46,341 research outputs found
One Action System or Two? Evidence for Common Central Preparatory Mechanisms in Voluntary and Stimulus-Driven Actions
Human behavior is comprised of an interaction between intentionally driven actions and reactions to changes in the environment. Existing data are equivocal concerning the question of whether these two action systems are independent, involve different brain regions, or overlap. To address this question we investigated whether the degree to which the voluntary action system is activated at the time of stimulus onset predicts reaction times to external stimuli.Werecorded event-related potentials while participants prepared and executed left- or right-hand voluntary actions, which were occasionally interrupted by a stimulus requiring either a left- or right-hand response. In trials where participants successfully performed the stimulus-driven response, increased voluntary motor preparation was associated with faster responses on congruent trials (where participants were preparing a voluntary action with the same hand that was then required by the target stimulus), and slower responses on incongruent trials. This suggests that early hand-specific activity in medial frontal cortex for voluntary action trials can be used by the stimulus-driven system to speed responding. This finding questions the clear distinction between voluntary and stimulus-driven action systems. © 2011 the authors
Optical preparation and measurement of atomic coherence at gigahertz bandwidth
We detail a method for the preparation of atomic coherence in a high density
atomic medium, utilising a coherent preparation scheme of gigahertz bandwidth
pulses. A numerical simulation of the preparation scheme is developed, and its
efficiency in preparing coherent states is found to be close to unity at the
entrance to the medium. The coherence is then measured non-invasively with a
probe field.Comment: 7 pages, 5 figure
Peer-to-peer:is deviant behavior the norm on P2P file-sharing networks?
P2P file-sharing networks such as Kazaa, eDonkey, and Limewire boast millions of users. Because of scalability concerns and legal issues, such networks are moving away from the semicentralized approach that Napster typifies toward more scalable and anonymous decentralized P2P architectures. Because they lack any central authority, these networks provide a new, interesting context for the expression of human social behavior. However, the activities of P2P community members are sometimes at odds with what real-world authorities consider acceptable. One example is the use of P2P networks to distribute illegal pornography. To gauge the form and extent of P2P-based sharing of illegal pornography, we analyzed pornography-related resource-discovery traffic in the Gnutella P2P network. We found that a small yet significant proportion of Gnutella activity relates to illegal pornography: for example, 1.6 percent of searches and 2.4 percent of responses are for this type of material. But does this imply that such activity is widespread in the file-sharing population? On the contrary, our results show that a small yet particularly active subcommunity of users searches for and distributes illegal pornography, but it isn't a behavioral norm
AGAPEROS: Searching for variable stars in the LMC Bar with the Pixel Method. I. Detection, astrometry and cross-identification
We extend the work developed in previous papers on microlensing with a
selection of variable stars. We use the Pixel Method to select variable stars
on a set of 2.5 x 10**6 pixel light curves in the LMC Bar presented elsewhere.
The previous treatment was done in order to optimise the detection of long
timescale variations (larger than a few days) and we further optimise our
analysis for the selection of Long Timescale and Long Period Variables
(LT&LPV). We choose to perform a selection of variable objects as comprehensive
as possible, independent of periodicity and of their position on the colour
magnitude diagram. We detail the different thresholds successively applied to
the light curves, which allow to produce a catalogue of 632 variable objects.
We present a table with the coordinate of each variable, its EROS magnitudes at
one epoch and an indicator of blending in both colours, together with a finding
chart.
A cross-correlation with various catalogues shows that 90% of those variable
objects were undetected before, thus enlarging the sample of LT&LPV previously
known in this area by a factor of 10. Due to the limitations of both the Pixel
Method and the data set, additional data -- namely a longer baseline and near
infrared photometry -- are required to further characterise these variable
stars, as will be addressed in subsequent papers.Comment: 11 pages with 10 figure
Double-impulse magnetic focusing of launched cold atoms.
We have theoretically investigated three-dimensional focusing of a launched cloud of cold atoms using a pair of magnetic lens pulses (the alternate-gradient method). Individual lenses focus radially and defocus axially or vice versa. The performance of the two possible pulse sequences are compared and found to be ideal for loading both 'pancake' and 'sausage' shaped magnetic/optical microtraps. It is shown that focusing aberrations are considerably smaller for double-impulse magnetic lenses compared to single-impulse magnetic lenses. An analysis of clouds focused by the double-impulse technique is presented
Current status of MCNP6 as a simulation tool useful for space and accelerator applications
For the past several years, a major effort has been undertaken at Los Alamos
National Laboratory (LANL) to develop the transport code MCNP6, the latest LANL
Monte-Carlo transport code representing a merger and improvement of MCNP5 and
MCNPX. We emphasize a description of the latest developments of MCNP6 at higher
energies to improve its reliability in calculating rare-isotope production,
high-energy cumulative particle production, and a gamut of reactions important
for space-radiation shielding, cosmic-ray propagation, and accelerator
applications. We present several examples of validation and verification of
MCNP6 compared to a wide variety of intermediate- and high-energy experimental
data on reactions induced by photons, mesons, nucleons, and nuclei at energies
from tens of MeV to about 1 TeV/nucleon, and compare to results from other
modern simulation tools.Comment: 4 pages, 3 figures, Proc. 11th Conference on the Intersections of
Particle and Nuclear Physics (CIPANP 2012), St. Petersburg, FL, May 28 - June
3, 201
Nonlinear photon transport in a semiconductor waveguide-cavity system containing a single quantum dot: Anharmonic cavity-QED regime
We present a semiconductor master equation technique to study the
input/output characteristics of coherent photon transport in a semiconductor
waveguide-cavity system containing a single quantum dot. We use this approach
to investigate the effects of photon propagation and anharmonic cavity-QED for
various dot-cavity interaction strengths, including weakly-coupled,
intermediately-coupled, and strongly-coupled regimes. We demonstrate that for
mean photon numbers much less than 0.1, the commonly adopted weak excitation
(single quantum) approximation breaks down, even in the weak coupling regime.
As a measure of the anharmonic multiphoton-correlations, we compute the Fano
factor and the correlation error associated with making a semiclassical
approximation. We also explore the role of electron--acoustic-phonon scattering
and find that phonon-mediated scattering plays a qualitatively important role
on the light propagation characteristics. As an application of the theory, we
simulate a conditional phase gate at a phonon bath temperature of K in the
strong coupling regime.Comment: To appear in PR
Phonon-dressed Mollow triplet in the regime of cavity-QED
We study the resonance fluorescence spectra of a driven quantum dot placed
inside a high semiconductor cavity and interacting with an acoustic phonon
bath. The dynamics is calculated using a time-convolutionless master equation
obtained in the polaron frame. We demonstrate pronounced spectral broadening of
the Mollow sidebands through cavity-emission which, for small cavity-coupling
rates, increases quadratically with the Rabi frequency. However, for larger
cavity coupling rates, this broadening dependence is found to be more complex.
This field-dependent Mollow triplet broadening is primarily a consequence of
the triplet peaks sampling different parts of the asymmetric phonon bath, and
agrees directly with recent experiments with semiconductor micropillars. The
influence from the detuned cavity photon bath and multi-photon effects is shown
to play a qualitatively important role on the fluorescence spectra.Comment: 4 pages, 4 figure
Multiple and variable speed electrical generator systems for large wind turbines
A cost effective method to achieve increased wind turbine generator energy conversion and other operational benefits through variable speed operation is presented. Earlier studies of multiple and variable speed generators in wind turbines were extended for evaluation in the context of a specific large sized conceptual design. System design and simulation have defined the costs and performance benefits which can be expected from both two speed and variable speed configurations
Quantum non-Gaussianity witnesses in the phase space
We address detection of quantum non-Gaussian states, i.e. nonclassical states
that cannot be expressed as a convex mixture of Gaussian states, and present a
method to derive a new family of criteria based on generic linear functionals.
We then specialise this method to derive witnesses based on -parametrized
quasiprobability functions, generalising previous criteria based on the Wigner
function. In particular we discuss in detail and analyse the properties of
Husimi Q-function based witnesses and prove that they are often more effective
than previous criteria in detecting quantum non-Gaussianity of various kinds of
non-Gaussian states evolving in a lossy channel.Comment: 9 pages, 6 figure
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