462 research outputs found
Convergence of dependent walks in a random scenery to fBm-local time fractional stable motions
It is classical to approximate the distribution of fractional Brownian motion
by a renormalized sum of dependent Gaussian random variables. In this
paper we consider such a walk that collects random rewards
for when the ceiling of the walk is located at The random reward (or scenery) is independent of the walk and
with heavy tail. We show the convergence of the sum of independent copies of suitably renormalized to a stable motion with integral representation,
whose kernel is the local time of a fractional Brownian motion (fBm). This work
extends a previous work where the random walk had independent increments
limits
Trouver du sens aux apprentissages :: les Ă©lĂšves face au savoir
La thĂ©matique du sens donnĂ© aux apprentissages est devenue, aujourdâhui « un lieu commun pavĂ© des meilleures intentions du monde » (Meirieu, 2016). Cependant, cet auteur relĂšve que « donner du sens » nâest pas une opĂ©ration facile, voire mĂ©canique, qui serait susceptible d'une systĂ©matisation grĂące Ă des « techniques » didactiques Ă©prouvĂ©es. De ce fait, certains Ă©lĂšves ne savent pas toujours ce quâils viennent faire Ă lâĂ©cole et nombre dâenseignants sont contraints de constater chez ces derniers, une forme de passivitĂ©, un manque de motivation, dâefforts ou de respect pour le travail accompli. En rĂ©action, je cherche en tant que future enseignante, Ă comprendre comment les Ă©lĂšves perçoivent le sens des apprentissages scolaires. Develay (1996) prĂ©tend quâil y a apprentissage, lorsque le sujet trouve du sens dans la situation dâenseignement. Pour la construction de mon mĂ©moire, je vais mâappuyer sur cette thĂ©orie et mâintĂ©resser au sens que les Ă©lĂšves trouvent Ă leur apprentissage. Pour ce faire, je souhaite mettre en place au cours de ma pratique professionnelle, un dispositif pĂ©dagogique qui dĂ©finit clairement les apprentissages et les envisage comme une source de progrĂšs. Mon objectif final sera de dĂ©celer si les Ă©lĂšves ont compris la matiĂšre enseignĂ©e et tenter de percevoir si ce dispositif leur a permis de lui donner un sens
Real-time quantum feedback prepares and stabilizes photon number states
Feedback loops are at the heart of most classical control procedures. A
controller compares the signal measured by a sensor with the target value. It
adjusts then an actuator in order to stabilize the signal towards its target.
Generalizing this scheme to stabilize a micro-system's quantum state relies on
quantum feedback, which must overcome a fundamental difficulty: the
measurements by the sensor have a random back-action on the system. An optimal
compromise employs weak measurements providing partial information with minimal
perturbation. The controller should include the effect of this perturbation in
the computation of the actuator's unitary operation bringing the incrementally
perturbed state closer to the target. While some aspects of this scenario have
been experimentally demonstrated for the control of quantum or classical
micro-system variables, continuous feedback loop operations permanently
stabilizing quantum systems around a target state have not yet been realized.
We have implemented such a real-time stabilizing quantum feedback scheme. It
prepares on demand photon number states (Fock states) of a microwave field in a
superconducting cavity and subsequently reverses the effects of
decoherence-induced field quantum jumps. The sensor is a beam of atoms crossing
the cavity which repeatedly performs weak quantum non-demolition measurements
of the photon number. The controller is implemented in a real-time computer
commanding the injection, between measurements, of adjusted small classical
fields in the cavity. The microwave field is a quantum oscillator usable as a
quantum memory or as a quantum bus swapping information between atoms. By
demonstrating that active control can generate non-classical states of this
oscillator and combat their decoherence, this experiment is a significant step
towards the implementation of complex quantum information operations.Comment: 12 pages, 4 figure
Towards quantum simulation with circular Rydberg atoms
The main objective of quantum simulation is an in-depth understanding of
many-body physics. It is important for fundamental issues (quantum phase
transitions, transport, . . . ) and for the development of innovative
materials. Analytic approaches to many-body systems are limited and the huge
size of their Hilbert space makes numerical simulations on classical computers
intractable. A quantum simulator avoids these limitations by transcribing the
system of interest into another, with the same dynamics but with interaction
parameters under control and with experimental access to all relevant
observables. Quantum simulation of spin systems is being explored with trapped
ions, neutral atoms and superconducting devices. We propose here a new paradigm
for quantum simulation of spin-1/2 arrays providing unprecedented flexibility
and allowing one to explore domains beyond the reach of other platforms. It is
based on laser-trapped circular Rydberg atoms. Their long intrinsic lifetimes
combined with the inhibition of their microwave spontaneous emission and their
low sensitivity to collisions and photoionization make trapping lifetimes in
the minute range realistic with state-of-the-art techniques. Ultra-cold
defect-free circular atom chains can be prepared by a variant of the
evaporative cooling method. This method also leads to the individual detection
of arbitrary spin observables. The proposed simulator realizes an XXZ spin-1/2
Hamiltonian with nearest-neighbor couplings ranging from a few to tens of kHz.
All the model parameters can be tuned at will, making a large range of
simulations accessible. The system evolution can be followed over times in the
range of seconds, long enough to be relevant for ground-state adiabatic
preparation and for the study of thermalization, disorder or Floquet time
crystals. This platform presents unrivaled features for quantum simulation
Phase Diversity Electro-optic Sampling: A new approach to single-shot terahertz waveform recording
THz spectroscopy is an emerging tool for detection of microorganisms and
harmful compounds in the food industry, the study of proteins in biomedicine
and the development of electron-beam X-ray sources for molecular imaging and
lithography. Recording of THz electric field evolution in single-shot is
crucially needed in terahertz spectroscopy of irreversible processes in such
applications as well as for data communication in the THz portion of the
spectrum where there is an abundance of untapped bandwidth. However, achieving
sub-picosecond resolution over a long time window has been an open problem for
electro-optic sampling -- the standard technique for recording terahertz
waveforms. We introduce a new conceptual framework for this open problem that
is inspired by time-stretch theory. The novel framework unveils a solution to
this 20 year-old problem leading to a dramatic enhancement of the achievable
temporal resolution. We validate this new technology in two applications.
First, we present single shot recordings of long free-propagating terahertz
transients with record time resolution. Second, we present recordings of
ultra-short relativistic electron bunches at the European X-ray Free Electron
Laser. These results show that electric signals may be now recorded with
terahertz bandwidth over arbitrarily long windows, thus enabling the
realization of "single-shot terahertz oscilloscopes" and single-shot
time-domain spectroscopy systems with an arbitrary time-bandwidth product
Development of a 3D model of clinically relevant microcalcifications
A realistic 3D anthropomorphic software model of microcalcifications may
serve as a useful tool to assess the performance of breast imaging applications
through simulations. We present a method allowing to simulate visually
realistic microcalcifications with large morphological variability. Principal
component analysis (PCA) was used to analyze the shape of 281 biopsied
microcalcifications imaged with a micro-CT. The PCA analysis requires the same
number of shape components for each input microcalcification. Therefore, the
voxel-based microcalcifications were converted to a surface mesh with same
number of vertices using a marching cube algorithm. The vertices were
registered using an iterative closest point algorithm and a simulated annealing
algorithm. To evaluate the approach, input microcalcifications were
reconstructed by progressively adding principal components. Input and
reconstructed microcalcifications were visually and quantitatively compared.
New microcalcifications were simulated using randomly sampled principal
components determined from the PCA applied to the input microcalcifications,
and their realism was appreciated through visual assessment. Preliminary
results have shown that input microcalcifications can be reconstructed with
high visual fidelity when using 62 principal components, representing 99.5%
variance. For that condition, the average L2 norm and dice coefficient were
respectively 10.5 m and 0.93. Newly generated microcalcifications with 62
principal components were found to be visually similar, while not identical, to
input microcalcifications. The proposed PCA model of microcalcification shapes
allows to successfully reconstruct input microcalcifications and to generate
new visually realistic microcalcifications with various morphologies
Zip Nucleic Acids: new high affinity oligonucleotides as potent primers for PCR and reverse transcription
Most nucleic acid-based technologies rely upon sequence recognition between an oligonucleotide and its nucleic acid target. With the aim of improving hybridization by decreasing electrostatic repulsions between the negatively charged strands, novel modified oligonucleotides named Zip nucleic acids (ZNAs) were recently developed. ZNAs are oligonucleotideâoligocation conjugates whose global charge is modulated by the number of cationic spermine moieties grafted on the oligonucleotide. It was demonstrated that the melting temperature of a hybridized ZNA is easily predictable and increases linearly with the length of the oligocation. Furthermore, ZNAs retain the ability to discriminate between a perfect match and a single base-pair-mismatched complementary sequence. Using quantitative PCR, we show here that ZNAs are specific and efficient primers displaying an outstanding affinity toward their genomic target. ZNAs are particularly efficient at low magnesium concentration, low primer concentrations and high annealing temperatures, allowing to improve the amplification in AT-rich sequences and potentially multiplex PCR applications. In reverse transcription experiments, ZNA gene-specific primers improve the yield of cDNA synthesis, thus increasing the accuracy of detection, especially for genes expressed at low levels. Our data suggest that ZNAs exhibit faster binding kinetics than standard and locked nucleic acid-containing primers, which could explain why their target recognition is better for rare targets
Novel graphene electrode for retinal implants : an in vivo biocompatibility study
Altres ajuts: this work has made use of the Spanish ICTS Network MICRONANOFABS partially supported by MICINN and the ICTS 'NANBIOSIS'.Evaluating biocompatibility is a core essential step to introducing a new material as a candidate for brain-machine interfaces. Foreign body reactions often result in glial scars that can impede the performance of the interface. Having a high conductivity and large electrochemical window, graphene is a candidate material for electrical stimulation with retinal prosthesis. In this study, non-functional devices consisting of chemical vapor deposition (CVD) graphene embedded onto polyimide/SU-8 substrates were fabricated for a biocompatibility study. The devices were implanted beneath the retina of blind P23H rats. Implants were monitored by optical coherence tomography (OCT) and eye fundus which indicated a high stability in vivo up to 3 months before histology studies were done. Microglial reconstruction through confocal imaging illustrates that the presence of graphene on polyimide reduced the number of microglial cells in the retina compared to polyimide alone, thereby indicating a high biocompatibility. This study highlights an interesting approach to assess material biocompatibility in a tissue model of central nervous system, the retina, which is easily accessed optically and surgically
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