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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - TwUnaGas (Two-dimensional Uniform Gas with tunable interactions)

Teaser

Summary

This project intended to experimentally study condensed matter physics and many-body effects in an ultra-cold gas of Potassium-39. Particular attention was given to the interface between three- and two-dimensional physics. The project is now at a stage where the role played by the interactions on the Berezinsky-Kosterlitz-Thouless (BKT) transition can be studied in a homogeneous ultracold two-dimensional gas.
Cold atom experiments allow to mimic most of the phenomena registered in condensed matter while ensuring almost absolute control over the properties of the system: temperature, density, presence or absence of impurities, inter-particle interaction strength as well as their nature (from contact interactions to long-range interactions for dipolar gases). Nonetheless, studies of homogeneous cold gases are still scarce due to the difficulty of creating â€œbox-potentialsâ€. Indeed within the cold-atom community, most ultra-cold samples are produced in harmonic traps, and therefore the density varies spatially. The possibility to provide, simple and under-control, homogeneous samples is of particular relevance for the condensed matter community where long-standing problems, such as, for instance, the superfluid fraction jump at the BKT transition, can be further investigated and understood.
Recent work performed at the hosting institution on creating box potentials led to the first realisation of a 3D homogeneous BEC (see Fig.1), a technique that during this project was applied by the beneficiary to the case of strongly-interacting gases with variable geometries: three- and two-dimensions.
The project led to a better understanding of the role played by interactions in cold atomic gases, from weakly-interacting systems all the way up to the unitary regime where interactions are as strong as allowed by quantum mechanics. Furthermore, the beneficiary designed and produced a new uniform two-dimensional potential that allows changing the strong confinement of the potential easily. This new design is currently used to generate, on daily-basis, two-dimensional homogeneous ultra-cold Bose gases with controllable interactions.

Work performed

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 â€œquantum depletionâ€ 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 â€œbox-potentialâ€ 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.

Final results

As reported above, the project led to several experimental observations that are at the frontier of our understanding of the strongly-interacting Bose gases. The homogeneity of the samples led the beneficiary to address several conceptual questions without facing the difficulties associated with spatially varying densities. The results contributed to a better understanding of the strongly-interacting Bose gas and improved the state-of-the-art of this topic considerably.
Moreover, the development of a tunable two-dimensional uniform trap for Potassium-39 atoms led to the possibility of studying strongly-interacting samples in both two- and three-dimensions. This feature is currently being implemented to probe the role of interactions at the BKT-transition and will soon result in another deliverable.