The aim of this thesis is to investigate different aspects of few-body interactions in cold Rydberg atoms. It has been realized in a co-tutelle program between the University of Paris-Saclay and the University of Pisa working on two different experimental set ups: one at Laboratoire Aimé Cotton on cold Cs Rydberg atoms and a second at Physics Department of Pisa on cold Rb Rydberg atoms. In Laboratoire Aimé Cotton we demonstrated the existence of new few-body interactions we observed in a frozen Rydberg gas of Cs atoms. These new resonances are a generalization of already known two-body Förster resonances. They act on the internal degrees of freedom of the Rydberg atoms leading to a resonant energy transfer analogous to the one in FRET (Fluorescence Resonance Energy Transfer). In analogy with Förster resonance, three-body FRETs are tuned with an external electric field and can be observed for different principal quantum number. The three-body interaction appeared in the absence of any two-body ones and for this reasons it has a Borromean character. The presence of this external electric field leads to additional resonances between Rydberg atoms supposedly forbidden. These resonances, we call quasi-forbidden Förster resonances, are due to dipole-dipole interaction as in the case of Förster resonance. We investigated these resonances finding a large number close to the allowed two-body and three-body FRET. A precise study was necessary in order to identify and discriminate these resonances from the allowed ones.In the experiment in Pisa we instead focus our attention on the mechanical effect of van der Waals repulsion between Rydberg atoms. We studied the spatial expansion due to a van der Waals interaction in a 1D chain of Rb Rydberg atoms excited with an off-resonant laser excitation. The comparison of the spatial expansion for different detuning of the laser excitation reveals the central role of the van der Waals interaction whose strength is equal to the detuning of the laser excitation.

This fourteenth volume in the Poincaré Seminar Series is devoted to Niels Bohr, his foundational contributions to understanding atomic structure and quantum theory and their continuing importance today. This book contains the following chapters: - Tomas Bohr, Keeping Things Open; - Olivier Darrigol, Bohr's Trilogy of 1913; -John Heilbron, The Mind that Created the Bohr Atom; - Serge Haroche & Jean-Michel Raimond, Bohr's Legacy in Cavity QED; - Alain Aspect, From Einstein, Bohr, Schrödinger to Bell and Feynman: a New Quantum Revolution?; - Antoine Browaeys, Interacting Cold Rydberg Atoms: A Toy Many-Body System; - Michel Bitbol & Stefano Osnaghi, Bohr ́s Complementarity and Kant ́s Epistemology. Dating from their origin in lectures to a broad scientific audience these seven chapters are of high educational value. This volume is of general interest to physicists, mathematicians and historians.

This graduate-level textbook gives an introductory overview of the fundamentals of quantum nonlinear optics. It deals with the organization of radiation field, interaction between electronic system and radiation field, statistics of light, and mutual manipulation of light and matter. It also covers laser oscillation, dynamics of light, nonlinear optical response, and nonlinear spectroscopy, as well as ultrashort and ultrastrong laser pulse. In addition, latest results of the frontier of this science are presented. Problems and solutions help the reader to master and review the material.

Au cours de cette thèse, la physique des systèmes en interaction à été étudié expérimentalement à partir de gaz froids d'atomes de Rydberg. Les atomes de Rydberg sont des atomes dans un état fortement excités et ils ont la propriété d'interagir fortement du fait d'interactions électrostatiques à longue portée. Le premier résultat majeur de cette thèse est l'observation expérimentale d'un processus à quatre corps. Ce processus consiste en l'échange d'énergie interne entre quatre atomes de Rydberg induit par leurs interactions mutuelles. Il a été possible, en plus de son observation expérimentale, de décrire théoriquement ce processus, au niveau quantique. L'excitation par laser des gaz d'atomes de Rydberg en forte interaction a aussi été étudiée durant cette thèse. Cette situation donne lieu à de très intéressants comportements à N-corps. Ce sujet d'intérêt fondamental pourrait aussi amener à d'éventuelles applications pour la réalisation de simulateurs quantiques ou de sources de lumière non classiques. Un second résultat majeur de cette thèse est l'observation expérimentale d'une statistique fortement sub-poissonienne, i.e corrélée de l'excitation Rydberg. Ce résultat confirme le caractère à N-corps de tels systèmes. Le troisième résultat majeur de cette thèse est le développement d'un modèle théorique pour l'excitation par laser des gaz d'atomes de Rydberg en forte interaction. En utilisant les états quantiques dit états collectifs de Dicke, il a été possible de mettre au jour de nouveaux mécanismes liés au comportement à N-corps de ces sytèmes atomiques en forte interaction.

Advances in Atomic, Molecular, and Optical Physics publishes reviews of recent developments in a field that is in a state of rapid growth, as new experimental and theoretical techniques are used on many old and new problems. Topics covered include related applied areas, such as atmospheric science, astrophysics, surface physics and laser physics. Articles are written by distinguished experts and contain relevant review material and detailed descriptions of important recent developments. International experts Comprehensive articles New developments

The properties of a Rydberg atom immersed in an ultracold environment were investigated. Two scenarios were considered, one of which involves the neighbouring ground-state atoms arranged in a spatially structured configuration, while the other involves them distributed randomly in space. To calculate the influence of the multiple ground-state atoms on the Rydberg atom, Fermi-pseudopotential was used, which simplified greatly the numerical effort. In many cases, the few-body interaction can be written down analytically which reveals the symmetry properties of the system. In the structured case, we report the first prediction of the formation of ``Rydberg Borromean trimers''. The few-body interactions and the dynamics of the linear A-B-A trimer, where A is the ground-state atom and B is the Rydberg atom, were investigated in the framework of normal mode analysis. This exotic ultralong-range triatomic bound state exists despite that the Rydberg-ground-state interaction is repulsive. Their lifetimes were estimated using both quantum scattering calculations and semi-classical approximations which are found to be typically sub-microseconds. In the disordered case, the Rydberg-excitation spectra of a frozen-gas were simulated, where the nuclear degrees of freedom can be ignored. The systematic change of the spectral shape with respect to the density of the gas and the excitation of the Rydberg atom were found and studied. Some parts of the spectral shape can be described by simple scaling laws with exponents given by the basic properties of the atomic species such as the polarizability and the zero-energy electron-atom scattering length.

After the development of the tunable laser, experimental studies in Rydberg states of atoms and molecules grew at a remarkable rate. Fundamental questions were resolved, opening doors for more experimental activity and theoretical inquiry. In this comprehensive summary of knowledge of Rydberg states, which was originally published in 1983, Professors Stebbings and Dunning brought together a select collection of experimental and theoretical discussions. Composed of works by the acknowledged leaders in the field, this volume will be of value for anyone with an interest in molecular physics.