Density and Spin Fluctuations Across the 2D BEC-BCS Crossover via Quantum Gas Microscopy

We report measurements of the static density and magnetic structure factors of a two-dimensional strongly interacting Fermi gas across the BEC-BCS crossover, using spin- and charge-resolved quantum gas microscopy in the continuum. By directly computing S(k) and S_m(k) from quantum gas microscope images of ⁶Li, we map the evolution of fluctuations from super-Poissonian in the molecular BEC regime to sub-Poissonian in the Fermi-liquid regime. At low momenta, we observe strong suppression of S(k) near unitarity beyond mean-field predictions, a signature of sound modes whose contribution we measure via temperature dependence. In the BEC regime, we identify non-monotonic charge fluctuations near the critical point. The magnetic structure factor S_m(k) reveals the emergence of long-range spin correlations and, through the fluctuation-dissipation theorem, directly yields the spin susceptibility across the crossover. The temperature dependence of the susceptibility at unitarity provides evidence for preformed-pair breaking, an indicator of the pseudogap phase in two-dimensional strongly interacting fermions.