. Fig, Il a une probabilité exp(?r/) de parcourir une distance r dans le milieu avant d'? etre diffusé, et sa direction de propagation après une diffusion est tirée au hasard suivant le diagramme de rayonnement de diffuseur On obtient un chemin de diffusion lorsque le photon sort du milieu diffusant. La méthode des " photons partiels " fì eches tiretées) consistèconsistè a considérer que l'on obtient un chemin donné après chaque diffusion. exp(?r/) de parcourir une distance r dans le milieu avant de subir une diffusion, ´ etant le libre parcours moyen. Le direction de propagation après une diffusion est tirée au hasard suivant le diagramme de rayonnement du diffuseur (dans le cas des atomes, ce sera un atome dans un sous-niveau donné, déterminé aléatoirement suivant la matrice densitéé caractérisant un atome) On continue de la sorte jusqu'` a ce que le photon sorte du milieu, et on obtient alors un chemin de diffusion, dont l'ordre est donné par le nombre de diffusions subies par le photon. La contribution de ce cheminàcheminà l'intensité incohérente, etàetà l'augmentation de rétrodiffusion (obtenue en calculantégalementàcalculantégalementcalculantégalementà partir de ce chemin l'amplitude du chemin inverse ) est alors calculée. Notons que l'on peut en pratique améliorer l'efficacité de la méthode en considérant que l'on a un chemin donné après chaque diffusion

. Le-principe-de-la-méthode-est-illustré-sur-la-figure-6, Par rapport aux méthodes présentées précédemment, celle-ci a le défaut de ne fournir aucune expression analytique, ni pour la dépendance des résultats en fonction de la transition atomique considérée (J et J e ), ni pour la forme du cône de rétrodiffusion. Il faut donc effectuer un nouveau calcul pour chaque valeur de J (et J e ) que l'on désiré etudier. Par contre, cette méthode a l'avantage de fournir efficacement l'intensitéintensitéà tous les ordres, avec une bonne précision. De plus, il est très facile de considérer une géométrie arbitraire pour le milieu diffusant, alors Bibliographie [1] M, 1989 Les Houches summer school on Chaos and Quantum Physics, 1991.

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