Détection de collision | EPFL Graph Search

Dans les simulations physiques, les jeux vidéo et la géométrie algorithmique, la détection de collision implique l'utilisation d'algorithmes pour tester les collisions (intersection de solides donnés), pour calculer des trajectoires, les dates d'impact et des points d'impact dans une simulation physique. Vue d'ensemble right|thumb|Des billes de billard s'entrechoquant est un exemple typique du domaine de la détection de collision. Dans la simulation physique, on souhaite procéder à des expériences, comme jouer au billard. La physique des billes de billard est bien décrite, à l'aide de modèles de mouvement de corps rigide et de collision élastique. Une description initiale de la scène devrait être donnée, avec une description physique précise de la table de billard et des billes, de même que les positions initiales de toutes les billes. Étant donné une certaine impulsion initiale sur la bille blanche (résultant probablement d'un joueur tapant la bille à l'aide de sa queue),

Functional Elements for Quantum-Dot-Based Integrated Quantum Photonics

Antoine Maxime Delgoffe

Quantum Integrated Photonics (QIP) harnesses quantum-states of light on tiny chips, from generation to processing and eventual detection. Within this context, this thesis explores functional QIP elements resulting from the monolithic integration of semiconductor quantum dots (QDs) in tailored photonic crystal systems of coupled cavities and waveguides. QDs present atom-like optical and electronic properties, which make them ideal for a deterministic, on-demand generation of single photons. Such QD-based devices can be expected to provide solutions for applications of QIP in quantum computing, communication and metrology. Here, we exploit the pyramidal site-controlled QD technology to reproducibly fabricate quantum photonic elements in which an arbitrary number of QD quantum emitters are embedded in photonic crystal (PhC) structures with alignment precision better than 50nm. Systematic investigation of reference structures with dense arrays of QDs to observe optical mode structures, and identical structures with only a few QDs for selective excitation and coupling to prescribed modes, is conducted using spatially-resolved micro-photoluminescence spectroscopy. Observations are consolidated by quantitative comparisons to 2D FDM numerical simulations. First, defect-potential PhC cavities are made by precise transverse shifts of selected holes in a PhC defect waveguide. In these structures, defect modes confined by a local mismatch in the photonic dispersion are observed, with peculiar polarization signatures. The fundamental defect modes display quality factors and mode volumes on par with conventional Ln=3 cavities. Defect modes can be selectively excited, and weak coupling of a single QD to cavity modes is demonstrated using thermal tuning. Next, tilted-potential PhC cavities supporting Airy-Bloch photonic modes are realized by staggered shifts of holes from 0 to smax along the axis. Such modes are suitable for local interaction and extraction of single photons from site-controlled QDs and their transmission to remote locations. Selective excitation of several modes with properly positioned QDs is demonstrated. Weak coupling of a QD to a single mode is achieved by thermal tuning, showing features of Purcell effect. The quasi linear energy-position dispersion of the Airy-Bloch modes maxima provides a mean for constructing an efficient wavelength-multiplexed single-photon injector. Finally, a monolithic model structure, consisting of two PhC waveguides side-coupled to a center Ln=3 cavity, with four ports connected to suspended nanobeams and grating couplers for light extraction/injection, is fabricated and tested. Five QDs are embedded in specific positions; four can act as on-chip single photon sources while the fifth, embedded in the center cavity, could be used for photon processing functions. The quasi-resonant excitation of this fifth QD via its p-shell is demonstrated using a pump laser beam that is injected into a waveguide through the grating coupler. Part of the s-shell emission is coupled to another targeted optical channel, demonstrating a complete photonic connectivity of all elements, a fundamental requirement towards on-chip processing. The results of this thesis consolidate the assets of the site-controlled pyramidal QD technology and tailored PhC confinement towards the realization of functional elements for quantum integrated photonics circuits.

EPFL2020

Dynamic triangulations for efficient 3D simulation of granular materials

Jean-Albert Ferrez

Granular materials are omnipresent in many fields ranging from civil engineering to food, mining and pharmaceutical industries. Often considered a fourth state of matter, they exhibit specific phenomena such as segregation, arching effects, pattern formation, etc. Due to its potential capability of realistically rendering these behaviors, the Distinct Element Method (DEM) is a very enticing simulation technique. Indeed it makes it possible to analyze and observe phenomena that are barely if at all accessible experimentally. DEM works by tracking every particle in the system individually, maintaining for each a trajectory influenced by external factors such as gravitation or contacts with boundary objects and by the interactions with other grains. The mathematical problem of identifying pairs of grains that interact and locating precisely where the contact occurs is highly dependent on the shape of the grains. We focus in this thesis on 3D spherical grains and use dynamic weighted Delaunay triangulations to track the collisions. The triangulation is built on the centers of the grains and evolves to follow their motion. We prove that all potentially colliding pairs of spheres are adjacent in the triangulation. As there are 6n to 8n edges for n spheres in most practical cases, the complexity of the collision detection becomes linear instead of quadratic in the number of particles, with a small overhead in maintaining the triangulation with efficient local operations. For the physical problem of realistically rendering the collision in a numerical contact model suitable for computer simulation, we have used widely accepted theories such as the viscoelastic model of Cundall, but have also tested some recent, more sophisticated developments in the field. The collision detection and contact models have been implemented in a modular DEM simulation code with advanced features in data structures storing the triangulation, in numerical robustness of the geometric computations, and in parallel processing on shared memory computers. Optimal packing of powders is important in many industrial processes, yet no theoretical result exists when dealing with grains of different sizes. We have performed simulations of such cases and could compare our results with experimental data. Preliminary results have been obtained regarding the relation between the size and proportion of grains and the density of the packing. Other simulations have also been performed, such as the granular flow through an hourglass. As no efficient simulation method is currently known for non-spherical 3D grains, we propose an intermediate approach of gluing spheres together into arbitrary shaped clusters and show some examples based on this approach.

EPFL2001