Realization of an Optical Microtrap for a Highly Degenerate Fermi Gas

This thesis reports on the realization of a micrometer-sized optical dipole trap for the preparation of a highly degenerate Fermi gas of 6Li atoms. The degenerate ensemble in the microtrap is the starting point for future experiments with a finite number of fermions. One phenomenon to study will be the formation of a shell structure in an interacting two-component spin-mixture. 6Li is especially well suited for such experiments due to the easy tuneability of the inter-particle interaction by means of Feshbach resonances. In the course of this thesis, we present our apparatus for the creation of ultracold Fermi gases. Thereby, the main subject is the assembly of our microtrap. After analyzing the assembly in an external test setup, we focus on the scheme for the transfer of atoms from our large volume optical dipole trap into the microtrap. Using a non-interacting Fermi gas we were able to determine the characteristics of the microtrap, such as lifetime and trap frequencies. Subsequently, we can give an estimation for the degeneracy of the Fermi gas inside. As an outlook, we present our first promising attempts to control the particle number, which is the next milestone on the way towards experiments on finite systems of fermions

From Few to Many: Observing the Formation of a Fermi Sea One Atom at a Time

Knowing when a physical system has reached sufficient size for its macroscopic properties to be well described by many-body theory is difficult. We investigate the crossover from few to many-body physics by studying quasi one-dimensional systems of ultracold atoms consisting of a single impurity interacting with an increasing number of identical fermions. We measure the interaction energy of such a system as a function of the number of majority atoms for different strengths of the interparticle interaction. As we increase the number of majority atoms one by one we observe the fast convergence of the normalized interaction energy towards a many-body limit calculated for a single impurity immersed in a Fermi sea of majority particles.Comment: 9 pages, 5 figure

Pairing in few-fermion systems with attractive interactions

We have studied quasi one-dimensional few-particle systems consisting of one to six ultracold fermionic atoms in two different spin states with attractive interactions. We probe the system by deforming the trapping potential and by observing the tunneling of particles out of the trap. For even particle numbers we observe a tunneling behavior which deviates from uncorrelated single-particle tunneling indicating the existence of pair correlations in the system. From the tunneling timescales we infer the differences in interaction energies of systems with different number of particles which show a strong odd-even effect, similar to the one observed for neutron separation experiments in nuclei.Comment: 9 pages, 6 figure

Fermionization of two distinguishable fermions

In this work we study a system of two distinguishable fermions in a 1D harmonic potential. This system has the exceptional property that there is an analytic solution for arbitrary values of the interparticle interaction. We tune the interaction strength via a magnetic offset field and compare the measured properties of the system to the theoretical prediction. At the point where the interaction strength diverges, the energy and square of the wave function for two distinguishable particles are the same as for a system of two identical fermions. This is referred to as fermionization. We have observed this phenomenon by directly comparing two distinguishable fermions with diverging interaction strength with two identical fermions in the same potential. We observe good agreement between experiment and theory. By adding one or more particles our system can be used as a quantum simulator for more complex few-body systems where no theoretical solution is available

Coherent molecule formation in anharmonic potentials near confinement-induced resonances

We perform a theoretical and experimental study of a system of two ultracold atoms with tunable interaction in an elongated trapping potential. We show that the coupling of center-of-mass and relative motion due to an anharmonicity of the trapping potential leads to a coherent coupling of a state of an unbound atom pair and a molecule with a center of mass excitation. By performing the experiment with exactly two particles we exclude three-body losses and can therefore directly observe coherent molecule formation. We find quantitative agreement between our theory of inelastic confinement-induced resonances and the experimental results. This shows that the effects of center-of-mass to relative motion coupling can have a significant impact on the physics of quasi-1D quantum systems.Comment: 7 pages, 4 figure