Polaritonic modes in a dense cloud of atoms

We analyze resonant light scattering by an atomic cloud in a regime where near-field interactions between scatterers cannot be neglected. We first use a microscopic approach and calculate numerically the eigenmodes of the cloud for many different realizations. It is found that there always exists a small number of polaritonic modes that are spatially coherent and superradiant. We show that scattering is always dominated by these modes. We then use a macroscopic approach by introducing an effective permittivity so that the atomic cloud is equivalent to a dielectric particle. We show that there is a one-to-one correspondence between the microscopic polaritonic modes and the modes of a homogeneous particle with an effective permittivity

Diffusion de la lumière dans les nuages denses mésoscopiques d'atomes froids

Lorsque l on place des atomes suffisamment proches les uns des autres, l interaction dipôle-dipôle résonante entre les atomes modifie leurs propriétés. Les atomes se comportent alors de manière collective. Ces effets collectifs se produisent lorsque les distances interparticulaires sont de l ordre de l/(2Pi), où l est la longueur d onde de la transition atomique. La densité atomique est alors de l ordre de 10^14 at/cm^3. Afin de créer des échantillons d atomes froids présentant des densités aussi élevées, nous avons mis en place plusieurs méthodes de chargement de nos pinces optiques de taille micrométrique. L une d elles utilise un processus d évaporation forcée qui amène les atomes proches de la dégénérescence quantique. En utilisant des nuages denses contenant quelques centaines d atomes à des densités spatiales élevées, et en étudiant les modifications de la diffusion de la lumière qui en résultent, nous avons pu mettre en évidence des effets collectifs entre les atomes. Nous avons par ailleurs mesuré le retard de Wigner associé à la diffusion élastique de la lumière par un atome unique de rubidium. Nous avons mesuré un retard proche de la valeur théorique, c est-à-dire deux fois la durée de vie de la transition atomique (52 ns).When several atoms are placed close to each other, the resonant dipole-dipole interactionbetween atoms modifies the atomic properties and atoms behave collectively. These collective effects occur for interatomic distances on the order of l/(2Pi) where l is the wavelength of the atomic transition. The atomic density is then on the order of 10^14 at/cm^3. To create such cold atomic samples, we load optical tweezers with a microscopic size according to several loading schemes. One of them uses forced evaporative cooling and brings the atoms close to quantum degeneracy. We have used dense clouds containing a few hundred atoms with a high spatial density to demonstrate collective effects between the atoms. In particular, we have studied how these effects modify the scattering of light by the cloud. Besides, we have also measured for the first time the time-delay associated to the elastic scattering of light by a single rubidium atom, the so-called Wigner delay. We have shown that this delay is close to the theoretical prediction of twice the lifetime of the atomic transition (52 ns).PARIS11-Inst. Optique (914712302) / SudocSudocFranceF

Two-dimensional transport and transfer of a single atomic qubit in optical tweezers

Quantum computers have the capability of out-performing their classical counterparts for certain computational problems1. Several scalable quantum-computing architectures have been proposed. An attractive architecture is a large set of physically independent qubits arranged in three spatial regions where (1) the initialized qubits are stored in a register, (2) two qubits are brought together to realize a gate and (3) the readout of the qubits is carried out2, 3. For a neutral-atom-based architecture, a natural way to connect these regions is to use optical tweezers to move qubits within the system. In this letter we demonstrate the coherent transport of a qubit, encoded on an atom trapped in a submicrometre tweezer, over a distance typical of the separation between atoms in an array of optical traps4, 5, 6. Furthermore, we transfer a qubit between two tweezers, and show that this manipulation also preserves the coherence of the qubit

Near-resonance light scattering by a cold ensemble of interacting atoms - Measurement of the coherent optical response (Orale)

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Construction d'une fontaine double a atomes froids de 87Rb et 133Cs (étude des effets dépendant du nombre d'atomes dans une fontaine)

PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Near resonance light scattering by an ensemble of interacting atoms (Orale)

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Light scattering by interacting two-level atoms (poster)

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Light induced dipole-dipole interactions (poster)

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