HAL Portal IOGS (nstitut d'Optique Graduate School)
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Contrôle de phase et façonnage d'impulsion de la diffraction de Bragg pour l'optique atomique quantique : des interférences d'ondes de matière à un test d'inégalité de Bell
No full textQuantum mechanics is a theory that describes the behavior of physical systems at the microscopic scale. Fundamentally different from classical physics, it predicts results sometimes contrary to classical intuition, including the phenomenon of entanglement, which suggests that for a system of two entangled particles, each particle cannot be independently described. The measurement of a physical observable on one particle also affects the other, regardless of the distance between the particles, leading to the observation of very strong correlations.One way to characterize an entangled system is to perform a Bell inequality test, which aims to exhibit correlations so strong that they cannot be explained not only by classical theory but by any local hidden variable theory. While such tests have been conducted with photons since the 1980s with the Aspect experiments in Orsay, the project described in this thesis aims to set up a Bell interferometer involving metastable helium atoms entangled in momentum. Starting with atoms prepared in a well-defined quantum state thanks to the phenomenon of Bose-Einstein condensation, pairs of highly correlated atoms are emitted at different momenta through the four-wave mixing process. These atoms are then sent through a two-particle, four-momentum mode interferometer using Bragg diffraction to coherently transfer momentum to the atoms and create atom mirrors and beam splitters.In this thesis, we study in detail Bragg diffraction and its influence on the realization of a Bell interferometer. We propose an original method to control the phase difference imprinted on the atoms between the two involved momentum doublets by temporally modulating the amplitude of the Rabi frequency. This technique is also used to shape the temporal profile of Bragg mirrors and beam splitters to improve their reflectivity and phase properties. A theoretical and experimental study is conducted to determine the achievable performance with these mirrors and beam splitters. Initial interferometric tests are reported (Mach-Zehnder, Ramsey, Hong-Ou-Mandel interferometers), validating the phase control technique in particular and showing promising results for the upcoming realization of a Bell test.La mécanique quantique est une théorie qui décrit le comportement des systèmes physiques à l'échelle microscopique. Fondamentalement différente de la physique classique, elle prédit des résultats parfois contraires à l'intuition classique, et notamment le phénomène d'intrication, qui prévoit que pour un système de deux particules dites intriquées, on ne peut pas décrire chaque particule indépendamment. La mesure d'une observable physique sur une particule affecte également l'autre, quelle que soit la distance entre les particules, donnant lieu à l'observation de très fortes corrélations.Une façon de caractériser un système intriqué est de réaliser un test d'inégalité de Bell, qui consiste à mettre en évidence des corrélations si fortes qu'elles ne peuvent pas être expliquées non seulement par la théorie classique, mais par aucune théorie à variables cachées locale. Si de tels tests ont été réalisés avec des photons depuis les années 1980 avec les expériences d'Aspect à Orsay, le projet décrit dans cette thèse se propose de mettre en place un interféromètre de Bell mettant en jeu des atomes d'hélium métastables intriqués en impulsion. A partir d'atomes préparés dans un état quantique bien défini grâce au phénomène de condensation de Bose-Einstein, des paires d'atomes fortement corrélées sont émises à des impulsions différentes par le processus de mélange à quatre ondes, puis les atomes sont envoyés dans un interféromètre à deux particules et quatre modes d'impulsion. Pour cela, on utilise la diffraction de Bragg, pour transférer de l'impulsion aux atomes de façon cohérente et ainsi former des miroirs et séparatrices à atomes.Dans cette thèse, on étudie en détails la diffraction de Bragg et son influence dans la réalisation d'un interféromètre de Bell. On propose une méthode originale pour contrôler la différence de phase imprimée sur les atomes entre les deux doublets d'impulsion en jeu, en modulant temporellement l'amplitude de la pulsation de Rabi. Cette technique est également mise à profit pour façonner le profil temporel des miroirs et séparatrices Bragg afin d'améliorer leurs propriétés de réflectivité et de phase. Une étude théorique et expérimentale est réalisée pour déterminer les performances accessibles avec ces miroirs et séparatrices. De premiers tests interférométriques sont reportés (interféromètres de type Mach-Zehnder, Ramsey, Hong-Ou-Mandel), validant la technique de contrôle de phase notamment et donnant des résultats prometteurs quant à la réalisation prochaine d'un test de Bell
FLoCoRA: Federated Learning Compression With Low-Rank Adaptation
No full textInternational audienceLow-Rank Adaptation (LoRA) methods have gained popularity in efficient parameter fine-tuning of models containing hundreds of billions of parameters. In this work, instead, we demonstrate the application of LoRA methods to train small-vision models in Federated Learning (FL) from scratch. We first propose an aggregation-agnostic method to integrate LoRA within FL, named FLoCoRA, showing that the method is capable of reducing communication costs by 4.8 times, while having less than 1% accuracy degradation, for a CIFAR-10 classification task with a ResNet-8. Next, we show that the same method can be extended with an affine quantization scheme, dividing the communication cost by 18.6 times, while comparing it with the standard method, with still less than 1% of accuracy loss, tested with on a ResNet-18 model. Our formulation represents a strong baseline for message size reduction, even when compared to conventional model compression works, while also reducing the training memory requirements due to the low-rank adaptation
Mechanisms of laser-based synthesis and modifications of nanomaterials
No full textInternational audienceLasers are known to be extremely versatile tools suitable both for the synthesis and modifications of numerous nanomaterials with unique and extremely interesting optical properties suitable for a wide range of applications in various fields ranging from optics and photonics to medical applications. Efficient control over these processes is still challenging and often requires numerical simulations because of the complex interplay of many physical and chemical processes involved that depend on the combination of both material properties and laser parameters. To simulate these processes, multi-physical modeling should be used including electromagnetic, thermal, mechanical, and chemical effects taking place at several time and space scales. Depending on the experimental conditions, nanoparticles can be formed, grow, aggregate, or on the contrary decay, so that a set of transient variations often take place, particularly when multipulse laser irradiation is applied. In the case of short and ultra-short laser pulses, strongly non-linear and time-dependent processes play a role involving not only ionization but also phase transitions, acoustic vibrations, shock waves, as well as void formation, and cavitation. If a considerable energy is released in a very short time, firstly aggregates decay, then nanoparticles are fragmented. Here, based on numerical calculations, the roles of several above-mentioned effects are analyzed. The performed simulations can be used for a better understanding of laser interactions with nanoobjects
Transient-assisted plasma etching (TAPE): Concept, mechanism, and prospects
No full textInternational audienceAtomic layer etching (ALE) schemes are often deemed economically unviable due to their slow pace and are not suited for every material/hard-mask combination. Conversely, plasma etching presents pattern profile challenges because of its inability to independently control ion and neutral flux. In this work, we introduce a new cyclic transient-based process, called transient-assisted plasma etching (TAPE). A cycle of TAPE is a short exposure step to a sustained flow of reactant before the reactant gas injection is stopped in the second step, resulting in a plasma transient. As the plasma ignites and a substantial amount of etchant remains, a chemically driven etching process occurs, akin to conventional etching. Later in the transient, the modified surface is exposed to a reduced etchant quantity and a sustained ion bombardment, in a similar way to ALE. The cointegration of conventional etching and atomic layer etching allows interesting compromises between etch control and processing time. Going for a transient plasma allows to provide the time and conditions needed for the necessary plasma-surface interactions to occur in one step. In this perspective, the mechanisms behind etch rate, profile correction, and conservation of surface composition using amorphous carbon, as a benchmark, are discussed
Mise en place expérimentale du protocole de cryptographie quantique BB84: Distribuer des clés quantiques
No full textInternational audienceQuantum key distribution (QKD) protocols offer a fundamentally secure method for sharing encryption keys, based on the principles of quantum mechanics. Among these, the BB84 protocol is one of the most well-known and widely implemented. Unlike classical cryptographic methods, which rely on computational hardness, QKD allows two distant parties to generate a shared secret key and detect any eavesdropping attempt during transmission.In this project, we present the experimental implementation of the BB84 protocol, designed specifically as a new educational labwork for second-year students at Institut d’Optique Graduate School. The objective was to create a robust, pedagogically valuable platform allowing students to explore the physics and engineering of quantum communication systems.Our setup employs a Twin Photon Source (TPS) based on spontaneous parametric down-conversion at a wavelength of 1550.3 nm, producing pairs of orthogonally polarized photons. One photon is directed toward Alice, where it is prepared in a polarization state using an Electro-Optic Modulator (EOM) controlled by a C++ interface and microcontroller. The second photon is sent to Bob, who randomly selects a measurement basis using a passive optical setup involving beam splitters and wave plates.Detection is performed using InGaAs Avalanche Photodiodes (APDs), and a correlator is used to time-tag photon arrivals and identify coincidence events, allowing for the extraction of a raw key. The correlator is controlled via Python, enabling automated data acquisition and cross-correlation analysis with high temporal resolution (down to 13 ps). A schematic and full system integration were developed by progressively testing each component independently, including wavelength-division multiplexers, polarization beam splitters (PBS), and polarization state controllers.This work demonstrates a practical and modular QKD setup that is both suitable for educational purposes and adaptable for further experimental developments. It provides students with hands-on experience in quantum optics and secure communication protocols, contributing to the training of future engineers and researchers in quantum technologies.Les protocoles de distribution quantique de clés (QKD) offrent une méthode fondamentalement sécurisée pour l’échange de clés de chiffrement, en s’appuyant sur les principes de la mécanique quantique. Parmi eux, le protocole BB84 est l’un des plus connus et des plus largement étudiés. Contrairement aux méthodes classiques de cryptographie, fondées sur la complexité algorithmique, la QKD permet à deux parties distantes de générer une clé secrète partagée tout en étant capables de détecter toute tentative d’interception.Dans ce projet, nous présentons l’implémentation expérimentale du protocole BB84 dans le cadre du développement d’un nouveau travail pratique destiné aux étudiants de deuxième année de l’Institut d’Optique Graduate School. L’objectif est de fournir une plateforme pédagogique robuste et concrète, permettant aux étudiants d’explorer les principes fondamentaux de la communication quantique.Notre dispositif repose sur une source de photons jumeaux (TPS), basée sur la conversion paramétrique spontanée dans un cristal non linéaire, produisant des paires de photons polarisés orthogonalement à une longueur d’onde de 1550,3 nm. Un photon est dirigé vers Alice, où il est préparé dans un état de polarisation défini via un modulateur électro-optique (EOM) contrôlé par une interface en C++ et une carte microcontrôleur. Le second photon est envoyé vers Bob, qui choisit aléatoirement une base de mesure grâce à un montage optique passif comprenant séparateurs de faisceaux et lames birefringentes.La détection est réalisée à l’aide de photodiodes à avalanche InGaAs (APD). Un corrélateur permet de mesurer les temps d’arrivée des photons et d’identifier les événements en coïncidence, condition nécessaire à la génération de la clé brute. Le corrélateur, contrôlé en Python, permet une acquisition automatisée des données avec une résolution temporelle de l’ordre de 13 ps. Chaque composant du montage (multiplexeurs WDM, séparateurs PBS, contrôleurs de polarisation...) a été testé individuellement avant d’être intégré au système global.Ce travail met en œuvre une plateforme modulaire et reproductible de distribution quantique de clé, à la fois adaptée à un usage pédagogique et ouverte à des explorations expérimentales plus avancées. Il offre aux étudiants une immersion concrète dans les technologies quantiques, et contribue à leur formation en optique quantique et en communication sécurisée
Fibered luminescent concentrator: A bridge between flashlamp devices and laser technologies for skin therapy
No full textInternational audienceBackground and objectives: Laser skin therapy and intense pulsed light (IPL) therapy are both light-based treatments used for various skin concerns. They have been used since decades and each system have their own specificity, advantages, and drawbacks. However specific treatment is still not accessible with standard techniques due to difficulties having a source with both laser and IPL advantages. We describe a new concept, the fibered luminescent concentrator—FLC, based on luminescent concentrators capable of concentrating spectrally and spatially an IPL source, resulting in a multi-color fibered device.Study design/materials and methods: The FLC utilizes luminescent materials arranged in parallelepiped shapes polished on all faces. The IPL broadband spectrum is absorbed by the luminescent molecules and is re-emitted to a red shifted wavelength. The emitted spectral bandwidth ranges from green to dark red, depending on the type of luminescent concentrator. This light is then spatially concentrated by total internal reflections in the parallelepiped and guided through a fiber to the final operator.Results: We have developed three different solid luminescent concentrators based on a transparent polymer sheet (PMMA) doped with luminescent organic dye molecules for yellow and red emission, and an alexandrite crystal for emission in the dark red spectrum. We demonstrate that our new non-laser FLC device can concentrate spectrally and spatially the light with no temporal deformation and offers real opportunities for treatments where the IPL is less well-adapted.Conclusion: The FLC is an additional tool for existing conventional systems such as laser or IPL sources. It is easily adaptable to any IPL source and is a very good complement, especially for wavelengths where the laser cannot easily produce light, such as the yellow band
DOCUMENT DE SÉCURITÉ PERSONNALISABLE ET SES PROCÉDÉS DE FABRICATION ET DEPERSONNALISATION
No full textA personalizable security document (1) includes a laser-engravable film (2) formed on a layer (6) of the security document (1) as a functional layer, which can be irradiated with laser light of a specific wavelength to form a color image. The functional layer includes metallic nanoparticles (20) that can be excited with the laser light of a specific wavelength to promote a growth, reshaping, or reorganization of the nanoparticles, which results in a change in the color of the laserengravable film (2) that can be observed, for example, in one or more observation modes. The laser-engravable film (2) is provided as a thin film in the nanometer range, which is very difficult to tamper with
Procesadores modernos con sólo unos Clicks
No full textInternational audienceAlgunos programas educativos en áreas de ingenierı́a y tecnologı́as a menudo tienen dificultades para proporcionar a sus estudiantes los instrumentos necesarios para realizar trabajos experimentales. Esto puede deberse a dificultades económicas o simplemente por falta de oportunidades para adquirir suficientes dispositivos. El diseño asistido por computadora podrı́a adoptarse para modelar un gran número de dispositivos con pequeños costos de operación. En este artı́culo se describe el uso del simulador gem5 como una herramienta que puede ser de utilidad para estudiantes de grado y posgrado en las áreas de ingenierı́a e informática. Revisaremos el reto que representa para los estudiantes el tener acceso a placas de prototipado modernas y describiremos cómo el uso de un simulador puede resolver este problema hasta cierto punto. Finalmente, ilustraremos paso a paso el uso del simulador gem5 para emular un microprocesador moderno