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Abstract

The emergence of collective and universal behaviour is at the heart of many of the exotic phases of matter that challenge our physical understanding until today. Fermionic superfluidity and superconductivity, for example, are found in a wide range of strongly correlated materials. And while it is understood that pairing is the fundamental prerequisite for their occurrence, the microscopic mechanisms for pair formation remain in many cases unknown. In this thesis, we study the emergence of collective behaviour and superfluidity at the most fundamental level -from the bottom up. To this end, we deterministically prepare the ground state of a mesoscopic Fermi gas consisting of up to 20 atoms in a two-dimensional harmonic potential. Our ultracold quantum gas allows us to freely tune the interactions from a completely non-interacting state to a regime of strong binding. We apply a novel fluorescence imaging technique to extract the momentum distribution of the strongly interacting Fermi gas and with full spin and single particle resolution. We observe a few-body precursor of a phase transition from a normal to a superfluid phase for a system consisting of as few as six interacting particles. It is revealed by the presence of Cooper pairs we detect directly as correlations between particles of opposite spin and momentum at the Fermi surface. When the attraction strength is increased, we observe how the pair character changes and a transition from Cooper pairs to tightly bound molecules occurs. The collective behaviour we discover in our mesoscopic system is closely related to observations in atomic and nuclear physics, superconducting grains or quantum dots. Our platform, with its completely programmability of interactions, particle numbers, the quantum state and the potential landscape, opens up new pathways to study such strongly correlated mesoscopic systems and their connection to the macroscopic world.