The kinetics of a phase transition has been studied by using a new dynamical Monte Carlo renormalization group method. Using a majority rule block-spin transformation in both space and contiguous times, we numerically renormalized the evolving configurations during the phase separation of a kinetic Ising model with spin-flip dynamics. We find that, in the scaling regime, the average domain size R(t) grows in time consistently with the $R sim t sp{1/2}$ Allen-Cahn antiphase boundary motion theory, although some correcting factors may exist. The same procedure has also been applied to the corresponding equilibrium critical system in order to find the critical exponent z. Our method yields values that are consistent with the ones obtained from a finite-size scaling analysis applied on the same data, thus showing that, in principle, this method can be successfully used to determine z in a more precise and consistent way.

The growth of walled cells is due to the cooperation of physical and chemical mechanisms leading to the controlled mechanical deformation of the cells. Plant cells, for example, need to expand the surface area of their cell wall in order to grow in size. This can be done in a uniform manner called diffusive growth or through tip-growth. Tip-growth is characterised by a mechanical deformation that is confined to a specific region of the cell wall, namely its tip. Tip-growth generally leads to tubular cell wall shapes and has been observed in plant cells such as pollen tubes and root hairs but also in fungal hyphae, algae and neuronal growth cones. The pollen tube is a protuberance growing on the pollen grain. As part of plant fertilisation, its purpose is to extend until it reaches the ovule of a host plant and allow for the passage of a sperm cell contained in the pollen grain. This thesis studies how physical principles direct and control the tip-growth of a pollen tube. The three particular phenomena that were addressed concern the effects of elastic forces and short-range electrical potentials on ion diffusion, actin protein aggregation and the viscous flow in growing pollen tubes. A very intriguing phenomenon, observed a decade ago, concerning growing pollen tubes is the oscillation in time of the growth rate, the concentration of signalling molecules such as calcium and the thickness of the cell wall. While the total interaction of the components of such a complex system is hard to asses, I show that a few key elements, when coupled adequately, lead to the steady oscillation similar to that observed experimentally. The growing cell was modelled as a fluid finger in the viscous regime with elastic properties depending on the material delivery to the growing region. The difference between the material delivered and the material required for growth led to the stable oscillation of the growth rate. A se / La croissance de cellules ayant une paroie cellulaire est due \'a la collaboration de processus physiques et chimiques menant \'a une déformation mécanique contrôlée de ces cellules. Les cellules végétales, par exemple, doivent augmenter la surface de leur paroie afin de croître. Ceci peut être accompli de façon uniforme appellée croissance diffuse ou par croissance apicale. La croissance apicale est caractérisée par une déformation mécanique qui est confinée à une région spécifique de la cellule, soit l'apex. La croissance apicale méne générallement à des paroies cellulaires de forme cylindrique. Elle est observée chez des cellules végétales telles que le tube pollinique ou des racines mais aussi chez des algues ou des hyphes fongiques. Le tube pollinique est une protubérance qui croît sur un grain de pollen. Lors de la fertilisation, son rôle est de s'allonger jusqu'à ce qu'il atteigne l'ovule de la plante hôte et de permettre le passage d'une cellule de sperme contenue dans le grain de pollen.Cette thése étudie les principes physiques qui contrôlent la croissance apicale d'une cellule végétale. Elle s'applique plus spécifiquement à la croissance d'un tube pollinique. Les trois systémes particuliers que nous étudierons concernent les effets élastiques et les potentiels électriques sur la diffusion d'ions, l'aggrégation de protéines et le flot visqueux dans les tubes de pollen en croissance. Un premier phénoméne, observé il y a une décennie, est l'oscillation dans le temps de la vitesse de croissance ainsi que de la concentration de calcium et de l'épaisseur de la paroie cellulaire d'un tube pollinique. Bien que les interactions entre toutes les composantes du tube pollinique sont complexes, nous avont démontré que quelles composantes, lorsque couplées de maniére adéquate, produisent l'oscillation stable observée en laboratoire.La c

Obtaining a deeper understanding of the processes that occur in elastomers during stress relaxation has been identified as a problem of great scientific importance. To this end, stress-strain measurements have been performed in situ with X-ray and neutron diffraction measurements. Here, we develop a mechanism for carrying out these measurements in a compressional strain configuration and compare these results with those in an extensional strain configuration. Measurements of the tensile relaxation moduli of polychloroprene (neoprene) and nitrile butadiene rubber (buna-N) elastomers are performed and analysed. Three techniques of data analysis are employed and contrasted: the standard linear solid model (SLSM), an extended Maxwell model, and the CONTIN algorithm for performing the regularized Laplace transform. Through these techniques, details regarding the magnitude and time-scale of various processes are extracted. / Obtenir une meilleure compréhension des processus qui se produisent chez les élastomères lors de la relaxation de contrainte a été identifiée comme étant une problématique scientifique de haute importance. À cette fin, des mesures de contrainte-déformation ont été effectuées in situ ainsi que des mesures de diffraction aux rayons X et aux neutrons. Dans la présente étude, il est question d'un mécanisme qui permet la réalisation de ces mesures en configuration de compression. Ces résultats sont comparés aux résultats de la configuration d'allongement. Des analyses de mesures du module de relaxation des élastomères, polychloroprène (néoprène) et butadiène (buna-N) y sont effectuées. Trois techniques différentes sont employées pour analyser les données : le modèle linéaire standard des solides (SLSM), un modèle de Maxwell étendu, et l'algorithm CONTIN pour effectuer la transformée régularisée de Laplace. À travers ces techniques, sont extraits des détails concernant l'ampleur et l'échelle de temps des processus divers présents dans la relaxation de contrainte.

A single quantum state can be shared by many distant parties. In this thesis, we try to characterize the information contents of such distributed states by defining the multiparty information and the multiparty squashed entanglement, two steps toward a general theory of multiparty quantum information. As a further step in that direction, we partially solve the multiparty distributed compression problem where multiple parties use quantum communication to faithfully transfer their shares of a state to a common receiver. We build a protocol for multiparty distributed compression based on the fully quantum Slepian-Wolf protocol and prove both inner and outer bounds on the achievable rate region. We relate our findings to previous results in information theory and discuss some possible applications. / Un état quantique peut être partagé entre plusieurs entités qui sont spatialement séparés. Dans ce mémoire, nous essayons de caractériser l'information quantique contenue dans de tels états distribués en définissant et utilisant les notions d'information multipartie (multiparty information) et d'intrication "écrasée" multipartie (multiparty squashed entanglement). Il s'agit de premiers pas vers une théorie générale de l'information quantique multipartie. Nous faisons aussi un autre pas dans cette direction en étudiant le problème de la compression distribuée d'information quantique. En particulier, nous proposons un protocole de compression distribuée basé sur la version quantique du protocole de Slepian et Wolf et analysons ses caractéristiques. Nous discutons aussi la relation entre nos résultats et les travaux précédents dans la théorie de l'information et soulignons quelques applications possibles de notre protocole.

Following work by Coleman and Lee using ordinary gravity, we obtain a low-energy effective Lagrangian for a supergravity system, in which superwormholes are represented by local operators in superspace. Supersymmetry is not broken by these new interactions.

We study the effective action that governs heterotic string theory compactified on a four-dimensional, N = 1 supersymmetric background. A scaling symmetry of the four-dimensional field T that represents the breathing mode on of internal manifold is introduced. This symmetry, being valid at each order in both perturbative series of the theory (string-loop and sigma model expansions), is used to find restrictions on the terms that can correct the truncated supersymmetric functions entering the effective action. We derive the general form of these corrections and show that, contrary to earlier conjectures in the literature, loops are not a priori counted by the ratio T/S, in which T and S represents the standard four-dimensional breathing mode and dilaton fields. Some other symmetry considerations allow us to restrict even more the corrections to the truncated results. Our conclusions are extended to the cases with matter fields, and more than one moduli for the internal manifold. / Some regularization techniques for problems related to those encountered in the quantization of string theories are also presented in an appendix.

A method for for simulating quantum computing circuits using stochastic processes is described and analyzed. Circuits are transformed into a complex action from which observables are computed. These averages are evaluated using Monte Carlo and complex Langevin methods. The transformation can be applied to any circuit with an input product state, and results in equations that are polynomial in storage. This method is unique in allowing for an efficient simulation that is also general. Three sample circuits are simulated. Standard simulations techniques are shown to yield poor estimates of the observables. An improved method is proposed by adding a coupling term to the action to stabilize the system. Results for this improved method are shown to be more accurate. Feasibility and future directions are discussed. / Une méthode pour simuler des circuits de calcul quantique par des processus stochastiques est décrite et analysée. Les circuits sont transformés en une action complexe par laquelle les observables sont calculés. Ces moyennes sont évaluées à l'aide de Monte Carlo et de méthodes complexes de Langevin. La transformation peut être appliquée sur n'importe quel circuit avec un état de produits d'entrée. Celle-ci résulte en équations qui sont polynomiales dans le stockage. Cette méthode est unique, car elle permet une simulation efficace et est à la fois généralisée. Trois circuits d'échantillonnage sont simulés. Les techniques de simulations courantes démontrent inadéquatement les estimations des observables. Une méthode améliorée est donc proposée par l'ajout d'un terme de couplage à l'action pour stabiliser le système. Les résultats démontrés via la méthode améliorée sont plus fiables. La faisabilité et les orientations futures sont discutées.

The main purpose of this thesis is to investigate a method for estimating loop integrals containing large masses. Working in Feynman parameter space, the method consists of determining the regions of integration that give the dominant contributions. The method is applied to some integrals that have been worked out in the literature, and it is found that the method can be simple to apply. The percent difference between the approximate and exact results using masses consistent with the Standard Model is typically roughly 30%. Also given are general formulae for arbitrary convergent and divergent one loop integrals with the external momenta set equal to zero.

I investigate the free field realization (FFR) of various extended conformal field theories (ECFT's). More specifically, I first present a systematic method that allows the construction of the exponential type screening currents in terms of free fields in the case of the ECFT's with Kac-Moody algebras. This method is explicitly illustrated through the $su(n) sb{k}$ and $sp(4) sb{k}$ Kac-Moody algebras. Then, I use the FFR to unravel the embedding structure of the Verma modules of the ECFT with a $W sb3$ algebra. This embedding structure is expressed through a set of intertwining diagrams, which in turn, is used to compute the irreducible characters of the $W sb3$ algebra. Next, I construct two FFR's for the ECFT with the $su(n) sb{k}$ parafermion algebra. Finally, I sketch the FFR of the coset model $su(n) sb{k} times su(n) sb ell/su(n) sb{k+ ell},$ which is given in terms of the fields realizing the $su(n) sb{k}$ parafermion model and an extra free field with a background charge.

The independent formation cross-sections of 30 min Cs^130, 6.5d Cs^132, 2.9 hr Cs^134m, 2.1 yr Cs^134g, 13 d Cs^136, 32 min Cs^138, 18 d Rb^86, and cumulative formation cross-sections of 30 yr Cs^137 and 17 min Xe^138 produced in the fission of Th^232 by 20 by 85 Mev. protons have been measured radiochemically. Charge distribution curves were constructed at 20, 30, 39, 50, 51, 65, 15 and 85 Mev. With increasing proton energy these curves broaden and the most probable charge shifts towards the line of beta stability. These effects are interpreted in terms of fission-spallation competition. Detailed calculations on the basis of two empirical postulates on charge distribution have been attempted. Previously published experimental results on proton induced fission of uranium have also been incorporated in these calculatlons to make a comparative study. It was also possible to determine the isomerratio Cs^134m/Cs^134g in these experiments and to compare the results with theoretical calculations at the lowest energies studied. Some qualitative conclusions could be drawn about the angular momentum distribution during the fission process.