Multiple scattering of light in cold atomic clouds with a magnetic field

Starting from a microscopic theory for atomic scatterers, we describe the scattering of light by a single atom and study the coherent propagation of light in a cold atomic cloud in the presence of a magnetic field B in the mesoscopic regime. Non-pertubative expressions in B are given for the magneto-optical effects and optical anisotropy. We then consider the multiple scattering regime and address the fate of the coherent backscattering (CBS) effect. We show that, for atoms with nonzero spin in their ground state, the CBS interference contrast can be increased compared to its value when B=0, a result at variance with classical samples. We validate our theoretical results by a quantitative comparison with experimental data.Comment: 16 pages, 7 figure

Magnetic Field Enhanced Coherence Length in Cold Atomic Gases

We study the effect of an external magnetic field on coherent backscattering of light from a cool rubidium vapor. We observe that the backscattering enhancement factor can be {\it increased} with $B$. This surprising behavior shows that the coherence length of the system can be increased by applying a magnetic field, in sharp contrast with ususal situations. This is mainly due to the lifting of the degeneracy between Zeeman sublevels. We find good agreement between our experimental data and a full Monte-Carlosimulation, taking into account the magneto-optical effects and the geometry of the atomic cloud

Photonic Hall effect in cold atomic clouds

On the basis of exact numerical simulations and analytical calculations, we describe qualitatively and quantitatively the interference processes at the origin of the photonic Hall effect for resonant Rayleigh (point-dipole) scatterers in a magnetic field. For resonant incoming light, the induced giant magneto-optical effects result in relative Hall currents in the percent range, three orders of magnitude larger than with classical scatterers. This suggests that the observation of the photonic Hall effect in cold atomic vapors is within experimental reach.Comment: 4 pages 4 figure

Innocent strategies as presheaves and interactive equivalences for CCS

Seeking a general framework for reasoning about and comparing programming languages, we derive a new view of Milner's CCS. We construct a category E of plays, and a subcategory V of views. We argue that presheaves on V adequately represent innocent strategies, in the sense of game semantics. We then equip innocent strategies with a simple notion of interaction. This results in an interpretation of CCS. Based on this, we propose a notion of interactive equivalence for innocent strategies, which is close in spirit to Beffara's interpretation of testing equivalences in concurrency theory. In this framework we prove that the analogues of fair and must testing equivalences coincide, while they differ in the standard setting.Comment: In Proceedings ICE 2011, arXiv:1108.014

Symmetric dialogue games in the proof theory of linear logic

This thesis develops an original approach to the interactive interpretation of the proof theory of linear logic. In contrast with the player/opponent dichotomy commonly associated with game semantics for logic, we propose a model in which the two players have symmetric roles. More precisely, we shift from a situation in which a player aims at proving a goal while the other one aims at refuting it to a situation in which the players aim at proving opposite goals. In the tradition of computation-as-proof-search, each step of the interaction is seen as a step in two orthogonal sequent calculus proof searches. To some extent, this work contributes to formalising the connections between proof search and proof normalisation. We first present a simple symmetric game for the additive fragment of linear logic, as a first introduction to dual proof search. We then introduce a much more involved symmetric game accounting for the multiplicative and additive fragment of linear logic. In order to get a full completeness result, we then develop a third, highly concurrent, symmetric game. Finally, we investigate a few extensions of our model.Cette thèse développe une approche originale de l'interprétation interactive de la théorie de la démonstration en logique linéaire. À l'inverse du cadre joueur/opposant communément associé aux sémantiques des jeux pour la logique, nous proposons un modèle dans lequel les deux joueurs ont des rôles symétriques. Plus précisément, nous passons d'une situation dans laquelle un joueur tente de démontrer un énoncé tandis que l'autre tente de le réfuter à une situation dans laquelle les deux joueurs tentent de démontrer des énoncés contraires. Dans la tradition du calcul vu comme comme recherche de démonstrations, chaque étape de l'interaction est vue comme une étape de deux recherches de démonstrations orthogonales en calcul des séquents. Ce travail contribue dans une certaine mesure à formaliser les liens entre la recherche de démonstrations et la normalisation de démonstrations. Nous présentons d'abord un jeu symétrique simple pour le fragment additif de la logique linéaire, en guise d'introduction à la recherche duale de démonstrations. Nous passons ensuite à un jeu symétrique bien plus complexe pour le fragment additif et multiplicatif de la logique linéaire. Afin d'obtenir un résultat de pleine complétude, nous développons ensuite un troisième jeu à la fois symétrique et concurrent. Enfin, nous étudions quelques extensions de notre modèle

Localisation faible de la lumière par un gaz d'atomes froids en présence d'un champ magnétique

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Jeux de dialogue symétriques dans la théorie de la démonstration de la logique linéaire

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Proof and refutation in MALL as a game

We present a setting in which the search for a proof of B or a refutation of B (i.e., a proof of ¬B) can be carried out simultaneously: in contrast, the usual approach in automated deduction views proving B or proving ¬B as two, possibly unrelated, activities. Our approach to proof and refutation is described as a two-player game in which each player follows the same rules. A winning strategy translates to a proof of the formula and a counter-winning strategy translates to a refutation of the formula. The game is described for multiplicative and additive linear logic (MALL). A game theoretic treatment of the multiplicative connectives is intricate and our approach to it involves two important ingredients. First, labeled graph structures are used to represent positions in a game and, second, the game playing must deal with the failure of a given player and with an appropriate resumption of play. This latter ingredient accounts for the fact that neither player might win (that is, neither B nor ¬B might be provable)