arXiv subjects

Sacha Barré

Publications and source records attributed to Sacha Barré.

5 recordsLinked to original sources

arXiv2026

Energy-resolved transport of ultracold atoms across the Anderson transition: theory and experiment

In a recent experiment [X. Yu et al., arXiv:2602.07654], energy-resolved measurements of an atomic matter wave spreading in a speckle potential enabled the direct observation of the three-dimensional Anderson transition. In this work, we present a quantitative theoretical description of the matter-wave dynamics based on a tailored implementation of the self-consistent theory of localization, which incorporates both the spectral and spatial properties of the state prepared in the disorder. We benchmark this theoretical approach against ab initio numerical simulations, and use it to analyze the atom density profiles observed experimentally in the localized, diffusive, and critical regimes. Particular emphasis is placed on the key role of the atomic energy distribution, especially on the distinct contributions of Bose-condensed and thermal atoms to interpret the experimental profiles. Our framework provides a versatile and efficient theoretical toolbox for quantitatively describing wave-packet dynamics in three-dimensional disordered quantum systems, which remain challenging for state-of-the-art large-scale numerical simulations.

cond-mat.quant-gas↗

arXiv2026

Direct Observation of the Three-Dimensional Anderson Transition with Ultracold Atoms in a Disordered Potential

Anderson localization of particles -- the complete halt of wave transport through multiple scattering and phase coherence -- is a paradigmatic manifestation of quantum interference in disordered media. In three dimensions, the scaling theory predicts a quantum phase transition at a critical energy, the mobility edge, separating localized from diffusive states and underpinning metal-insulator transitions in electronic systems. Despite decades of experimental efforts, a direct observation of this emblematic transition for matter waves has remained elusive. Previous attempts with ultracold atoms were hindered by strong and uncontrolled energy broadening, resulting in indirect, sometimes inaccurate, and model-dependent estimates of the mobility edge. Here we implement a novel energy-resolved scheme to prepare atomic matter waves with a narrow energy distribution and track their expansion dynamics over long timescales. This allows for a direct observation of the three-dimensional Anderson transition in a laser-speckle disordered potential, and for a precise measurement of the mobility edge that is independent of any underlying theoretical modeling. Our measurements show excellent agreement with state-of-the-art numerical predictions over a wide range of disorder strengths, resolving long-standing discrepancies between prior experiments and theory. Beyond the three-dimensional Anderson transition, our approach opens new avenues for quantitative investigations of quantum critical phenomena in spatially disordered systems, including the roles of dimensionality, symmetry class, and interactions.

cond-mat.quant-gas↗

arXiv2024

Study of energy deposition in the coolant of LFR

The determination of the fraction of energy deposited in the coolant is required for the setup of accurate thermal-hydraulic calculations in reactor core analysis. This study focuses on assessing this fraction and analysing the neutronic and photonic processes contributing to energy deposition in Lead-cooled Fast Reactors (LFRs). Using OpenMC, coupled neutron-photon transport calculations were performed within a fuel pin cell geometry, representative of the one under development at \textsl{new}cleo. Additionally, the implementation of lattice geometry was tested to gauge the impact of reflective boundary conditions on computational efficiency. In the context of a surface-based algorithm, the pin geometry has proven to be computationally more cost-effective. The fraction of energy deposited in the LFR coolant was evaluated at $\sim5.6$\%, surpassing that of pressurised water Reactors ($\lessapprox 3\%$), with photon interactions emerging as the principal contributor. The influence of bremsstrahlung radiation was also considered, revealing minor impact compared to other photonic processes. Given the continuous exploration of various core designs and the expectation of diverse operational conditions, a parametric analysis was undertaken by varying the coolant temperature and pitch values. Temperature changes did not significantly affect the results, while modifying the pitch induced a rise in the fraction of deposited energy in lead, highlighting its dependence on the coolant mass. This mass effect was explored in various fuel assembly designs based on the ALFRED benchmark and on the typical assembly model proposed by \textsl{new}cleo, leading to a correlation function for the prediction of coolant heating in realistic assemblies from simple pin cell calculations.

physics.ins-det↗

arXiv2024

FunTuple: A new N-tuple component for offline data processing at the LHCb experiment

The offline software framework of the LHCb experiment has undergone a significant overhaul to tackle the data processing challenges that will arise in the upcoming Run 3 and Run 4 of the Large Hadron Collider. This paper introduces FunTuple, a novel component developed for offline data processing within the LHCb experiment. This component enables the computation and storage of a diverse range of observables for both reconstructed and simulated events by leveraging on the tools initially developed for the trigger system. This feature is crucial for ensuring consistency between trigger-computed and offline-analysed observables. The component and its tool suite offer users flexibility to customise stored observables, and its reliability is validated through a full-coverage set of rigorous unit tests. This paper comprehensively explores FunTuple's design, interface, interaction with other algorithms, and its role in facilitating offline data processing for the LHCb experiment for the next decade and beyond.

physics.data-an↗

arXiv2023

Optical Photon Simulation with Mitsuba3

Optical photon propagation is an embarrassingly parallel operation, well suited to acceleration on GPU devices. Rendering of images employs similar techniques -- for this reason, a pipeline to offload optical photon propagation from Geant4 to the industry-standard open-source renderer Mitsuba3 has been devised. With the creation of a dedicated plugin for single point multi-source emission, we find a photon propagation rate of $2\times10^{5}$ photons per second per CPU thread using LLVM and $1.2\times10^{6}$ photons per second per GPU using CUDA. This represents a speed-up of 70 on CPU and 400 on GPU over Geant4 and is competitive with other similar applications. The potential for further applications is discussed.

physics.comp-ph↗