3,986 research outputs found
Classification in postural style
This article contributes to the search for a notion of postural style,
focusing on the issue of classifying subjects in terms of how they maintain
posture. Longer term, the hope is to make it possible to determine on a case by
case basis which sensorial information is prevalent in postural control, and to
improve/adapt protocols for functional rehabilitation among those who show
deficits in maintaining posture, typically seniors. Here, we specifically
tackle the statistical problem of classifying subjects sampled from a two-class
population. Each subject (enrolled in a cohort of 54 participants) undergoes
four experimental protocols which are designed to evaluate potential deficits
in maintaining posture. These protocols result in four complex trajectories,
from which we can extract four small-dimensional summary measures. Because
undergoing several protocols can be unpleasant, and sometimes painful, we try
to limit the number of protocols needed for the classification. Therefore, we
first rank the protocols by decreasing order of relevance, then we derive four
plug-in classifiers which involve the best (i.e., more informative), the two
best, the three best and all four protocols. This two-step procedure relies on
the cutting-edge methodologies of targeted maximum likelihood learning (a
methodology for robust and efficient inference) and super-learning (a machine
learning procedure for aggregating various estimation procedures into a single
better estimation procedure). A simulation study is carried out. The
performances of the procedure applied to the real data set (and evaluated by
the leave-one-out rule) go as high as an 87% rate of correct classification (47
out of 54 subjects correctly classified), using only the best protocol.Comment: Published in at http://dx.doi.org/10.1214/12-AOAS542 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Spikes in quantum trajectories
A quantum system subjected to a strong continuous monitoring undergoes
quantum jumps. This very well known fact hides a neglected subtlety: sharp
scale-invariant fluctuations invariably decorate the jump process even in the
limit where the measurement rate is very large. This article is devoted to the
quantitative study of these remaining fluctuations, which we call spikes, and
to a discussion of their physical status. We start by introducing a classical
model where the origin of these fluctuations is more intuitive and then jump to
the quantum realm where their existence is less intuitive. We compute the exact
distribution of the spikes for a continuously monitored qubit. We conclude by
discussing their physical and operational relevance.Comment: 8 pages, 8 figure
Zooming in on Quantum Trajectories
We propose to use the effect of measurements instead of their number to study
the time evolution of quantum systems under monitoring. This time redefinition
acts like a microscope which blows up the inner details of seemingly
instantaneous transitions like quantum jumps. In the simple example of a
continuously monitored qubit coupled to a heat bath, we show that this
procedure provides well defined and simple evolution equations in an otherwise
singular strong monitoring limit. We show that there exists anomalous
observable localised on sharp transitions which can only be resolved with our
new effective time. We apply our simplified description to study the
competition between information extraction and dissipation in the evolution of
the linear entropy. Finally, we show that the evolution of the new time as a
function of the real time is closely related to a stable Levy process of index
1/2.Comment: 5 pages, 2 figure
Estimation of the Brownian dimension of a continuous It\^{o} process
In this paper, we consider a -dimensional continuous It\^{o} process which
is observed at regularly spaced times on a given time interval .
This process is driven by a multidimensional Wiener process and our aim is to
provide asymptotic statistical procedures which give the minimal dimension of
the driving Wiener process, which is between 0 (a pure drift) and . We
exhibit several different procedures, all similar to asymptotic testing
hypotheses.Comment: Published in at http://dx.doi.org/10.3150/07-BEJ6190 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Computing the Rates of Measurement-Induced Quantum Jumps
Small quantum systems can now be continuously monitored experimentally which
allows for the reconstruction of quantum trajectories. A peculiar feature of
these trajectories is the emergence of jumps between the eigenstates of the
observable which is measured. Using the Stochastic Master Equation (SME)
formalism for continuous quantum measurements, we show that the density matrix
of a system indeed shows a jumpy behavior when it is subjected to a tight
measurement (even if the noise in the SME is Gaussian). We are able to compute
the jump rates analytically for any system evolution, i.e. any Lindbladian, and
we illustrate how our general recipe can be applied to two simple examples. We
then discuss the mathematical, foundational and practical applications of our
results. The analysis we present is based on a study of the strong noise limit
of a class of stochastic differential equations (the SME) and as such the
method may be applicable to other physical situations in which a strong noise
limit plays a role.Comment: 9 pages, 2 figures, close to the published version. The text has been
profoundly rewritten. The concept of "quantum spikes" is no longer discussed
and will be studied in a subsequent articl
Temporal ghost imaging with twin photons
We use twin photons generated by spontaneous parametric down conversion to perform temporal ghost imaging of a single time signal. The retrieval of a binary signal containing eight bits is performed with an error rate below 1%
From bare interactions, low--energy constants and unitary gas to nuclear density functionals without free parameters: application to neutron matter
We further progress along the line of Ref. [Phys. Rev. {\bf A 94}, 043614
(2016)] where a functional for Fermi systems with anomalously large -wave
scattering length was proposed that has no free parameters. The
functional is designed to correctly reproduce the unitary limit in Fermi gases
together with the leading-order contributions in the s- and p-wave channels at
low density. The functional is shown to be predictive up to densities
fm that is much higher densities compared to the Lee-Yang
functional, valid for fm. The form of the functional
retained in this work is further motivated. It is shown that the new functional
corresponds to an expansion of the energy in and to all
orders, where is the effective range and is the Fermi momentum. One
conclusion from the present work is that, except in the extremely low--density
regime, nuclear systems can be treated perturbatively in with
respect to the unitary limit. Starting from the functional, we introduce
density--dependent scales and show that scales associated to the bare
interaction are strongly renormalized by medium effects. As a consequence, some
of the scales at play around saturation are dominated by the unitary gas
properties and not directly to low-energy constants. For instance, we show that
the scale in the s-wave channel around saturation is proportional to the
so-called Bertsch parameter and becomes independent of . We also
point out that these scales are of the same order of magnitude than those
empirically obtained in the Skyrme energy density functional. We finally
propose a slight modification of the functional such that it becomes accurate
up to the saturation density fm
Computational temporal ghost imaging
Ghost imaging is a fascinating process, where light interacting with an
object is recorded without resolution, but the shape of the object is
nevertheless retrieved, thanks to quantum or classical correlations of this
interacting light with either a computed or detected random signal. Recently,
ghost imaging has been extended to a time object, by using several thousands
copies of this periodic object. Here, we present a very simple device, inspired
by computational ghost imaging, that allows the retrieval of a single
non-reproducible, periodic or non-periodic, temporal signal. The reconstruction
is performed by a single shot, spatially multiplexed, measurement of the
spatial intensity correlations between computer-generated random images and the
images, modulated by a temporal signal, recorded and summed on a chip CMOS
camera used with no temporal resolution. Our device allows the reconstruction
of either a single temporal signal with monochrome images or
wavelength-multiplexed signals with color images
Emergence of macroscopic directed motion in populations of motile colloids
From the formation of animal flocks to the emergence of coordinate motion in
bacterial swarms, at all scales populations of motile organisms display
coherent collective motion. This consistent behavior strongly contrasts with
the difference in communication abilities between the individuals. Guided by
this universal feature, physicists have proposed that solely alignment rules at
the individual level could account for the emergence of unidirectional motion
at the group level. This hypothesis has been supported by agent-based
simulations. However, more complex collective behaviors have been
systematically found in experiments including the formation of vortices,
fluctuating swarms, clustering and swirling. All these model systems
predominantly rely on actual collisions to display collective motion. As a
result, the potential local alignment rules are entangled with more complex,
often unknown, interactions. The large-scale behavior of the populations
therefore depends on these uncontrolled microscopic couplings. Here, we
demonstrate a new phase of active matter. We reveal that dilute populations of
millions of colloidal rollers self-organize to achieve coherent motion along a
unique direction, with very few density and velocity fluctuations. Identifying
the microscopic interactions between the rollers allows a theoretical
description of this polar-liquid state. Comparison of the theory with
experiment suggests that hydrodynamic interactions promote the emergence of
collective motion either in the form of a single macroscopic flock at low
densities, or in that of a homogenous polar phase at higher densities.
Furthermore, hydrodynamics protects the polar-liquid state from the giant
density fluctuations. Our experiments demonstrate that genuine physical
interactions at the individual level are sufficient to set homogeneous active
populations into stable directed motion
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