In order to address the case of a homogeneous strongly-interacting two-dimensional Bose gas, several conceptual difficulties had to be overcome.
First, it became fundamental to understand the behaviour of the gas when interactions are as strong as allowed by quantum mechanics (unitary regime). While for mass-balanced two-component Fermi gases, thermodynamic relations between macroscopic observables are described by universal properties of the short-range particle correlations, this ceases to be true for Bose gases. Indeed, in the latter case, due to the presence of three-body states associated with the Efimov physics, three-particle correlations that cannot be deduced from the knowledge of pairwise ones exist. These new correlations lead to the appearance of non-universal microscopic contributions to the description of macroscopic quantities of the system, contribution which is maximised at unitarity.
While the unitary regime has been extensively studied in the case of Fermi gases, both experimental and theoretical studies of the unitary Bose gas are only recently emerging.
For that purpose, several experimental studies were carried at unitarity as well as for moderate interaction strength. These studies led to several major contributions to the state-of-the-art of strongly-interacting Bose gases. In particular, we were able, through a Ramsey interferometric technique (see Fig.2) to extract, for the first time, the value of C3, quantity that describes three-particle correlations associated to the Efimov physics. We also observed universal dynamical properties of the unitary Bose gas, for both condensed and thermal clouds, prepared in a uniform potential. These studies led to a better understanding of the transition between a degenerate unitary Bose gas and its thermal counterpart.
A study of the quasi-particle energy through Bragg diffraction was also performed. The beneficiary reported substantial deviations from the commonly used Bogoliubov dispersion relation for the case of strongly-interacting Bose-Einstein condensates. Although partial agreement with different theoretical models was reported, no full agreement was found with any available model over the entire interaction range, thus inviting further theoretical effort. These results allowed to probe the phenomenon of “quantum depletion” in a homogeneous Bose gas. This phenomenon, although believed to explain the small condensed fraction in liquid helium, had, until now, never been quantitatively verified.
We also developed a two-dimensional uniform “box-potential” for Potassium-39 with a tunable strong-confinement frequency. This extra-flexibility combined with the tunability of the interaction strength inherent to the species sets a unique experimental platform in which to probe the interface between two- and three-dimensional realm with tunable interactions.
Overall, the project led to 5 publications of which 4 Physical Review Letters and one Letter in Science.
