3,654 research outputs found
Steering in-plane shear waves with inertial resonators in platonic crystals
Numerical simulations shed light on control of shear elastic wave propagation
in plates structured with inertial resonators. The structural element is
composed of a heavy core connected to the main freestanding plate through tiny
ligaments. It is shown that such a configuration exhibits a complete band gap
in the low frequency regime. As a byproduct, we further describe the asymmetric
twisting vibration of a single scatterer via modal analysis, dispersion and
transmission loss. This might pave the way to new functionalities such as
focusing and self-collimation in elastic plates
A Reproducible Study on Remote Heart Rate Measurement
This paper studies the problem of reproducible research in remote
photoplethysmography (rPPG). Most of the work published in this domain is
assessed on privately-owned databases, making it difficult to evaluate proposed
algorithms in a standard and principled manner. As a consequence, we present a
new, publicly available database containing a relatively large number of
subjects recorded under two different lighting conditions. Also, three
state-of-the-art rPPG algorithms from the literature were selected, implemented
and released as open source free software. After a thorough, unbiased
experimental evaluation in various settings, it is shown that none of the
selected algorithms is precise enough to be used in a real-world scenario
BEAT: An Open-Source Web-Based Open-Science Platform
With the increased interest in computational sciences, machine learning (ML),
pattern recognition (PR) and big data, governmental agencies, academia and
manufacturers are overwhelmed by the constant influx of new algorithms and
techniques promising improved performance, generalization and robustness.
Sadly, result reproducibility is often an overlooked feature accompanying
original research publications, competitions and benchmark evaluations. The
main reasons behind such a gap arise from natural complications in research and
development in this area: the distribution of data may be a sensitive issue;
software frameworks are difficult to install and maintain; Test protocols may
involve a potentially large set of intricate steps which are difficult to
handle. Given the raising complexity of research challenges and the constant
increase in data volume, the conditions for achieving reproducible research in
the domain are also increasingly difficult to meet.
To bridge this gap, we built an open platform for research in computational
sciences related to pattern recognition and machine learning, to help on the
development, reproducibility and certification of results obtained in the
field. By making use of such a system, academic, governmental or industrial
organizations enable users to easily and socially develop processing
toolchains, re-use data, algorithms, workflows and compare results from
distinct algorithms and/or parameterizations with minimal effort. This article
presents such a platform and discusses some of its key features, uses and
limitations. We overview a currently operational prototype and provide design
insights.Comment: References to papers published on the platform incorporate
Long-term & large-scale viscous evolution of dense planetary rings
We investigate the long-term and large-scale viscous evolution of dense
planetary rings using a simple 1D numerical code. We use a physically realistic
viscosity model derived from N-body simulations (Daisaka et al., 2001), and
dependent on the disk's local properties (surface mass density, particle size,
distance to the planet). Particularly, we include the effects of gravitational
instabilities (wakes) that importantly enhance the disk's viscosity. We show
that common estimates of the disk's spreading time-scales with constant
viscosity significantly underestimate the rings' lifetime. With a realistic
viscosity model, an initially narrow ring undergoes two successive evolutionary
stages: (1) a transient rapid spreading when the disk is self-gravitating, with
the formation of a density peak inward and an outer region marginally
gravitationally stable, and with an emptying time-scale proportional to 1/M_0^2
(where M_0 is the disk's initial mass) (2) an asymptotic regime where the
spreading rate continuously slows down as larger parts of the disk become
not-self-gravitating due to the decrease of the surface density, until the disk
becomes completely not-self-gravitating. At this point its evolution
dramatically slows down, with an emptying time-scale proportional to 1/M_0,
which significantly increases the disk's lifetime compared to the case with
constant viscosity. We show also that the disk's width scales like t^{1/4} with
the realistic viscosity model, while it scales like t^{1/2} in the case of
constant viscosity, resulting in much larger evolutionary time-scales in our
model. We find however that the present shape of Saturn's rings looks like a
100 million-years old disk in our simulations. Concerning Jupiter's, Uranus'
and Neptune's rings that are faint today, it is not likely that they were much
more massive in the past and lost most of their mass due to viscous spreading
alone.Comment: 18 pages, 18 figures, 2 tables. Accepted for publication in Icaru
High-Fidelity and Ultrafast Initialization of a Hole Spin Bound to a Te Isoelectronic Center in ZnS
We demonstrate the optical initialization of a hole-spin qubit bound to an
isoelectronic center (IC) formed by a pair of Te impurities in ZnSe, an
impurity/host system providing high optical homogeneity, large electric dipole
moments, and long coherence times. The initialization scheme is based on the
spin-preserving tunneling of a resonantly excited donor-bound exciton to a
positively charged Te IC, thus forming a positive trion. The radiative decay of
the trion within less than 50 ps leaves a heavy hole in a well-defined
polarization-controlled spin state. The initialization fidelity exceeds 98:5 %
for an initialization time of less than 150 ps.Comment: 5 pages, 3 figures, 1 supplemental information sectio
Cold Atom Clock Test of Lorentz Invariance in the Matter Sector
We report on a new experiment that tests for a violation of Lorentz
invariance (LI), by searching for a dependence of atomic transition frequencies
on the orientation of the spin of the involved states (Hughes-Drever type
experiment). The atomic frequencies are measured using a laser cooled
Cs atomic fountain clock, operating on a particular combination of
Zeeman substates. We analyze the results within the framework of the Lorentz
violating standard model extension (SME), where our experiment is sensitive to
a largely unexplored region of the SME parameter space, corresponding to first
measurements of four proton parameters and improvements by 11 and 13 orders of
magnitude on the determination of four others. In spite of the attained
uncertainties, and of having extended the search into a new region of the SME,
we still find no indication of LI violation.Comment: 4 pages, accepted for Physical Review Letter
BEAT: Web-based open science platorm for all
This presentation was part of Day 5 of the Open Science in Practice Summer School #osip2017. Additional information can be found on osip2017.epfl.ch
Numerical forecasts for lab experiments constraining modified gravity:The chameleon model
Current acceleration of the cosmic expansion leads to coincidence as well as fine-tuning issues in the framework of general relativity. Dynamical scalar fields have been introduced in response of these problems, some of them invoking screening mechanisms for passing local tests of gravity. Recent lab experiments based on atom interferometry in a vacuum chamber have been proposed for testing modified gravity models. So far only analytical computations have been used to provide forecasts. We derive numerical solutions for chameleon models that take into account the effect of the vacuum chamber wall and its environment. With this realistic profile of the chameleon field in the chamber, we refine the forecasts that were derived analytically. We finally highlight specific effects due to the vacuum chamber that are potentially interesting for future experimentsinfo:eu-repo/semantics/publishe
International trade under attack: what strategy for Europe? Bruegel Policy Contribution Issue nË12 | August 2018
The multilateral trading system is seriously threatened by the country which has been its main inspirer, the United States. The US position is focused on bilateral trade imbalances presumably resulting from unbalanced trade policies, but it is flawed. Not only does it make little sense given the existence of global value chains, but it also misses its target: what matters most are aggregate trade surpluses and deficits, which depend above all on the differential between domestic investment and savings, and little on trade policy
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