1,945 research outputs found
Quantum limits in interferometric measurements
Quantum noise limits the sensitivity of interferometric measurements. It is
generally admitted that it leads to an ultimate sensitivity, the ``standard
quantum limit''. Using a semi-classical analysis of quantum noise, we show that
a judicious use of squeezed states allows one in principle to push the
sensitivity beyond this limit. This general method could be applied to large
scale interferometers designed for gravitational wave detection.Comment: 4 page
Casimir-Polder shifts on quantum levitation states
An ultracold atom above a horizontal mirror experiences quantum reflection
from the attractive Casimir-Polder interaction, which holds it against gravity
and leads to quantum levitation states. We analyze this system by using a
Liouville transformation of the Schr\"odinger equation and a Langer coordinate
adapted to problems with a classical turning point. Reflection on the
Casimir-Polder attractive well is replaced by reflection on a repulsive wall
and the problem is then viewed as an ultracold atom trapped inside a cavity
with gravity and Casimir-Polder potentials acting respectively as top and
bottom mirrors. We calculate numerically Casimir-Polder shifts of the energies
of the cavity resonances and propose a new approximate treatment which is
precise enough to discuss spectroscopy experiments aiming at tests of the weak
equivalence principle on antihydrogen. We also discuss the lifetimes by
calculating complex energies associated with cavity resonances.Comment: Accepted in PR
A multi-channel optical plug-in module for gigabit data reception
A plug-in module has been built for reception of optically transmitted data by gigabit applications. The optical receiving module is based on a 12-channel optical receiver and an FPGA with embedded high-speed deserializers. It is compatible with the serializer ASIC used by many LHC systems. Due to its compact design, several of these modules could be plugged into VME readout systems. This module will be the principle element for both the CMS Preshower data concentrator card and the TOTEM front-end driver
Group Analysis of Self-organizing Maps based on Functional MRI using Restricted Frechet Means
Studies of functional MRI data are increasingly concerned with the estimation
of differences in spatio-temporal networks across groups of subjects or
experimental conditions. Unsupervised clustering and independent component
analysis (ICA) have been used to identify such spatio-temporal networks. While
these approaches have been useful for estimating these networks at the
subject-level, comparisons over groups or experimental conditions require
further methodological development. In this paper, we tackle this problem by
showing how self-organizing maps (SOMs) can be compared within a Frechean
inferential framework. Here, we summarize the mean SOM in each group as a
Frechet mean with respect to a metric on the space of SOMs. We consider the use
of different metrics, and introduce two extensions of the classical sum of
minimum distance (SMD) between two SOMs, which take into account the
spatio-temporal pattern of the fMRI data. The validity of these methods is
illustrated on synthetic data. Through these simulations, we show that the
three metrics of interest behave as expected, in the sense that the ones
capturing temporal, spatial and spatio-temporal aspects of the SOMs are more
likely to reach significance under simulated scenarios characterized by
temporal, spatial and spatio-temporal differences, respectively. In addition, a
re-analysis of a classical experiment on visually-triggered emotions
demonstrates the usefulness of this methodology. In this study, the
multivariate functional patterns typical of the subjects exposed to pleasant
and unpleasant stimuli are found to be more similar than the ones of the
subjects exposed to emotionally neutral stimuli. Taken together, these results
indicate that our proposed methods can cast new light on existing data by
adopting a global analytical perspective on functional MRI paradigms.Comment: 23 pages, 5 figures, 4 tables. Submitted to Neuroimag
Roughness correction to the Casimir force : Beyond the Proximity Force Approximation
We calculate the roughness correction to the Casimir effect in the parallel
plates geometry for metallic plates described by the plasma model. The
calculation is perturbative in the roughness amplitude with arbitrary values
for the plasma wavelength, the plate separation and the roughness correlation
length. The correction is found to be always larger than the result obtained in
the Proximity Force Approximation.Comment: 7 pages, 3 figures, v2 with minor change
Casimir torque between corrugated metallic plates
We consider two parallel corrugated plates and show that a Casimir torque
arises when the corrugation directions are not aligned. We follow the
scattering approach and calculate the Casimir energy up to second order in the
corrugation amplitudes, taking into account nonspecular reflections,
polarization mixing and the finite conductivity of the metals. We compare our
results with the proximity force approximation, which overestimates the torque
by a factor 2 when taking the conditions that optimize the effect. We argue
that the Casimir torque could be measured for separation distances as large as
1 Comment: 7 pages, 3 figures, contribution to QFEXT07 proceeding
Renormalon disappearance in Borel sum of the 1/N expansion of the Gross-Neveu model mass gap
The exact mass gap of the O(N) Gross-Neveu model is known, for arbitrary ,
from non-perturbative methods. However, a "naive" perturbative expansion of the
pole mass exhibits an infinite set of infrared renormalons at order 1/N,
formally similar to the QCD heavy quark pole mass renormalons, potentially
leading to large perturbative ambiguities. We examine the
precise vanishing mechanism of such infrared renormalons, which avoids this
(only apparent)contradiction, and operates without need of (Borel) summation
contour prescription, usually preventing unambiguous separation of perturbative
contributions. As a consequence we stress the direct Borel summability of the
(1/N) perturbative expansion of the mass gap. We briefly speculate on a
possible similar behaviour of analogous non-perturbative QCD quantities.Comment: 16 pp., 1 figure. v2: a few paragraphs and one appendix added, title
and abstract slightly changed, essential results unchange
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