Test de l'invariance d'échelle dans un gaz de bosons bidimensionnel : préparation et caractérisation d'ondes solitaires

Abstract

At a sufficiently large phase-space density, a Bose gas can be described by a macroscopic wave function subject to nonlinear dynamics. Degenerate Bose gases now constitute a major platform in the study of self-trapped nonlinear fields known as solitons or solitary waves. Up to now, most experimental observations of solitons have been restricted to one-dimensional situations, where they are generically stable. On the contrary, multidimensional solitary waves are more prone to instabilities and thus more challenging to observe experimentally. In this thesis, we produce solitary waves in a two-dimensional ultracold bosonic system. After presenting our experimental setup, we demonstrate our ability to produce deterministically a Townes soliton, a celebrated solution of the nonlinear Schrödinger equation in two dimensions. Our novel approach is based on a two-component mixture: starting from a uniform bath of atoms in a given internal state, we imprint the soliton wave function using an optical transfer to another state. We show that the soliton can exist with various sizes, a hallmark of the scale invariance present in the underlying model. We then confirm the relation linking the soliton atom number to the interaction strength. Our experimental observations are supported by further numerical and theoretical considerations. These studies also allow us to go beyond the simplest model sustaining a Townes soliton, by considering effects beyond the mean-field description. Finally, we propose a few experiments aimed at characterizing excited, moving, as well as colliding solitons.