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dissertation (10)


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2024 (8)

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Dissertation
Dynamique quantique dissipative et application à la superradiance
Authors: ---
Year: 2012 Publisher: [S.l.] : [chez l'auteur],

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Dissertation
Cooperative spontaneous emission with quantized atomic motion
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Year: 2016

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Dissertation
Mémoire
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Year: 2024 Publisher: Liège Université de Liège (ULiège)

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With the aim of contributing to the advancement and development of the emerging field of spin wave optics, we investigate the propagation of magnetostatic surface spin waves in a uniform permalloy waveguide with grooves and constrictions created by in-situ atomic force microscopy nanolithography. Our findings reveal that the introduction of these artificial defects in the spin waves path leads to an unexpected reduction in the signal intensity of the transmitted spin wave compared with a waveguide interrupted by a full slit. This remarkable extinction phenomenon dominates the tunnel-like propagation facilitated by magnetic dipolar coupling across the gap. Combining experimental and numerical analyses, the complex interplay between spin wave tunneling, diffraction and reflection at the edges of the waveguide is comprehensibly addressed. These results provide new insights into the controllable manipulation of spin waves, opening up promising avenues for the development of spin wave switches and interferometers.


Dissertation
Mémoire
Authors: --- --- --- ---
Year: 2024 Publisher: Liège Université de Liège (ULiège)

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This thesis aims to present the various coding methods and applications related to quantum key distribution. Quantum key distribution enables unconditional security for data exchanges, and its development is therefore a crucial issue in our digital world.


Dissertation
Mémoire
Authors: --- --- --- --- --- et al.
Year: 2024 Publisher: Liège Université de Liège (ULiège)

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Étude des propriétés physiques de l'ambre dissous utilisé en peinture artistique.


Dissertation
Mémoire
Authors: --- --- --- ---
Year: 2024 Publisher: Liège Université de Liège (ULiège)

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The study of quantum open systems usually relies on numerical methods of resolution for their equations of motion. In this master's thesis, we present a new formalism to obtain an exact analytical resolution for these equations, the “third quantization”. We use this mathematical framework to study the properties of several quantum transport systems.


Dissertation
Mémoire
Authors: --- --- --- --- --- et al.
Year: 2024 Publisher: Liège Université de Liège (ULiège)

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This master thesis explores the use of quantum annealing process to solve the number partitioning problem, which is a NP-Complete problem. For this master thesis, we explore the effect that can have different parameters on the probability of finding the right answer after measurement at the end of the quantum annealing process. We first study the system of qubit as a closed system, and as an open system, with Markovian interactions with the environment.


Dissertation
Mémoire
Authors: --- --- --- --- --- et al.
Year: 2024 Publisher: Liège Université de Liège (ULiège)

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In quantum cryptography, the first secure protocol to have been formalized is the BB84 protocol. In this master's thesis, we analyze its security under dissipation, weak measurements combined with artificial intelligence, and quantum feedback.


Dissertation
Research master thesis
Authors: --- --- --- --- --- et al.
Year: 2024 Publisher: Liège Université de Liège (ULiège)

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In this study, I investigate layered oxide perovskites, specifically n=2 Dion-Jacobson phases with the formula AA′B2O7, using first-principles Density Functional Theory simulations. These materials are of interest due to their potential ferroelectric properties but are often under-explored and poorly characterized. I carefully calculate the force constants associated with phonon modes by utilizing the frozen-phonon technique in the symmetry-adapted mode basis. The coupling of different phonon modes happens only if it is symmetry-allowed, giving a natural tool to study phase transitions. Unstable modes are identified by a negative force constant; the eigenvalues of these modes can be frozen into the structure, which, after relaxation, results in a more stable material. The materials studied in this work are compounds with the formula ANdNb2O7 where A is Rb, Na, or NH4 cation. The ammonia molecule-containing phase is the primary compound of this study. The presence of various unstable phonon modes complicates the prediction of a definitive ground state; thus, I present several potential lower symmetry structures derived from the aristotype. These models serve as a basis for further experimental characterization and enhance our understanding of structure-property relationships, particularly the influence of aspherical cations on the material’s ground state structure. This research provides foundational insights into the dynamic properties of layered perovskites, paving the way for future studies on their practical applications in ferroelectric and other functional materials.


Dissertation
Mémoire
Authors: --- --- --- ---
Year: 2024 Publisher: Liège Université de Liège (ULiège)

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Cette thèse de master présente le développement et l’application de réseaux neuronaux pour la
reconnaissance de structures topologiques, en particulier les skyrmions et anti-skyrmions magnétiques.
Les phases topologiques de la matière, caractérisées par leurs propriétés géométriques plutôt
que par des paramètres d’ordre locaux, ont des implications significatives en spintronique en raison
de leurs propriétés de transport particulières. Par exemple, les skyrmions peuvent être déplacés
par des courants électriques et servir de porteurs d’informations potentiels. Ces applications potentielles
nécessitent une analyse précise de leurs propriétés dynamiques. Cependant, les méthodes
traditionnelles pour localiser et analyser la dynamique des structures topologiques impliquent des
algorithmes complexes et ne sont valides qu’à 0 Kelvin. Pour surmonter ces limitations, cette
recherche exploite des techniques d’apprentissage profond, y compris les réseaux neuronaux Feedforward,
les réseaux de neurones convolutionnels et les architectures UNet. Des bases de données de
structures magnétiques synthétiques ont été générées et étiquetées, servant de base à l’entraînement
de ces réseaux. Les résultats démontrent l’efficacité de ces modèles pour classifier avec précision
les structures topologiques avec une précision supérieure à 97% sur des données inédites. Malgré
ce succès, des défis tels que les biais dans les ensembles de données d’entraînement et le surapprentissage
du modèle ont été identifiés, suggérant la nécessité d’affinements supplémentaires. Ce
travail jette les bases de futures recherches sur le comportement dynamique des skyrmions et leurs
applications potentielles dans les dispositifs spintroniques de nouvelle génération. This master thesis presents the development and application of neural networks for the recognition
of topological structures, specifically magnetic skyrmions and anti-skyrmions. Topological phases
of matter, characterized by their geometric properties rather than local order parameters, have
significant implications in spintronics as they have special transport properties. For example,
skyrmions can be moved by electrical currents and serve as potential information carriers. These
potential applications require a precise analysis of their dynamical properties. However, traditional
methods to locate and analyse topological structures dynamics are involving complex algorithm
and are only valid at 0 Kelvin. To overcome these limitations, this research leverages deep learning
techniques, including Feedforward Neural Networks, Convolutional Neural Networks, and UNet
architectures. Databases of synthetic magnetic structures were generated and labelled, serving
as the foundation for training these networks. The results demonstrate the effectiveness of these
models in accurately classifying topological structures with over 97% accuracy on unseen data.
Despite the success, challenges such as bias in the training datasets and model overfitting were
identified, suggesting the need for further refinement. This work lays the groundwork for future
research on the dynamic behaviour of skyrmions and their potential applications in next-generation spintronic devices.

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