Detecting Superfluids, Exciting the Higgs Mode and Enhanced Cooling of Dimers in the BEC-BCS Crossover

Abstract

In this thesis, the BEC-BCS crossover is experimentally investigated using a quantum simulator apparatus. We prepare a degenerate, interacting fermionic sample by cooling atoms in two of the lowest hyperfine states of ⁶Li in a crossed optical dipole trap. Interactions between the two states are controlled by means of a broad magnetic Feshbach resonance, and we adjust the samples' temperature and density by preciely tuning the trapping potential. This setup allows us to access and probe the entire BEC-BCS crossover. <br /> A key property of the BEC-BCS crossover is the superfluid critical temperature, predicted to have a maximum on the BEC side of the strongly interacting regime. However, accurately measuring the critical temperature is challenging due to difficulties in determining a reliable temperature scale in the presence of strong interactions. In this thesis, we determine the critical temperature in the crossover with high accuracy by reconstructing the density distribution and incorporating interaction effects in the low-density wings when fitting to the virial expansion of the equation of state. This requires precise identification of the superfluid phase transition onset, for which we have developed two novel advanced image recognition techniques based on machine learning. Our improved methodology confirms, for the first time, an increase in the critical temperature from the BCS limit, extending beyond the unitarity point and approaching the BEC limit. <br /> Crossing the superfluid phase transition is accompanied by spontaneous symmetry breaking, creating an energy landscape that supports two distinct excitation modes: the Goldstone and Higgs modes. Here, we probe the Higgs mode using two distinct excitation methods: a quench and a modulation of the interaction strength. This enables us to observe the Higgs mode throughout the crossover, revealing a gradual fading of the mode as it approaches the BEC regime, where particle-hole symmetry vanishes. Notably, we observe no temperature dependence of the Higgs mode, prompting further research. <br /> Finally, we present a novel cooling method for a strongly interacting Fermi gas on the BEC side of the crossover, where a composite dimer bound state exists. By applying a modulation of the magnetic field at frequencies close to, but higher than the bound state energy, we selectively dissociate and remove high-energy dimers from the trap, thus realising evaporative cooling of the sample. This method does not require any changes to the trapping potential and facilitates staying in the efficient runaway regime. We demonstrate cooling for a wide range of interactions on the BEC side of the crossover, achieving high efficiencies that match or exceed all previously reported forced evaporation cooling near Feshbach resonances.