Fermionization of two distinguishable fermions
Prof. Dr. Selim Jochim (Physikalisches Institut Universität Heidelberg)

As the building blocks of matter, few-fermion systems such as atoms and nuclei play an essential role in nature.
We have prepared systems containing 1-10 ultracold atoms with exquisite control over the many-body quantum state with fidelities exceeding 90% [1]. These systems are particularly interesting as the interaction strength can be tuned using Feshbach resonances, enabling the realization of strongly correlated few-body systems.

We prepare these systems starting from a degenerate spin mixture of 6Li atoms in an optical dipole trap. With this trap we overlap a µm-sized tightly focused dipole trap resulting in a substantial enhancement of the degeneracy inside the microtrap. After thermalization we remove the reservoir that contains some 10000 atoms. We spill most of the remaining ~600 atoms in a controlled way by applying a tilt to the microtrap using a magnetic field gradient, such that only very few quantum states remain in the trap. Due to the aspect ratio of 10:1, the small number of atoms in this trap is in the quasi 1-D regime.

In a first set of experiments we compare the energy of two distinguishable fermions in the ground state (spin up and down) with tunable repulsive interactions to the energy of two identical spin-up fermions in the two lowest vibrational states. In this way we identify the point where the two distinguishable atoms become "fermionized", i.e., they possess the same spatial correlations as the identical ones.

Next steps include the study of attractive systems, where one expects paring to strongly depend on the exact particle number. It should be possible to address longstanding questions in nuclear physics, such as the competition between on-shell and intra-shell paring.

[1] F. Serwane et al., Science 332, 336 (2011)