255 research outputs found
Topologically decoherence-protected qubits with trapped ions
We show that trapped ions can be used to simulate a highly symmetrical
Hamiltonian with eingenstates naturally protected against local sources of
decoherence. This Hamiltonian involves long range coupling between particles
and provides a more efficient protection than nearest neighbor models discussed
in previous works. Our results open the perspective of experimentally realizing
in controlled atomic systems, complex entangled states with decoherence times
up to nine orders of magnitude longer than isolated quantum systems.Comment: 4 page
Caractérisation technologique et valorisation en bois d' uvre du pin d'Alep de la région Provence-Alpes-Côte d'Azur -
Actuellement, la ressource bois est peu exploitée dans nos régions méditerranéennes, alors que les bois de la région possèdent des potentialités technologiques certaines. Dans cet article, les auteurs nous montrent que le bois de pin d'Alep, bien que souvent déprécié, présente des caractéristiques physiques et mécaniques satisfaisantes pour une utilisation en menuiserie ou en charpente, voire en bois d'industri
Recoil-Induced-Resonances in Nonlinear, Ground-State, Pump-Probe Spectroscopy
A theory of pump-probe spectroscopy is developed in which optical fields
drive two-photon Raman transitions between ground states of an ensemble of
three-level atoms. Effects related to the recoil the atoms undergo
as a result of their interactions with the fields are fully accounted for in
this theory. The linear absorption coefficient of a weak probe field in the
presence of two pump fields of arbitrary strength is calculated. For subrecoil
cooled atoms, the spectrum consists of eight absorption lines and eight
emission lines. In the limit that , where and
are the Rabi frequencies of the two pump fields, one recovers the
absorption spectrum for a probe field interacting with an effective two-level
atom in the presence of a single pump field. However when , new interference effects arise that allow one to selectively turn on
and off some of these recoil induced resonances.Comment: 30 pages, 8 figures. RevTex. Submitted to Phys. Rev. A, Revised
versio
Comparison of Recoil-Induced Resonances (RIR) and Collective Atomic Recoil Laser (CARL)
The theories of recoil-induced resonances (RIR) [J. Guo, P. R. Berman, B.
Dubetsky and G. Grynberg, Phys. Rev. A {\bf 46}, 1426 (1992)] and the
collective atomic recoil laser (CARL) [ R. Bonifacio and L. De Salvo, Nucl.
Instrum. Methods A {\bf 341}, 360 (1994)] are compared. Both theories can be
used to derive expressions for the gain experienced by a probe field
interacting with an ensemble of two-level atoms that are simultaneously driven
by a pump field. It is shown that the RIR and CARL formalisms are equivalent.
Differences between the RIR and CARL arise because the theories are typically
applied for different ranges of the parameters appearing in the theory. The RIR
limit considered in this paper is , while the CARL
limit is , where is the magnitude of the
difference of the wave vectors of the pump and probe fields, is the
width of the atomic momentum distribution and is a recoil
frequency. The probe gain for a probe-pump detuning equal to zero is analyzed
in some detail, in order to understand how the gain arises in a system which,
at first glance, might appear to have vanishing gain. Moreover, it is shown
that the calculations, carried out in perturbation theory have a range of
applicability beyond the recoil problem. Experimental possibilities for
observing CARL are discussed.Comment: 16 pages, 1 figure. Submitted to Physical Review
Density, extractives and decay resistance variabilities within branch wood from four agroforestry hardwood species
Agroforestry practices like pruning trees to control the light flux to crops produce every year a large volume of branches which is valorized by farmers as mulching or energy fuel. However, according to the literature, the wood of branches shows higher rates of polyphenols than stem wood and this can open some new perspectives for branch exploitation. In this study, the wood properties (density, mechanical properties, extractive content and decay resistance) were determined on branches of different sizes from oak, chestnut, poplar and walnut trees collected in two agroforestry systems. These properties were evaluated according to the wood age and the sampling position along the radial and longitudinal axes of the branch. All samples were analyzed by NIR-Spectroscopy and a predicting model aimed to assess the branch wood properties has been developed. Wood characteristics largely vary between species and do not exactly follow the same trends from one species to another. Overall, hardwood density of branches is similar to that of trunks, the content in wood extractives follows similar evolutions, and the decay resistance of branch wood does not seem to be really impacted by its position along the branch. Reliable NIRS models were built to easily predict the wood density and extractives content of agroforestry branches. The extractives content and the decay resistance of branch hardwood appear to be substantially lower than those of trunks, which suggests a non-suitability of branch wood for developing highvalued green chemistry
Transport of small anionic and neutral solutes through chitosan membranes: Dependence on cross-linking and chelation of divalent cations
Chitosan membranes were prepared by solvent casting and cross-linked with glutaraldehyde at several ratios
under homogeneous conditions. The cross-linking degree, varying from 0 to 20%, is defined as the ratio between
the total aldehyde groups and the amine groups of chitosan. Permeability experiments were conducted using a
side-by-side diffusion cell to determine the flux of small molecules of similar size but with different chemical
moieties, either ionized (benzoic acid, salicylic acid, and phthalic acid) or neutral (2-phenylethanol) at physiological
pH. The permeability of the different model molecules revealed to be dependent on the affinity of those structurally
similar molecules to chitosan. The permeability of the salicylate anion was significantly enhanced by the presence
of metal cations commonly present in biological fluids, such as calcium and magnesium, but remained unchanged
for the neutral 2-phenylethanol. This effect could be explained by the chelation of metal cations on the amine
groups of chitosan, which increased the partition coefficient. The cross-linking degree was also correlated with
the permeability and partition coefficient. The change in the permeation properties of chitosan to anionic solutes
in the presence of these metallic cations is an important result and should be taken into consideration when trying
to make in vitro predictions of the drug release from chitosan-based controlled release systems
Characterisation and categorisation of the diversity in viscoelastic vibrational properties between 98 wood types
International audienceContext Increased knowledge on diversity in wood properties would have implications both for fundamental research and for promoting a diversification of uses as material. *Aims The objective is to contribute to overcoming the critical lack of data on the diversity of wood dynamic mechanical/viscoelastic vibrational properties, by testing lesser-known species and categorizing sources of variability. *Methods Air-dry axial specific dynamic modulus of elasticity (E'/γ) and damping coefficient (tanδ) were measured on a wide sampling (1792 specimens) of 98 wood types from 79 species. An experimental device and protocol was designed for conducting systematic (i.e. rapid and reproducible) characterizations. *Results Diversity at the specimens' level corroborates the "standard" relationship between tanδ and E'/γ, which is discussed in terms of orientation of wood elements and of chemical composition. Diversity at the species level is expressed on the basis of results for normal heartwood, with specific gravity (γ) ranging from 0.2 to 1.3. Axial E'/γ ranges from 9 to 32 GPa and tanδ from 4×10-3 to 19×10-3. Properties distribution follows a continuum, but with group characteristics. The lowest values of tanδ are only found in certain tropical hardwoods. Results can also suggest alternative species for musical instruments making
Cyclic AMP Responsive Element Binding Proteins Are Involved in ‘Emergency’ Granulopoiesis through the Upregulation of CCAAT/Enhancer Binding Protein β
In contrast to the definitive role of the transcription factor, CCAAT/Enhancer binding protein α (C/EBPα), in steady-state granulopoiesis, previous findings have suggested that granulopoiesis during emergency situations, such as infection, is dependent on C/EBPβ. In this study, a novel lentivirus-based reporter system was developed to elucidate the molecular switch required for C/EBPβ-dependency. The results demonstrated that two cyclic AMP responsive elements (CREs) in the proximal promoter region of C/EBPβ were involved in the positive regulation of C/EBPβ transcription during granulocyte-macrophage colony-stimulating factor (GM-CSF)–induced differentiation of bone marrow cells. In addition, the transcripts of CRE binding (CREB) family proteins were readily detected in hematopoietic stem/progenitor cells. CREB was upregulated, phosphorylated and bound to the CREs in response to GM-CSF stimulation. Retroviral transduction of a dominant negative CREB mutant reduced C/EBPβ mRNA levels and significantly impaired the proliferation/differentiation of granulocyte precursors, while a constitutively active form of CREB facilitated C/EBPβ transcription. These data suggest that CREB proteins are involved in the regulation of granulopoiesis via C/EBPβ upregulation
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