Quantum systems of ultracold atoms with tunable interactions

Roberto B. Diener

Konstantinou, Konstantinos

This thesis details the development of a new ultracold atom experiment hosting (bosonic) ³⁹K, from the drawing board to its eventual utilisation in studying atom-light scattering in a uniform system of ultracold atoms with tunable interactions. This work is conceptually divided into three parts. The first part (Inferno), covering Chapters 1 - 3, establishes the theoretical framework for ultracold Bose gases and details the design philosophy and construction of the machine at the heart of this work. The primary benchmarks of the experiment include the production of harmonically trapped condensates of up to 1.2×10⁶ atoms in ≈ 12 s, uniform 3D degenerate gases of 0.8×10⁶ atoms in ≈ 14 s, and uniform 2D systems of 1.5×10⁵ atoms in ≈ 20 s. The second part (Purgatorio), covering Chapters 4 and 5, builds upon the machine's established capabilities. First, we detail the process by which we generate a uniform condensate used for our atom-light scattering experiments. Second, we demonstrate an alternative, all-optical route to quantum degeneracy that bypasses the usage of a magnetic trap in the sequence which is a central component of conventional methods (such as the one in Chapter 3). This technique is an intermediate step between sub-Doppler cooling and optical dipole trap (ODT) loading, designed to optimise the initial conditions for optical evaporation in the ODT. In particular, our method uses the simultaneous compression and cooling of a sub-Doppler cloud within a dynamically tunable blue-detuned tube trap, thereby enhancing the atom loading fraction into the ODT, where the final evaporation towards degeneracy occurs. The improvement of the initial conditions is the critical step that allows this all-optical sequence to successfully reach quantum degeneracy. The third and final part (Paradiso), covering Chapter 6, presents the main scientific result of this thesis. Here, we study the scattering between off-resonant light and a uniform Bose gas with tunable interactions, and illustrate the extreme sensitivity of atom-light scattering to local atomic correlations. We begin by showing that for the (quasi-)ideal (non-interacting) gas, the observed enhancement in atom-light scattering is captured by quantum statistical effects. This picture changes dramatically however, once we introduce atomic interactions. We find that due to the presence of local correlation effects, even weak repulsive interactions can completely suppress the enhancement in atom-light scattering, while attractive ones increase it above the ideal-gas level. Moreover, by studying the dynamics after an interaction quench, we show that the post-quench correlation dynamics are orders of magnitude faster than the dynamics associated with changes to momentum-space occupations or the density distribution. Our results thus establish off-resonant light scattering as a powerful and sensitive probe of equilibrium and non-equilibrium many-body physics.

Rikard Lundmark, C. Forssén, J. Rotureau

The creation of tunable open quantum systems is becoming feasible in current experiments with ultracold atoms in low-dimensional traps. In particular, the high degree of experimental control over these systems allows detailed studies of tunneling dynamics, e.g., as a function of the trapping geometry and the interparticle interaction strength. In order to address this exciting opportunity we present a theoretical framework for two-body tunneling based on the rigged Hilbert space formulation. In this approach, bound, resonant, and scattering states are included on an equal footing and we argue that the coupling of all these components is vital for a correct description of the relevant threshold phenomena. In particular, we study the tunneling mechanism for two-body systems in one-dimensional traps and different interaction regimes. We find a strong dominance of sequential tunneling of single particles for repulsive and weakly attractive systems, while there is a signature of correlated pair tunneling in the calculated many-particle flux for strongly attractive interparticle interaction.

W. K. Hensinger, Hartmut Häffner, Antoine Browaeys +7 more

Qingze Guan, D. Blume, Vincent M. Klinkhamer +3 more

Florian Meinert, Manfred J. Mark, Katharina Lauber +2 more

We report on the experimental implementation of tunable occupation-dependent tunneling in a Bose-Hubbard system of ultracold atoms via time-periodic modulation of the on-site interaction energy. The tunneling rate is inferred from a time-resolved measurement of the lattice site occupation after a quantum quench. We demonstrate coherent control of the tunneling dynamics in the correlated many-body system, including full suppression of tunneling as predicted within the framework of Floquet theory. We find that the tunneling rate explicitly depends on the atom number difference in neighboring lattice sites. Our results may open up ways to realize artificial gauge fields that feature density dependence with ultracold atoms.