Probing the Higgs-amplitude mode from weakly to strongly interacting superfluids

The transition to superfluidity and superconductivity involves the emergence of a bosonic field which represents the order parameter in the Ginsburg Landau theory. The connection between these concepts lies in the BEC-BCS crossover, where this field represents the pair wave function of the electrons forming a composite particle

that evolves from a Cooper pair to a bosonic diatomic molecule. In these phases, the U(1) phase-symmetry spontaneously breaks, giving rise to two distinct collective excitations: the Nambu-Goldstone (phase) mode and the massive Higgs (amplitude) mode. Understanding Higgs-type excitations is essential in comprehending quantum many-body systems, and addressing their stability in the absence of particle-hole symmetry, which is lost during the crossover, has long been a fundamental question. Here we show that an ultracold quasi-2D Fermi gas exhibits a long-lived excitation of the bosonic field throughout the entire crossover. Spectroscopic analysis via trapping modulation reveals a well-defined Higgs-amplitude mode on the BCS side located at the pairing gap energy 2Δ. As we approach the strongly correlated regime, this amplitude mode smoothly transitions into a coherent breathing-like excitation of the bosonic field along the confined direction. The mode resonance broadens with increasing temperature and becomes overdamped close to the critical temperature. Experimental

evidence, supported by an effective field theory indicates that the bosonic excitation maps into a non-relativistic to relativistic harmonic oscillator, transitioning from a Gross-Pitaevskii to a Klein-Gordon energy functional. This work provides an approach to simulate relativistic quantum field theories in a quantum simulator.