Mean-density Bogoliubov description of inhomogeneous Bose-condensed gases

Excitation spectrum and quasiparticles in quantum gases. A rigorous approach

This thesis is devoted to a rigorous study of interacting quantum gases. The main objects of interest are the closely related concepts of excitation spectrum and quasiparticles. The immediate motivation of this work is to propose a spectral point of view concerning these two concepts. In the first part of this thesis we discuss the concepts of excitation spectrum and quasiparticles. We provide an overview of physical motivations and based on that we propose a spectral and Hamiltonian-based approach towards these terms. Based on that, we formulate definitions and propositions related to these c…

Cavity-enhanced superradiant Rayleigh scattering with ultra-cold and Bose-Einstein condensed atoms

We report on the observation of collective atomic recoil lasing and superradiant Rayleigh scattering with ultracold and Bose-Einstein condensed atoms in an optical ring cavity. Both phenomena are based on instabilities evoked by the collective interaction of light with cold atomic gases. This publication clarifies the link between the two effects. The observation of superradiant behavior with thermal clouds as hot as several tens of $μ\textrm{K}$ proves that the phenomena are driven by the cooperative dynamics of the atoms, which is strongly enhanced by the presence of the ring cavity.

Hybrid synchronization in coupled ultracold atomic gases

We study the time evolution of two coupled many-body quantum systems one of which is assumed to be Bose condensed. Specifically, we consider two ultracold atomic clouds populating each two localized single-particle states, i.e. a two-component Bosonic Josephson junction. The cold atoms cloud can retain its coherence when coupled to the condensate and displays synchronization with the latter, differing from usual entrainment. We term this effect among the ultracold and the condensed clouds as {\it hybrid synchronization}. The onset of synchronization, which we observe in the evolution of averag…

玻色气体从低温到临界温度的重整化热力学

Renormalised thermodynamics for Bose gases from low to critical temperatures

研究通过非微扰近似计算稀薄玻色气体的热力学性质，提出自洽描述的重整化方法，确定超流序参量在临界点的行为。

Rigorous approach to the problem of ultraviolet divergencies in dilute Bose gases

We have found an expression for the full many-body Green's function of N pairwise finite-range interacting atoms, in a form of a chain fraction involving two-body T-matrices only, with no explicit presence of the interaction potentials themselves. We show that in the limit of infinitely small potential range, this expression reduces to the Green's function for N atoms interacting via a generalized pseudo-potential, function of a free parameter Λ. Using this Λ-freedom we resolve all inconsistensies of the Hartree-Fock-Bogoliubov formalism known so far, with no ad hoc modifications of the theory…

Taming the entanglement in the dynamical theory of weakly interacting Bose gases

I show that the dynamics of the weakly interacting bose gas can be described by a modified time dependent Bogoliubov theory. The novelty of the approach is to include decoherence steps that gradually transform the entanglement entropy of the pure state into the von Neumann entropy of a statistical mixture. This approximation drastically reduces the entanglement that is needed in order to represent the system's state while becoming exponentially accurate in the mean field limit. I argue that this scheme can be extended to all quantum systems whose ground state can be well approximated by a vari…