Localisation faible de la lumière dans un gaz d´atomes froids

Recent experimental results show that the interference contrast observed in coherent backscattering (CBS) of light by cold atoms is drastically reduced with respect to classical disordered media. In the present theoretical contribution, we study the impact of the degeneracy of the atomic dipole transition on weak localisation of light. The non-scalar components of the atomic scattering operator are characteristic of the internal structure, and strongly modify the interference properties of multiple light scattering. A systematic analysis in terms of irreducible tensors permits to calculate exact analytical expressions for the single and double scattering contributions to the CBS signal for arbitrarily degenerate atomic dipole transitions. Furthermore, we sum up the series of ladder and crossed diagrams that describe the average scattered light intensity and the weak localisation corrections, respectively. We find that the degeneracy of the atomic transition has negligeable impact on the average light amplitude, small impact on the average intensity, but decisive impact on the interference corrections. The internal degrees of freedom very effectively reduce the interference leading to weak localisation and, therefore, the CBS signal for any degenerate atomic dipole transition.Experimentelle Daten zur kohärenten Rückstreuung von Licht an kalten Atomen zeigen, daß der Interferenzkontrast im Vergleich zu klassischen Streumedien deutlich reduziert ist. In der vorliegenden theoretischen Arbeit untersuchen wir den Einfluß der Entartung des atomaren Dipolübergangs auf die schwache Lokalisierung von Licht. Die nicht-skalaren Komponenten des atomaren Streutensors sind charakteristisch für die interne Struktur des Dipolübergangs und modifizieren insbesondere die Interferenzeigenschaften. Eine systematische Entwicklung nach irreduziblen Tensoren gestattet es, die Einfach- und Doppelstreuung an Atomen mit beliebiger Entartung analytisch exakt zu berechnen und die Reihe der Leiter- und Kreuzdiagramme für die mittlere Intensität sowie die Interferenzterme der schwachen Lokalisierung geschlossen aufzusummieren. Es zeigt sich, daß die Entartung des atomaren Dipolübergangs für die mittlere Lichtamplitude unerheblich ist und die mittlere Lichtintensität nur wenig beeinflußt. Dagegen werden der Interferenzbeitrag der schwachen Lokalisierung und damit das kohärente Rückstreusignal drastisch verringert, in schöner Übereinstimmung mit dem experimentellen Resultat

Localisation faible de la lumière dans un gaz d´atomes froids

Recent experimental results show that the interference contrast observed in coherent backscattering (CBS) of light by cold atoms is drastically reduced with respect to classical disordered media. In the present theoretical contribution, we study the impact of the degeneracy of the atomic dipole transition on weak localisation of light. The non-scalar components of the atomic scattering operator are characteristic of the internal structure, and strongly modify the interference properties of multiple light scattering. A systematic analysis in terms of irreducible tensors permits to calculate exact analytical expressions for the single and double scattering contributions to the CBS signal for arbitrarily degenerate atomic dipole transitions. Furthermore, we sum up the series of ladder and crossed diagrams that describe the average scattered light intensity and the weak localisation corrections, respectively. We find that the degeneracy of the atomic transition has negligeable impact on the average light amplitude, small impact on the average intensity, but decisive impact on the interference corrections. The internal degrees of freedom very effectively reduce the interference leading to weak localisation and, therefore, the CBS signal for any degenerate atomic dipole transition.Experimentelle Daten zur kohärenten Rückstreuung von Licht an kalten Atomen zeigen, daß der Interferenzkontrast im Vergleich zu klassischen Streumedien deutlich reduziert ist. In der vorliegenden theoretischen Arbeit untersuchen wir den Einfluß der Entartung des atomaren Dipolübergangs auf die schwache Lokalisierung von Licht. Die nicht-skalaren Komponenten des atomaren Streutensors sind charakteristisch für die interne Struktur des Dipolübergangs und modifizieren insbesondere die Interferenzeigenschaften. Eine systematische Entwicklung nach irreduziblen Tensoren gestattet es, die Einfach- und Doppelstreuung an Atomen mit beliebiger Entartung analytisch exakt zu berechnen und die Reihe der Leiter- und Kreuzdiagramme für die mittlere Intensität sowie die Interferenzterme der schwachen Lokalisierung geschlossen aufzusummieren. Es zeigt sich, daß die Entartung des atomaren Dipolübergangs für die mittlere Lichtamplitude unerheblich ist und die mittlere Lichtintensität nur wenig beeinflußt. Dagegen werden der Interferenzbeitrag der schwachen Lokalisierung und damit das kohärente Rückstreusignal drastisch verringert, in schöner Übereinstimmung mit dem experimentellen Resultat

Speed of sound in disordered Bose-Einstein condensates

Disorder modifies the sound-wave excitation spectrum of Bose-Einstein condensates. We consider the classical hydrodynamic limit, where the disorder correlation length is much longer than the condensate healing length. By perturbation theory, we compute the phonon lifetime and correction to the speed of sound. This correction is found to be negative in all dimensions, with universal asymptotics for smooth correlations. Considering in detail optical speckle potentials, we find a quite rich intermediate structure. This has consequences for the average density of states, particularly in one dimension, where we find a "boson dip" next to a sharp "boson peak" as function of frequency. In one dimension, our prediction is verified in detail by a numerical integration of the Gross-Pitaevskii equation.Comment: final, extended version with 2 new figure

Localisation faible de la lumière dans un gaz d'atomes froids (rétrodiffusion cohérente et structure quantique interne)

Des résultats récents de rétrodiffusion cohérente (CBS) de la lumière sur un nuage d'atomes froids ont montre que le contraste d'interférence est fortement réduit par rapport aux milieux désordonnés classiques. Dans le présent travail théorique, nous étudions l'impact de la dégénérescence de la transition dipolaire atomique sur la localisation faible de la lumière. Les composantes non-scalaires du tenseur de diffusion atomique, caractéristiques de la structure interne, modifient la diffusion multiple de la lumière. Une analyse systématique en termes de tenseurs irréductibles permet d'obtenir des expressions analytiques exactes pour les contributions de la diffusion simple et double au signal CBS. Nous résommons analytiquement la série des diagrammes échelle de l'intensité diffusée et la série des diagrammes croises de la localisation faible, pour toute transition atomique dégénérée. La structure interne n'a qu'une influence négligeable sur l'amplitude lumineuse moyenne, et une influence faible sur l'intensité moyenne. Par contre, la dégénérescence de la transition atomique réduit drastiquement l'interférence de localisation faible et donc la rétrodiffusion cohérente, et explique ainsi les résultats expérimentaux.Recent experimental results show that the interference contrast observed in coherent backscattering (CBS) of light by cold atoms is drastically reduced with respect to classical disordered media. In the present theoretical contribution, we study the impact of the degeneracy of the atomic dipole transition on weak localization of light. The nonscalar components of the atomic scattering operator are characteristics of the internal structure, and strongly modify the interference properties of multiple light scattering. A systematic analysis in terms of irreducible tensors permits to calculate exact analytical expressions for the single and double scattering contributions to the CBS signal for arbitrarily degenerate atomic dipole transitions. Furthermore, we sum up the series of ladder and crossed diagrams that describe the average scattered light intensity and the weak localization corrections, respectively. We find that the degeneracy of the atomic transition has negligible impact on the average light amplitude, small impact on the average intensity, but decisive impact on the interference corrections. The internal degrees of freedom very effectively reduce the interference of weak localization and, therefore, the CBS signal for any degenerate atomic dipole transition.NICE-BU Sciences (060882101) / SudocSudocFranceGermanyFRD

Cdc42-dependent actin dynamics controls maturation and secretory activity of dendritic cells

Cell division cycle 42 (Cdc42) is a member of the Rho guanosine triphosphatase family and has pivotal functions in actin organization, cell migration, and proliferation. To further study the molecular mechanisms of dendritic cell (DC) regulation by Cdc42, we used Cdc42-deficient DCs. Cdc42 deficiency renders DCs phenotypically mature as they up-regulate the co-stimulatory molecule CD86 from intracellular storages to the cell surface. Cdc42 knockout DCs also accumulate high amounts of invariant chain–major histocompatibility complex (MHC) class II complexes at the cell surface, which cannot efficiently present peptide antigens (Ag’s) for priming of Ag-specific CD4 T cells. Proteome analyses showed a significant reduction in lysosomal MHC class II–processing proteins, such as cathepsins, which are lost from DCs by enhanced secretion. As these effects on DCs can be mimicked by chemical actin disruption, our results propose that Cdc42 control of actin dynamics keeps DCs in an immature state, and cessation of Cdc42 activity during DC maturation facilitates secretion as well as rapid up-regulation of intracellular molecules to the cell surface