At : Amphithéâtre Pierre Gilles de Gennes - Bâtiment Condorcet-4 Rue Elsa Morante, 75013 Paris , on 15:30

As with the study of electronic circuits, the study of complex systems is often facilitated by breaking them down into simpler sub-systems. Two sub-problems then arise: 1) the study of each sub-system separately; 2) the emergence of new behaviours when they are reassembled. The theory of non-equilibrium circuits is best understood for a single pair of current (electric) and conjugate thermodynamic force (voltage), as is the case in electronics. Each subsystem is then described by a current-voltage characteristic, summarised in the concept of scalar impedance. The current-voltage characteristic of the whole system is then obtained using the conservation laws within and at the interface of each subsystem (for example, Kirchoff’s laws). The aim of this thesis is to pave the way for the generalisation of stationary electronics to non-equilibrium stationary thermodynamic machines with an arbitrary number of currents and conjugate forces subject to different couplings (e.g. thermoelectric). To do this, we identify tools for treating conservation laws within a complex network. We replace the notion of scalar impedance by a matrix object, the non-equilibrium conductance matrix, which takes into account the coupling between the different types of currents flowing through the system. In particular, we show that the law of addition of resistances (resp. conductances) for series (resp. parallel) associations remains valid in the context of non-equilibrium thermodynamic machines. We illustrate this general result for a wide class of systems (thermoelectric converters, chemical reaction networks, energy conversion models with Markov jump processes).

At : Amphithéâtre du Centre de Nanosciences et de Nanotechnologies, 10 Bd Thomas Gobert, 91120 Palaiseau , on 10:00

Quantum technologies promise to bring several improvements in the fields of sensing, communication and computing. Single-photon-emitting devices are a promising platform for quantum applications, having already demonstrated an advantage over their classical counterparts. The highest-quality emitter for the purpose are epitaxial quantum dots (QD) made of III-V semiconductors. To reach the best performances the QDs are embedded in microcavities and are electrically controlled and stabilized. In this thesis are described the basics of QDs properties and the processes to transform them into efficient sources. This includes multiple advancements toward more functional sources with broader applications, requiring standardization to efficiently scale the fabrication yield. Major advancements have been taken on two key parts of the process, controlling the spatial and spectral density of QDs, and improving the deterministic fabrication by in-situ lithography. Motivated by simplifying the use of these devices, the development of directly fiber coupled sources of single photon is presented. This is a crucial step toward plug and play devices, allowing a simpler and more reliable operation. The fiber pigtailed devices are shown to conserve the state-of-the-art performances over months of operation. In parallel, a different type of QDs have been developed based on droplet-etching epitaxy. This unlocks the expansion of the wavelength range of available single-photon emitters. The first deterministic devices are fabricated based on the new droplet-etched QDs.

At : campus jussieu, amphithéâtre à préciser , on 14:00

During morphogenesis, bio-chemical signaling and mechanical cues are at play in order to give rise to shape and function to future formed tissues, influencing each other through a dynamic feedback loop. Still, the role of such mechanical cues on the emergence of structure is not well understood. In particular, in the context of adhesive tissues or cell aggregates, it is known that the hierarchy of cell-cell adhesion, together with cellular motility, can lead to the self-organization of identified structures, a phenomenon described within the framework of the differential adhesion hypothesis. Nevertheless, these kinds of processes have been mainly studied in biological systems in vivo or in vitro where deciphering the exact influence of individual factors like cell-cell adhesion or forces remain challenging. In order to tackle this problem, we choose to work in a simplified framework. We use adhesive emulsions as a biomimetic system of epithelial tissues, in which each oil droplet mimics a cell in the tissue and we introduce differentials of adhesion using palindromic DNA sequences as the binders between the droplets. The obtained “proto-tissues” are then flowed in micro-channels of various geometry that directly define the stress field. We seek to experimentally and theoretically elucidate the influence of local droplet deformations and T1 events on the global behavior of the emulsion flow. Next, we try to correlate those observations to the evolving structure of the emulsion along the channel as a function of the interdroplet adhesion hierarchy. By comparing our findings with structural features described in biological tissues we will thus be able to infer a potential role of adhesion regulations to facilitate the emergence of structure in developing tissues.

At : Observatoire de Paris , on 09:30

Cette thèse présente les derniers travaux réalisés sur le gravimètre absolu à atomes froids du LNE-SYRTE. Il s’agit d’un appareil utilisant l’interférométrie atomique à l’aide d’impulsions Raman sur un nuage d’atomes froids de Rubidium 87 en chute libre, dont la phase est sensible à l’accélération de pesanteur g. Nous connaissons les limites en sensibilité comme en exactitude, qui sont liées aux aberrations du front d’onde du faisceau réalisant les impulsions Raman. Dans le but de réduire ces effets et d’atteindre une exactitude de 1×10-9 g, le dispositif a été déménagé du LNE-Trappes à l’Observatoire de Paris, afin d’apporter les modifications nécessaires. Après de nombreuses réparations dans le but de remettre l’expérience en marche, nous parvenons à réaliser des mesures de g avec une sensibilité de 20×10-9 g en 1 s, comparable à celles réalisées dans le laboratoire du LNE-Trappes. Nous présentons également une étude détaillée de l’effet de l’impulsion miroir sur les signaux de mesures. Nous montrons que dû à la présence d’inhomogénéités sur le profil d’intensité, cette impulsion est une source de perte critique du contraste des mesures, pouvant amener à une limite de sensibilité. Ce phénomène cause également un biais sur la mesure de g, de l’ordre de 5×10-9 g, qui n’est pas éliminé de l’algorithme de mesure et doit donc être pris en compte dans le bilan d’incertitude.

At : Institut de Physique Théorique, Itzykson room , on 16:00

Correlation functions are central objects in Physics and Mathematics. We define them in the context of integrable systems using determinantal formulas and explore their properties and applications on intersection theory in the moduli space of curves. Moreover, we present the relation between topological recursion, integrable systems and matrix models. This thesis comprises two articles and code implementation. The first article presents a method for deriving differential equations and recurrence relations for correlation functions in integrable systems and random matrices, allowing for the construction of coefficients that are rational functions. The second article introduces a new formula for intersection numbers of psi classes over the moduli space of curves, solving a conjecture from previous work. Python implementation for computing the following integrals over the moduli space of curves: simple and triple Hodge integrals with or without theta classes. We begin by introducing random matrix theory from a historical perspective.

At : Salle Ferdinand Berthier (U207) 29 rue d’Ulm, 75005 Paris , on 09:30

At : Salle Favard 46 rue d’Ulm 75005 Paris , on 14:00

At : , on 14:30

At : Amphithéatre de l’Institut Pierre-Gilles de Gennes Institut Pierre-Gilles de Gennes 6 rue Jean Calvin 75005 Paris , on 14:00

At : 4 place Jussieu, 75005 Paris , on 14:00

Les nanoplaquettes colloïdales semi-conductrices se distinguent par leurs propriétés optiques exceptionnelles, faisant d’elles des candidates prometteuses pour des applications en optoélectronique, photovoltaïque et information quantique. Il est donc essentiel de comprendre les propriétés collectives découlant de leurs interactions lorsqu’elles sont assemblées. Dans cette thèse, j’ai étudié, à l’aide d’un dispositif de micro-photoluminescence, des chaînes unidimensionnelles de nanoplaquettes de CdSe auto-assemblées. Il est bien établi que, dans ces chaînes, le transfert d’énergie par FRET (pour Förster Resonance Energy Transfer) domine la dynamique de luminescence, introduisant ainsi de nombreux effets collectifs, comme le scintillement. J’ai d’abord étudié la photoluminescence des nanoplaquettes individuelles. Les mesures sur des objets uniques ont permis de mieux comprendre la dynamique des nanoplaquettes ainsi que l’importance des états pièges dans ces systèmes. J’ai ensuite mis en évidence un phénomène collectif dans les assemblages, avec l’observation d’un dégroupement de photons et d’un scintillement collectif, que nous interprétons comme un processus d’annihilation exciton-exciton favorisé par le transfert d’énergie par FRET. La diffusion de l’énergie excitonique le long de la chaîne augmente la probabilité de rencontre de deux excitons sur une même nanoplaquette, favorisant ainsi les recombinaisons de type Auger et, par conséquent, améliorant le dégroupement. Un modèle de marche aléatoire a permis de décrire ce phénomène. Enfin, des études préliminaires à basse température ont révélé un décalage vers le rouge entre l’émission des nanoplaquettes individuelles et celles des chaînes, ainsi que l’apparition de nombreux pics dans le spectre des chaînes, que nous supposons être liés à l’émission d’états pièges.

At : Collège de France, 11 Place Marcelin Berthelot, Paris 5è, Salle 2 , on 15:00

Cold atom experiments have traditionally employed bulk imaging techniques, such as absorption imaging, to measure atomic sample densities. However, in recent years, single-atom imaging methods have emerged as powerful tools to probe and manipulate quantum many-body systems at the level of individual particles. This work focuses specifically on quantum gas microscopy using Raman sideband cooling in an optical lattice. This technique, which has been used extensively in several cold atoms groups, has mostly been applied in the context of discrete systems, such as the celebrated Bose- and Fermi-Hubbard models and their generalization. In this thesis, we show how quantum gas microscopy can be extended beyond its original scope to probe continuous many-body systems in a quantitative manner. Our proposed protocol consists in a rapid ramp of the pinning lattice intensity, projecting atoms from the continuum onto discrete lattice sites, followed by a Raman sideband cooling stage that confines atoms near the motional ground state and prevents them from escaping during imaging. To validate the effectiveness of this new method, we characterize the pinning and imaging process by using continuous quantum gas microscopy to reconstruct the wavefunction of a single-atom Gaussian wave packet during its free expansion. Under optimal conditions, we achieve an imaging fidelity in excess of 95%, defined as the probability of projecting an atom from the continuum to the nearest lattice site and successfully imaging it. We also apply this technique to a quasi-two-dimensional non-interacting Fermi gas, enabling us to extract two- and three-body correlation functions, g2 and g3, with unprecedented spatial resolution and finding an excellent agreement with the theoretical prediction. In particular, our measurements reveal a 90% reduction in the probability of finding two atoms in close proximity for the coldest samples, offering one of the most pristine in-situ observations of the Pauli exclusion principle in a continuous system. Finally, we propose a thermometry method based on atom number fluctuations, which are readily obtained from our imaging procedure. Crucially, this thermometry technique could be generalized to strongly interacting systems, where temperature estimation is typically challenging. The quantum gas microscopy technique developed here opens new avenues for studying continuous many-body systems with unprecedented resolution, offering a powerful tool for future cold atom experiments.

At : Petit amphi, LPTMS – Bâtiment Pascal n° 530 rue André Rivière – Université Paris-Saclay 91405 Orsay CEDEX, FRANCE , on 10:00

This thesis investigates the role of human behavior in epidemic modeling. Epidemics are influenced by both individuals’ spontaneous responses and government-imposed measures, creating a feedback loop that shapes the outbreak’s progression. However, many existing models treat human behavior as an external factor, limiting their realism. To address this gap, the thesis applies the Mean Field Game (MFG) framework, a theoretical tool for incorporating individual decision-making into epidemic models. The first part of the thesis applies the MFG approach to an SIR (Susceptible-Infected-Recovered) model with a social structure. Individuals balance infection risk against the social and economic costs of reducing contact. Numerical simulations with realistic parameters identify a Nash equilibrium, reflecting self-interested behavior, and we compare it to the social optimum, which minimizes overall societal costs. The gap between these scenarios is narrowed through constrained Nash equilibria, which include government interventions. The analysis also reveals that changes in population size or model duration can lead to phase transitions in optimal strategies, emphasizing the need for adaptive policies. In the second part, the MFG framework is extended to complex networks, where individuals differ in connectivity. Using pairwise approximation, we derive epidemic dynamics and integrate MFG principles. Simulations on realistic contact networks highlight how variations in social cost structures influence individual behavior, with significant differences observed based on connectivity levels. A related project provides new analytical insights into the SIR model on regular networks.

At : Amphithéâtre de l’IPGP, 1 rue Jussieu, Paris 75005 , on 14:00