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Accueil du site > Séminaires > Année en cours > Séminaire doctorants 2e année

Séminaire doctorants 2e année

Jeudi 28 septembre

10h30 — Khalid OUBAHA

Titre : Microwave transport in complex environments - towards wireless communication on chip level
Encadrement : Ulrich Kuhl, Olivier Legrand
Résumé : Reverberation chambers (RC) are used in the industry to test the immunity or the susceptibility of on-board electronic systems (plane, automobile, smartphone) towards the electromagnetic waves present in their environment. The international standards fixed a number of statistical criteria with which the RC have to comply. The main criterion requires that the field inside the cavity is isotropic and the field components follow a bivariate Gaussian distribution. In the electromagnetic community this so called Hill’s hypothesis[1] is typically realized when the resonance overlap is large. It is also realized in chaotic cavities once one is sufficiently far from the lowest eigenfrequency and is related to Berry’s ansatz [2]. The strong overlap regime, known as Ericson regime [3] in nuclear physics, has also been known in room acoustics for a long time [4]. Here, we study experimentally several statistics of the electromagnetic response in a chaotic reverberation chamber (RC) (see figure) and investigate the effects of the modal overlap on the distributions of reflection and transmission and compare them to predictions using random matrix theory (RMT) numerics. We also verify the prediction by Schroeder and Kuttruff concerning the relation between the mean frequency spacing of maxima and the average decay rate of the system [5]. To go towards communication we included an established communication channels, experimentally imposed by directed horn antennas, which is coupled to our CRC. We find a good agreement between the distribution of transmission and a theory developed in collaboration with D. Savin [6].

[1] D.A. Hill, ’Plane wave integral representation for fields in reverberation chambers,’ IEEE Trans. on Electromagnetic Compatibility 40, 209 (1998).
[2] M.V. Berry, ’Regular and irregular semiclassical wavefunctions,’ J. Phys. A 10, 2083 (1977).
[3] T.Ericson, ’A theory of fluctuations in nuclear cross sections,’ Ann. Phys. (N.Y.) 23, 390 (1963).
[4] P. Lee and A. Stone, ’Universal conductance fluctuations in metals,’ Phys. Rev. Lett. 55, 1622 (1985).
[5] M.R. Schroeder and K.H. Kuttruff, ’On frequency response curves in rooms. Comparison of experimental, theoretical, and Monte Carlo results for the average frequency spacing between maxima,’ J. Acoust. Soc. Am. 34, 76 (1962).
[6] D.V. Savin, M. Richter, U. Kuhl, O. Legrand, and F. Mortessagne, ’Fluctuations in an established transmission in the presence of a complex environment,’ Phys. Rev. E 96, 032221 (2017).

11h — Aurélien ELOY

Titre : Diffusing-wave spectroscopy with a cold atomic cloud
Encadrement : Robin Kaiser
Résumé : While in conventional lasers feedback is provided by the optical cavity, in a so-called random laser it is provided by multiple scattering in the gain medium itself. This kind of laser has been studied for many years [1]. It is usually based on condensed matter systems, where gain and scattering are obtained by two different physical systems. But one can also use cold atoms to provide gain and scattering at the same time. as recently demonstrated by our team [2]. However the characterization of such a laser is challenging. The coherence properties of this cold-atom random laser remain to be studied in more details.

The first step towards this characterization was the study of the frequency noise power spectral density of conventional lasers. We have recently shown the role of the atoms in the conversion from frequency noise to intensity noise [3]. Now we propose to move forward by analyzing the second-order correlation function of the emitted light in different regimes, first in a passive medium and finally by adding gain. A probe is sent in the cold-atomic cloud and the scattered photons are collected via a Hanbury Brown – Twiss detection set-up. We are able to probe the atoms either in the single scattering regime, when the probe is far detuned, or in the multiple scattering regime. This is the first demonstration of the diffusing wave spectroscopy [4] in a cold atomic cloud.

[1]. D. S. Wiersma, Nat. Phys., 4, 359 (2008)
[2]. Q. Baudouin, N. Mercadier, V. Guarrera, W. Guerin and R. Kaiser, Nat. Phys., 9, 357 (2013)
[3]. S. Vartabi-Kashanian, A. Eloy, W. Guerin, M. Lintz, M. Fouché and R. Kaiser, Phys. Rev. A, 94, 043622 (2016)
[4]. D. J. Pine, D. A. Weitz, P. M. Chaikin and E ; Herbolzheimer, Phys. Rev. Lett., 60, 1134 (1988)

11h30 — Florent MAZEAS

Titre : Ingénierie de structures photoniques intégrées pour la communication quantique
Encadrement : Laurent Labonté, Sébastien Tanzilli
Résumé : L’intrication est une des ressources fondamentales pour les nombreuses applications qu’offrent les technologies quantiques. Parmi celles-ci, la communication quantique exploite cette propriété afin de garantir une sécurité inconditionnelle lors de la transmission de données entre différents utilisateurs. Dans le cadre de ma thèse, nous nous intéressons à la génération de ces états intriqués sur des puces photoniques intégrées. Après avoir démontré la possibilité de fabriquer des sources de paires de photons intriqués de haute qualité, nous montrerons la capacité d’intégration de telles sources sur la plateforme Silicium. Durant cet exposé, serons discutés les différentes étapes menant à une intégration complète et notamment l’étape de filtrage du faisceau de pompe par l’intermédiaire de filtres de Bragg intégrés aux designs innovants. Une source de paires de photons hyperintriqués réalisée sur Niobate de Lithium sera également présentée. Tout comme les travaux menés sur Silicium, nous exploitons une stratégie de démultiplexage spectral à l’aide de composants fibrés pour rendre la source compatible avec les réseaux des télécommunications. Ces sources de haute qualité d’intrication constituent un pas supplémentaire vers la possibilité de réaliser des réseaux de communications quantiques.