La nécessité de synchroniser les différentes étapes des protocoles d’information et de communication quantiques implique l’utilisation de mémoires quantiques. Différents systèmes physiques sont aujourd’hui explorés, parmi lesquels les ions en matrice cristalline, les atomes froids et les vapeurs atomiques. Le protocole de stockage le plus couramment utilisé se fonde sur le phénomène de Transparence Electromagnétiquement Induite (EIT) : une impulsion lumineuse est gravée dans la cohérence Raman entre les deux états fondamentaux d’un système atomique à trois niveaux en Lambda. Bien qu’elle ouvre des perspectives prometteuses, en termes d’efficacité, de fidélité et de temps de stockage, cette technique est néanmoins sensible aux effets déphasants, tels que des gradients de champs magnétiques.Dans ce mémoire, j’étudie tout d’abord le stockage d’impulsions lumineuses classiques par EIT dans une vapeur d’hélium métastable à température ambiante. Les résultats expérimentaux obtenus sont en accord avec les simulations numériques des équations de Maxwell-Bloch complètes du système et montrent notamment l’existence d’une phase supplémentaire acquise par l’impulsion restituée en configuration désaccordée. Cette phase s’explique par la propagation du faisceau sonde dans un milieu dispersif. Dans une deuxième partie, je mets expérimentalement en évidence, dans le même système, une nouvelle forme de stockage basée sur le phénomène d’Oscillations Cohérentes de Population (CPO), par nature plus robuste aux effets déphasants que l’EIT. Les simulations numériques permettent d’analyser plus précisément les mécanismes à l’œuvre dans une mémoire CPO et, notamment, l’influence de la phase relative entre les faisceaux signal et de couplage sur les efficacités de stockage. / The need to synchronise quantum information and communication protocols implies the use of quantum memories. Different physical systems are investigated nowadays, among which ions in crystals, cold atoms and atomic vapours. The most common protocol is based on the Electromagnetically Induced Transparency (EIT) phenomenon: a light pulse is engraved in the Raman coherence of both ground states of an atomic Lambda–type three-level system. Though it opens promising perspectives, with respect to efficiency, fidelity and storage time, this technique is, however, sensitive to dephasing effects such as magnetic field gradients.In this thesis, I first study the storage of classical light pulses via EIT in a room- temperature metastable helium vapor. The obtained experimental results agree with the numerical simulation of the complete Maxwell-Bloch equations of the system. In particular, the existence of an extra phase acquired by the retrieved pulse is demonstrated in the detuned configuration, which can be explained by the propagation of the signal beam in the medium. In the second part, I experimentally isolate, in the same system, a new storage protocol based on the Coherent Population Oscillation (CPO) phenomenon, which is by nature more robust than EIT to dephasing effects. The numerical simulations allow us to precisely analyse the mechanisms involved in a CPO memory and, in particular, the influence of the relative phase between the signal and coupling beams on the storage efficiencies.

The density amplitude of an isolated quantum vortex line in superfluid 4He is calculated using a generalized Gross-Pitaevskii (G-P) equation. The generalized G-P equation for the order parameter extends the usual mean-field approach by replacing the interatomic potential in the ordinary G-P equation by a local, static T matrix, which takes correlations between the particles into account. The T matrix is a sum of ladder diagrams appearing in a diagrammatic expansion of the mean field term in an exact equation for the order parameter. It is an effective interaction which is much softer than the realistic interatomic Morse dipole-dipole potential from which it is calculated. A numerical solution of the generalized G-P equation is required since it is a nonlinear integro-differential equation with infinite limits. For the energy denominator in the T matrix equation, a free-particle spectrum and the observed phonon-roton spectrum are each used. For the fraction of particles in the zero-momentum state (Bose-Einstein dondensate) which enters the equation, both a theoretical value of 0.1 and an experimental value of 0.024 are used. The chemical potential is adjusted so that the density as a function of distance from the vortex core approaches the bulk density asymptotically. Solutions of the generalized G-P equation are not very dependent on the choice of energy denominator or condensate fraction. The density profile is a monotonically increasing function of the distance from the vortex core. The core radius, defined to be the distance to half the bulk density, varies from 3.7 A to 4.7 A, which is over three times the experimental value of 1.14 A at absolute zero.

Experimental investigations of liquid helium flows Selected turbulent flows of He II, the superfluid phase of liquid 4 He, are inves- tigated experimentally. The second sound attenuation technique is employed to directly probe the tangle of quantized vortices, thin topological defects within the superfluid, while relatively small particles made of solid hydrogen are dispersed in He II to visualize the overall flow of the liquid via the particle tracking ve- locimetry. Considering the known particle-vortex interaction mechanisms, steady thermal counterflow in a square channel is investigated. Significant inhomogene- ity of the vortex tangle density along the channel height (near the flow-generating heater) is shown to develop. The means of energy transport in turbulent flows of He II are found strikingly different from those taking place in turbulent flows of viscous fluids. Moreover, individual particles in counterflow are observed to intermittently switch between two distinct motion regimes along their trajecto- ries. The regimes are identified and qualitatively described. Steady counterflow jets in He II are realized and the spatial arrangement of the underlying vortex tangle is explored. Finally, macroscopic vortex rings are thermally generated and observed in He II. A method for tracking their...

The project compares the technology of welding of CO2 laser and fiber laser in terms of engineering study, the welds will be made by same welding conditions on two materials (S235JRG2 a X5CrNi 18-10), by using different protective gases (Helium and Argon). In the terms of literal background research both the technologies and questions of laser welding are described. The geometry of welds is carried out in the experimental part. The final results of the experiment are concluded at the end of the diploma thesis.

Cette thèse est constituée de deux parties, dont l'axe commun est la modélisation de phénomènes de petites échelles pour des écoulements turbulents.Dans une première partie on s'intéresse à l'influence des rotons sur la dynamique d'un modèle d'hélium superfluide. On commence par une calibration d'un modèle non-local d'interaction dans le but de reproduire la relation de dispersion expérimentale de l'hélium, mesurée par diffraction de neutrons. On utilise ensuite ce modèle calibré pour réaliser des simulations numériques directes (DNS) de l'équation de Gross-Pitaevskii, afin de sonder le phénomène de reconnexion des tourbillons quantiques. Ce phénomène est étudié en détail via une analyse géométrique et énergétique des résultats des DNS. On compare alors systématiquement ces résultats à ceux du modèle local afin d'étudier l'influence des rotons sur l'écoulement aux échelles de l'ordre de l'Angstrom.Dans un second temps on cherche à décrire la structure spatio-temporelle de la turbulence homogène et isotrope. Pour cela on commence par une analyse des propriétés statistiques du champ eulérien de vitesse, basée sur l'évaluation de ses incréments spatio-temporels. On utilise les données issues d'une DNS des équations de Navier-Stokes mises à disposition par l'Université Johns Hopkins. On propose ensuite un champ aléatoire spatio-temporel pour la vitesse eulérienne, en caractérisant d'abord la structure de ses corrélations par une approximation gaussienne. On modifie ensuite cette approximation par une mesure multi-fractale afin de reproduire les aspects non-gaussiens observés dans la DNS, tels que les hauts niveaux des coefficients d'asymétrie et d'aplatissement. / This thesis consists of two parts that share a common theme : the modeling of small-scale phenomena in turbulent flows.In a first part we focus on the influence of rotons on the dynamics of a model of superfluid helium. We begin by a calibration of a nonlocal model of the interaction, aiming at reproducing the experimental dispersion relation of helium, as measured by neutron scattering methods. This model is then used to perform Direct Numerical Simulations (DNS) of the Gross-Pitaevskii equation, in order to probe the reconnection of quantum vortices. This phenomenon is studied quantitatively through a geometrical and energetical analysis of the results of the DNS. We then systematically compare these results with those of the local model, so as to study the influence of rotons on flow scales of the order of the Angtstrom.The goal of the second part is to describe the spatio-temporal structure of homogeneous and isotropic turbulence. To achieve it we start by a standard analysis of the statistical properties of the eulerian velocity field, by computing its spatio-temporal increments. We use the data from a DNS of the Navier-Stokes equations, hosted and made available by the Johns Hopkins University. We then propose a random, spatio-temporal eulerian velocity field, by first characterizing the structure of its correlations through a gaussian approximation. This approximation is then modified by a multifractal measure in order to reproduce the non-gaussian features, as they are demanded by the observed high level of skewness and flatness of increments.

Steady state and decay of quantum turbulence generated in channel flows and detected by second sound attenuation Bc. Emil Varga Abstract Quantum turbulence is studied in superfluid 4 He under classical flow condi- tions. Turbulence is generated by a flow through a 7 × 7 mm square channel with a flow conditioner either with an additional grid or without it. The flow is generated mechanically by squeezing a stainless steel bellows. Vortex line den- sity is measured by attenuation of second sound in both steady state and decay for a range of temperatures 1.17 - 2.16 K. In the steady state, temperature- independent scaling of the vortex line density with flow velocity of the form L ∝ V 3/2 is observed. In the decay the expected late-time behaviour L ∝ t−3/2 is observed. Explanation for both of these observations is based on a quasi- classical model of quantum turbulence, that allows the extraction of the effective kinematic viscosity νeff, which approximately agree with the values available in the literature. Two models based on counterflow theory are also explored and the effect of inhomogeneous vortex line distribution on the measurement tech- nique is studied.

Helium can significantly influence solar wind dynamics. The changes of its relative abundance are usually associated with crossings of the boundaries between adjacent flux tubes. However, the recent studies of the data from the BMSW instrument onboard the Spektr-R spacecraft show that the relative helium abundance could vary also inside the flux tubes. The differential motion of proton and helium solar wind components was suggested as a source of turbulence inside the flux tube. The thesis is devoted to the long-term statistical study of the fast helium abundance variations with respect to parameters of the solar wind and changes of its source region. For this purpose, the plasma data from the instruments onboard the Wind and the Spektr-R spacecraft and magnetic field measurements from Wind were used. Simultaneous changes of the relative helium abundance and solar wind parameters were investigated in the long-term Wind observations as well as in the observations prior to and behind interplanetary shocks. Finally, the study of differences between proton and helium velocities during solar minima and solar maxima was conducted.

Neutron irradiated copper-boron alloys are employed to study the mutual interaction between metallic crystalline lattices and inert gases. Inert gases precipitate to form gas bubbles and their growth induces dilation of the matrix. This dilation, technically designated as swelling, affects the structural integrity of nuclear fuels during their service. The estimated enthalpy of solution of helium in copper is 5.5 eV/atom. As a Consequence, its solubility in the copper matrix is extremely poor and it cannot enter a copper based matrix via any thermo-chemical route. Hence, recourse to a nuclear reaction is taken to impregnate copper with helium. Helium is produced in situ through neutron irradiation in copper-boron alloys as a result of (n, alpha) nuclear reaction between boron atoms and neutrons. The characteristic feature of the growth of helium gas bubbles driven by isothermal annealing of the metallic matrix is that their rate of growth is highly sensitive to the distance of the bubbles from the external surface of the specimen. The growth of gas bubbles as a function of time and temperature is modulated by the flow of vacancies from the free surface of the specimen. A theory for the surface induced growth of helium gas bubbles in the neutron irradiated copper-boron alloys is presented here.

The rates of dissolution of heavy crude oil in liquid solvents and rates of desorption of solvents from oil have been measured. The crude oil used is a non-volatile heavy oil of 4253 mPa.s viscosity at room temperature. The solvents used are hexane, heptane and toluene. When the oil (black) is contacted with a solvent (transparent) an interface is seen which moves with time and takes a very long time to become fuzzy. The rate of movementof the front is measured. The dissolution experiments give very consistent results, but there are two parameters involved, Do, the diffusivity at infinite dilution and alpha which determines the concentration dependence. As a result it is necessary to do desorption experiments to be able to calculate both constants from the rate of movement of the front data. However, desorption experiments could not be performed under conditions of low concentrations suitable for the present case because of the very viscous nature of the oil. As a result, although the desorption experiments also showed good results, they could not be used to obtain good values of the parameters. When Stokes-Einstein equation was used to calculate Do, excellent results were obtained with alpha ~ 10 for the dissolution experiments and good deal smaller for the desorption experiments. That result is used to conclude that the above form for concentration dependent diffusivity is correct and concentration dependence is very high at low solvent concentrations explaining the sharp interfaces during dissolution.Other evidences have also been offered.

Le rapport isotopique de l'hélium atmosphérique (RA = 3He/4He = 1.39 × 10-6) est lié aux flux naturels et anthropiques de 3He et de 4He en provenance de la Terre (et de l'espace pour 3He). Les activités industrielles ont émis des gaz riches en 4He radiogénique (exploitation des combustibles fossiles) et en 3He (activités nucléaires). Du fait de ces flux d'hélium, il a été proposé que le rapport 3He/4He dans l'air a varié dans le temps et dans l'espace. De telles variations pourraient permettre l'utilisation de ce rapport comme nouveau traceur des émissions atmosphériques de polluants anthropiques et/ou naturelles. Pour vérifier ces possibilités, nous avons entrepris la mesure haute-précision (2-6‰, 2σ) de la composition isotopique de l'hélium atmosphérique au Centre de Recherches Pétrographiques et Géochimiques (Université de Lorraine et Centre National de la Recherche Scientifique, Vandoeuvre-lès-Nancy, France). Nous avons effectué des analyses d'intercalation avec un spectromètre de masse double collecteur (Helix Split Flight Tube de la société Thermo Instruments). Au cours de ces analyses, plusieurs analyses individuelles d'un échantillon d'air sont effectuées en alternance avec un standard d'air. Nous n’avons détecté aucune évidence de variation temporelle du rapport 3He/4He dans l’air de France piégé dans des boules de pétanque (1965, 1990, 2010, 2013), dans des réservoirs métalliques (2010, 2016) et dans un carburateur de voiture (1910). Nous avons également re-analysé des échantillons d'air ancien prélevés dans des réservoirs métalliques depuis 1978 à Cape Grim (Tasmanie, Australie). En incluant les mesures de Mabry et al. (2015) pour certains de ces réservoirs, nous obtenons une tendance commune de -0.05 ± 2.46 ‰, montrant l'absence de variation statistiquement significative sur une période de 106 ans. Au niveau mondial, nous avons seulement détecté deux échantillons d’air sur seize (Dôme C, Antarctique; Tokyo, Japon) dont les compositions sont statistiquement distinctes, marquées par des excès en 3He de 2.0 ± 1.4 ‰, et 1.7 ± 1.5 ‰, respectivement (intervalle de confiance de 95%). Ces excès pourraient être liés aux flux d’hélium locaux par : (i) les précipitations aurorales en Antarctique ; (ii) les volcans et/ou les déchets nucléaires (Fukushima Daï-Ichi) au Japon. Dans la région de l’Afar (Éthiopie), nous avons détecté des excès en 3He de ~1 % au-dessus du lac de lave du volcan Erta Ale, pouvant être utilisés pour tracer des gaz réactifs (e.g. CO2) simultanément émis. Ces excès sont toutefois rapidement dilués dans les masses d’air régionales. L'isolation des gaz libérés par le sol du cratère de l'Erta Ale à l'aide d'un contenant inversé placé à sa surface a permis d'évaluer les flux diffusifs en 3He (via l'augmentation du rapport 3He/4He dans l'air isolé) et en CO2. Globalement, cette thèse conforte l’utilisation de RA comme standard inter-laboratoire. Dans certaines régions (Dôme C, Japon), une investigation plus poussée dans le temps devrait être effectuée pour contraindre les sources d’hélium atmosphérique pouvant engendrer de faibles variations locales / The helium isotopic composition of air (RA = 3He/4He = 1.39 × 10-6) is related to the natural and anthropogenic fluxes of 3He and 4He from the Earth (and from space for 3He). Industrial activities emit gases enriched in radiogenic 4He (exploitation of fossil fuels) and in 3He (nuclear activities). It has been proposed that the 3He/4He ratio in air has varied over time and also spatially because of these helium fluxes. Such variations could allow the use of this ratio as a new tracer of anthropogenic and/or natural pollutants. To verify these possibilities, we undertook high-precision measurements of the composition of atmospheric helium (2-6‰, 2σ) at the Centre de Recherches Pétrographiques et Géochimiques (University of Lorraine, Centre National de Recherche Scientique, Vandoeuvre-lès-Nancy, France). We performed sample-standard bracketing analyses with a double collector Helix SFT Mass Spectrometer. During each of these analyses, several individual analyses of an air sample are conducted in alternation with an air standard. We found no evidence of temporal variation of the 3He/4He ratio in French air trapped in petanque balls (1965, 1990, 2010, 2013), metallic tanks (2010, 2016) and in a carburettor (1910). Including the measurements of Mabry et al. (2015) for the Cape Grim air archive (Tasmania, Australia), we obtained a trend of -0.05 ± 2.46‰ over 106 years. At the global level, we only detected two air samples out of sixteen (Dôme C-Antarctica, Tokyo-Japan) whose compositions are statistically distinct, marked by 3He excesses of 2.0 ± 1.4‰, and 1.7 ± 1.5‰, respectively (95% confidence interval). These excesses could be related to local and regional helium fluxes input such as: (i) auroral precipitation in Antarctica; (ii) volcanoes and/or nuclear releases (Fukushima Daï-Ichi) in Japan. In the Afar region (Ethiopia), we detected 3He excesses of ~1% above the lava lake of the Erta Ale volcano, which can be used to trace reactive gases (e.g. CO2) released simultaneously. These excesses are quickly affected by air mixing with the regional air of Afar. The diffusive soil fluxes of 3He (via increase of the 3He/4He ratio) and CO2 have been estimated from the accumulation of gases released from the soil of the Erta Ale crater in an inverted chamber placed on its surface. Overall, this thesis supports the use of RA as an inter-laboratory standard. In some areas (Dôme C, Japon), a more thorough investigation should be carried out over time to constrain the sources that may cause small helium isotopic variations in air