginary
has underpinned the western fashion discourse for some decades now.
For example, pertaining to this imaginary is the ideal of the female body\u2019s
thinness (Bordo 1993); the positive value attributed to the youthful body; and
the aspiration to the beautiful-and-new as a source of distinction (Lipovetsky
1987), as well as the sur-representation of Caucasian ethnic groups in images
of fashion (Entwistle and Wissinger 2006). Also pertaining to this imaginary
of fashion are usually implicit assumptions about human life. For instance,
assumptions about the temporal organization of the day and the week into
work time (office), leisure time (in the countryside) and social time (evening),
or the belief that the possession of certain consumer goods certifies
social status. These are fragments of representations of the world consistent
with the project of western modernity to achieve the ideal of a world in which
technology and science enable humans to fulfil themselves as independent
adults with the capacity to choose. This, in fact, was the promise of the
Enlightenment, with industrial capitalism and the bourgeoisie embodying
its. values and assuming the task of realizing it. Fashion as an institution of western
modernity (Wilson 1985; Lehmann 2000) has contributed significantly to
this project \u2013 and is an explicit manifestation of it. Recently, however, the western fashion system seems to have been able
to include meanings that it had thus far marginalized. A number of factors are
altering the ordinary metabolism of this system; new ways to do things and
new representations (discourses, visual contents, values) appear that seem to
provide the dominant fashion imaginary with new contents and avenues.
The need to take stock of these new developments prompted the conference
entitled Fashion Tales 2015: Feeding the Imaginary, organized in June 2015
by Centro Modacult of the Catholic University of Milan, in collaboration with
this journal. The conference \u2013 of which this issue of the International Journal of
Fashion Studies collects some contributions \u2013 identified three main directions
along which innovative experiences occur. Two of them have to do with the
impact of new technologies on the structure of the fashion system itself; in
particular, the technologies arising from advances in chemical research, and
digital technologies. While the former are transforming the fashion industry
under the banner of sustainability, the latter are leading to the widespread
mediatization of fashion (Rocamora 2016). The third direction concerns nonwestern fashio
Class of PPT bound entangled states associated to almost any set of pure entangled states
We analyze a class of entangled states for bipartite systems,
with non-prime. The entanglement of such states is revealed by the
construction of canonically associated entanglement witnesses. The structure of
the states is very simple and similar to the one of isotropic states: they are
a mixture of a separable and a pure entangled state whose supports are
orthogonal. Despite such simple structure, in an opportune interval of the
mixing parameter their entanglement is not revealed by partial transposition
nor by the realignment criterion, i.e. by any permutational criterion in the
bipartite setting. In the range in which the states are Positive under Partial
Transposition (PPT), they are not distillable; on the other hand, the states in
the considered class are provably distillable as soon as they are Nonpositive
under Partial Transposition (NPT). The states are associated to any set of more
than two pure states. The analysis is extended to the multipartite setting. By
an opportune selection of the set of multipartite pure states, it is possible
to construct mixed states which are PPT with respect to any choice of bipartite
cuts and nevertheless exhibit genuine multipartite entanglement. Finally, we
show that every -positive but not completely positive map is associated to a
family of nondecomposable maps.Comment: 12 pages, 3 figures. To appear in Phys. Rev.
Correlation functions of a Lieb-Liniger Bose gas
The ground-state correlation functions of a one-dimensional homogeneous Bose
system described by the Lieb-Liniger Hamiltonian are investigated by using
exact quantum Monte Carlo techniques. This article is an extension of a
previous study published in Phys. Rev. A {\bf 68}, 031602 (2003). New results
on the local three-body correlator as a function of the interaction strength
are included and compared with the measured value from three-body loss
experiments. We also carry out a thorough study of the short- and long-range
behavior of the one-body density matrix.Comment: 10 pages, 8 figures, contribution to Cortona BEC JPB special issu
Universal resources for approximate and stochastic measurement-based quantum computation
We investigate which quantum states can serve as universal resources for
approximate and stochastic measurement-based quantum computation, in the sense
that any quantum state can be generated from a given resource by means of
single-qubit (local) operations assisted by classical communication. More
precisely, we consider the approximate and stochastic generation of states,
resulting e.g. from a restriction to finite measurement settings or from
possible imperfections in the resources or local operations. We show that
entanglement-based criteria for universality obtained for the exact,
deterministic case can be lifted to the much more general approximate,
stochastic case, moving from the idealized situation considered in previous
works, to the practically relevant context of non-perfect state preparation. We
find that any entanglement measure fulfilling some basic requirements needs to
reach its maximum value on some element of an approximate, stochastic universal
family of resource states, as the resource size grows. This allows us to rule
out various families of states as being approximate, stochastic universal. We
provide examples of resources that are efficient approximate universal, but not
exact deterministic universal. We also study the robustness of universal
resources for measurement-based quantum computation under realistic assumptions
about the (imperfect) generation and manipulation of entangled states, giving
an explicit expression for the impact that errors made in the preparation of
the resource have on the possibility to use it for universal approximate and
stochastic state preparation. Finally, we discuss the relation between our
entanglement-based criteria and recent results regarding the uselessness of
states with a high degree of geometric entanglement as universal resources.Comment: 17 pages; abstract shortened with respect to the published version to
respect the arXiv limit of 1,920 character
Conditioning of extreme learning machine for noisy data using heuristic optimization
This article provides a tool that can be used in the exact sciences to obtain good approximations to reality when noisy data is inevitable. Two heuristic optimization algorithms are implemented: Simulated Annealing and Particle Swarming for the determination of the extreme learning machine output weights. The first operates in a large search space and at each iteration it probabilistically decides between staying at its current state or moving to another. The swarm of particles, it optimizes a problem from a population of candidate solutions, moving them throughout the search space according to position and speed. The methodology consists of building data sets around a polynomial function, implementing the heuristic algorithms and comparing the errors with the traditional computation method using the Moore–Penrose inverse. The results show that the heuristic optimization algorithms implemented improve the estimation of the output weights when the input have highly noisy data
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