Precise photoexcitation measurement of Tan's contact in the entire BCS-BEC crossover

Abstract

We study two-body correlations in a spin-balanced ultracold harmonically trapped Fermi gas of $^6$Li atoms in the crossover from the Bardeen-Cooper-Schrieffer (BCS) to the Bose-Einstein-Condensate (BEC) regime. For this, we precisely measure Tan's contact using a novel method based on photoexcitation of atomic pairs, which was recently proposed by Wang et al. [Phys. Rev. A 104 063309 (2021)]. We map out the contact in the entire phase diagram of the BCS-BEC crossover for various temperatures and interaction strengths, probing regions in phase-space that have not been investigated yet. Our measurements reach an uncertainty of $\approx 2 \%$ and thus represent a precise quantitative benchmark. We compare our data to theoretical predictions and interpolations and localize the regions in phase space where the latter give valid results. In regions where the contact is already well known we find excellent agreement with our measurements. Thus, our results demonstrate that photoinduced loss is a precise probe to measure quantum correlations in a strongly interacting Fermi gas.