264 research outputs found
La construction de l'insertion socioprofessionnelle des jeunes Ă l'Ă©preuve du temps : une enquĂȘte longitudinale: Rapport final
La recherche dont nous rendons compte ici est une recherche au long cours, qui s'inscrit dans la durĂ©e et procĂšde par enquĂȘtes rĂ©pĂ©tĂ©es. Il convient donc dans un premier temps de rappeler quelques Ă©lĂ©ments de sa construction initiale, qui ont Ă©tĂ© prĂ©sentĂ©s plus longuement dans le premier rapport de recherche rĂ©digĂ© en 20001. Certains de ces Ă©lĂ©ments ne bougent pas, restent stables et permettent la comparaison ; d'autres Ă©voluent dans le temps, au fur et Ă mesure que se construisent les trajectoires d'insertion. Nous commencerons donc ici par rappeler les principes de base, les hypothĂšses et les choix mĂ©thodologiques effectuĂ©s dĂšs le dĂ©but de cette entreprise. Nous ferons ensuite le point sur la mĂ©thodologie de l'enquĂȘte et sur l'Ă©volution de la population du panel, lors des trois vagues d'enquĂȘte qui ont dĂ©jĂ Ă©tĂ© rĂ©alisĂ©es. Nous prĂ©ciserons ensuite les questions traitĂ©es ici, autour du thĂšme central des Ă©volutions des façons dont les jeunes abordent le monde du travail. Entrant ensuite dans la comparaison temporelle proprement dite, nous prĂ©senterons et discuterons des mouvements qui se sont produits dans la rĂ©partition des jeunes dans des catĂ©gories de trajectoires. Celles-ci avaient Ă©tĂ© identifiĂ©es il y a trois ans. Nous les reprenons pour voir comment les jeunes s'y situent aujourd'hui. Nous prĂ©ciserons ensuite plus rapidement quelques points-clĂ©s de ces Ă©volutions : les bifurcations biographiques et la notion de projet, l'Ă©volution des rĂ©seaux de relations personnelles au moment de l'entrĂ©e dans la vie professionnelle, l'utilisation Ă©ventuelle de ces relations pour trouver du travail, les autres dimensions de l'entrĂ©e dans la vie adulte et les trajectoires de dĂ©cohabitation parentale, pour discuter enfin du rĂŽle relatif du travail dans l'accĂšs au statut d'adulte
Les liens invisibles entre agriculture de proximité et commerce alimentaire dit "ethnique"
International audienceLa littĂ©rature scientifique concernant les agricultures de proximitĂ©, i.e. celles qui se pratiquent Ă proximitĂ© des marchĂ©s quâelles desservent, sâest beaucoup renouvelĂ©e ces derniĂšres annĂ©es sous lâeffet dâun intĂ©rĂȘt rĂ©affirmĂ© pour lâagriculture urbaine et pĂ©riurbaine . Si ces Ă©tudes ont surtout Ă©tĂ© attentives aux logiques de reterritorialisation que traduisent ces agricultures, certains auteurs soulignent la nĂ©cessitĂ©, dâune part, de mieux apprĂ©hender la diversitĂ© de ces agricultures de proximitĂ©, de leurs protagonistes, de leurs facteurs dâĂ©volution et, dâautre part, de reconsidĂ©rer leurs transformations du point de vue agricole (gĂ©ographie et sociologie agricoles, agronomie) . Elles appellent de surcroĂźt une mise en tension avec les situations dâinjustice socio-spatiale qui naissent tant de lâĂ©volution des relations villes / campagnes que de la dynamique des systĂšmes agro-alimentaires . De nouvelles formes de vulnĂ©rabilitĂ© se repĂšrent en effet : difficultĂ©s dâaccĂšs Ă une alimentation de qualitĂ© pour les consommateurs ; mise Ă lâĂ©cart dâune partie des consommateurs et des producteurs des nouveaux rĂ©seaux malgrĂ© la proximitĂ© gĂ©ographique ; prĂ©carisation des conditions de travail de certains agriculteurs .La recherche exploratoire dont nous proposons de prĂ©senter les rĂ©sultats dans cette communication se penche sur des pratiques agricoles qui se dĂ©ploient en marge des agricultures de proximitĂ© classiquement Ă©tudiĂ©es et des reprĂ©sentations positives associĂ©es actuellement aux agricultures urbaines et pĂ©riurbaines du point de vue des « populations aisĂ©es et Ă©duquĂ©es » des centres-villes. Elle vient souligner la diversitĂ© de cette nouvelle catĂ©gorie tant en termes dâacteurs que de modes dâexploitation (statut foncier, main dâĆuvre, assolements, commercialisationâŠ.)Un premier travail sur deux paysages dâ« agriculture de filiĂšres longues » (la Plaine de France dans le Val-dâOise et le Comtat Venaissin dans les Bouches-du-RhĂŽne) a ainsi permis de mettre Ă jour lâexistence de certaines formes dâexploitations agricoles directement liĂ©es Ă des marchĂ©s urbains de niche du commerce dit « ethnique » (alimentation mĂ©diterranĂ©enne ou asiatique). Le caractĂšre « invisible » ou cachĂ© de ces formes dâagricultures de proximitĂ© (lieux dâinstallation, parcellaire singulier, statut prĂ©caire, informalitĂ©âŠ) a impliquĂ© de mobiliser plusieurs mĂ©thodes dâinvestigation : exploration empirique, observation participante sur lâexploitation, entretiens semi-directifs et photo-interprĂ©tation, enquĂȘtes sur les marchĂ©s de revente. Les premiers rĂ©sultats obtenus permettent de montrer lâexistence de liens originaux entre le commerce dit « ethnique » implantĂ© dans les centres urbains et la rĂ©organisation spatiale des espaces agricoles de proximitĂ©. Ils apportent aussi des Ă©lĂ©ments sur les diffĂ©rentes trajectoires des travailleurs agricoles au sein de ces rĂ©seaux dâagriculture de proximitĂ©
High throughput methodology for synthesis, screening, and optimization of solid state Lithium ion electrolytes
A study of the lithium ion conductor Li3xLa2/3âxTiO3 solid solution and the surrounding composition space was carried out using a high throughput physical vapor deposition system. An optimum total ionic conductivity value of 5.45 Ă 10â4 S cmâ1 was obtained for the composition Li0.17La0.29Ti0.54 (Li3xLa2/3âxTiO3x = 0.11). This optimum value was calculated using an artificial neural network model based on the empirical data. Due to the large scale of the data set produced and the complexity of synthesis, informatics tools were required to analyze the data. Partition analysis was carried out to determine the synthetic parameters of importance and their threshold values. Multivariate curve resolution and principal component analysis were applied to the diffraction data set. This analysis enabled the construction of phase distribution diagrams, illustrating both the phases obtained and the compositional zones in which they occur. The synthetic technique presented has significant advantages over other thin film and bulk methodologies, in terms of both the compositional range covered and the nature of the materials produce
Early adaptation to oxygen is key to the industrially important traits of Lactococcus lactis ssp. cremoris during milk fermentation
Background: Lactococcus lactis is the most used species in the dairy industry. Its ability to adapt to technological stresses, such as oxidative stress encountered during stirring in the first stages of the cheese-making process, is a key factor to measure its technological performance. This study aimed to understand the response to oxidative stress of Lactococcus lactis subsp. cremoris MG1363 at the transcriptional and metabolic levels in relation to acidification kinetics and growth conditions, especially at an early stage of growth. For those purposes, conditions of hyper-oxygenation were initially fixed for the fermentation. Results: Kinetics of growth and acidification were not affected by the presence of oxygen, indicating a high resistance to oxygen of the L. lactis MG1363 strain. Its resistance was explained by an efficient consumption of oxygen within the first 4 hours of culture, leading to a drop of the redox potential. The efficient consumption of oxygen by the L. lactis MG1363 strain was supported by a coherent and early adaptation to oxygen after 1 hour of culture at both gene expression and metabolic levels. In oxygen metabolism, the over-expression of all the genes of the nrd (ribonucleotide reductases) operon or fhu (ferrichrome ABC transports) genes was particularly significant. In carbon metabolism, the presence of oxygen led to an early shift at the gene level in the pyruvate pathway towards the acetate/2,3-butanediol pathway confirmed by the kinetics of metabolite production. Finally, the MG1363 strain was no longer able to consume oxygen in the stationary growth phase, leading to a drastic loss of culturability as a consequence of cumulative stresses and the absence of gene adaptation at this stage. Conclusions: Combining metabolic and transcriptomic profiling, together with oxygen consumption kinetics, yielded new insights into the whole genome adaptation of L. lactis to initial oxidative stress. An early and transitional adaptation to oxidative stress was revealed for L. lactis subsp. cremoris MG1363 in the presence of initially high levels of oxygen. This enables the cells to maintain key traits that are of great importance for industry, such as rapid acidification and reduction of the redox potential of the growth media
Optical control of the spin of a magnetic atom in a semiconductor quantum dot
International audienceThe control of single spins in solids is a key but challenging step for any spin-based solid-state quantum-computing device. Thanks to their expected long coherence time, localized spins on magnetic atoms in a semiconductor host could be an interesting media to store quantum information in the solid state. Optical probing and control of the spin of individual or pairs of Mn atoms (S=5/2) have been obtained in II-VI and III-V semiconductor quantum dots during the last years. In this paper , we review recently developed optical control experiments of the spin of an individual Mn atoms in II-VI semiconductor self-assembled or strain free quantum dots. We first show that the fine structure of the Mn atom and especially a strained induced magnetic anisotropy is the main parameter controlling the spin memory of the magnetic atom at zero magnetic field. We then demonstrate that the energy of any spin state of a Mn atom or pairs of Mn atom can be independently tuned by using the optical Stark effect induced by a resonant laser field. The strong coupling with the resonant laser field modifies the Mn fine structure and consequently its dynamics. We then describe the spin dynamics of a Mn atom under this strong resonant optical excitation. In addition to standard optical pumping expected for a resonant excitation, we show that the Mn spin population can be trapped in the state which is resonantly excited. This effect is modeled considering the coherent spin dynamics of the coupled electronic and nuclear spin of the Mn atom optically dressed by a resonant laser field. Finally, we discuss the spin dynamics of a Mn atom in strain free quantum dots and show that these structures should permit a fast optical coherent control of an individual Mn spin
A many-body singlet prepared by a central spin qubit
Controllable quantum many-body systems are platforms for fundamental
investigations into the nature of entanglement and promise to deliver
computational speed-up for a broad class of algorithms and simulations. In
particular, engineering entanglement within a dense spin ensemble can turn it
into a robust quantum memory or a computational platform. Recent experimental
progress in dense central spin systems motivates the design of algorithms that
use a central-spin qubit as a convenient proxy for the ensemble. Here we
propose a protocol that uses a central spin to initialize two dense spin
ensembles into a pure anti-polarized state and from there creates a many-body
entangled state -- a singlet -- from the combined ensemble. We quantify the
protocol performance for multiple material platforms and show that it can be
implemented even in the presence of realistic levels of decoherence. Our
protocol introduces an algorithmic approach to preparation of a known many-body
state and to entanglement engineering in a dense spin ensemble, which can be
extended towards a broad class of collective quantum states.Comment: 11 pages, 6 figures, and supplementary material
Collective Quantum Memory Activated by a Driven Central Spin
Coupling a qubit coherently to an ensemble is the basis for collective quantum memories. A single driven electron in a quantum dot can deterministically excite low-energy collective modes of a nuclear spin ensemble in the presence of lattice strain. We propose to gate a quantum state transfer between this central electron and these low-energy excitationsâspin wavesâin the presence of a strong magnetic field, where the nuclear coherence time is long. We develop a microscopic theory capable of calculating the exact time evolution of the strained electron-nuclear system. With this, we evaluate the operation of quantum state storage and show that fidelities up to 90% can be reached with a modest nuclear polarization of only 50%. These findings demonstrate that strain-enabled nuclear spin waves are a highly suitable candidate for quantum memory.We thank E. Chekhovich for helpful discussions. This work was supported by the ERC PHOENICS grant (617985), the EPSRC Quantum Technology Hub NQIT (EP/M013243/1), and the Royal Society (RGF/EA/181068). D.âA.âG. acknowledges support from St. Johnâs College Title A Fellowship. E.âV.âD. and J.âM. acknowledge funding from the Danish Council for Independent Research (Grant No. DFF-4181-00416). C.âL.âG. acknowledges support from a Royal Society Dorothy Hodgkin Fellowship
APOE genotype influences the gut microbiome structure and function in humans and mice: relevance for Alzheimerâs disease pathophysiology
Apolipoprotein E (APOE) genotype is the strongest prevalent genetic risk factor for Alzheimerâs disease (AD). Numerous studies have provided insights into the pathologic mechanisms. However, a comprehensive understanding of the impact ofAPOEgenotype onmicroflora speciation and metabolismis completely lacking. In this study,we investigated the association between APOE genotype and the gut microbiome composition in human and APOEâtargeted replacement (TR) transgenic mice. Fecal microbiota amplicon sequencing from matched individuals with different APOE genotypes revealed no significant differences in overall microbiota diversity in group aggregated human APOE genotypes. However, several bacterial taxa showed significantly different relative abundance between APOE genotypes. Notably, we detected an association of Prevotellaceae and Ruminococcaceae and several butyrate-producing genera abundances with APOE genotypes. These findings were confirmed by comparing the gutmicrobiota ofAPOE-TRmice. Furthermore, metabolomic analysis of murine fecalwater detected significant differences in microbe-associated amino acids and short-chain fatty acids between APOE genotypes. Together, these findings indicate that APOE genotype is associated with specific gut microbiome profiles in both humans and APOE-TR mice. This suggests that the gut microbiome is worth further investigation as a potential target to mitigate the deleterious impact of the APOE4 allele on cognitive decline and the prevention of A
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