Direct observation of non-local fermion pairing in an attractive Fermi-Hubbard gas

TL;DR

This study directly visualizes non-local fermion pairing in an attractive Fermi-Hubbard gas, revealing pairing mechanisms, pair sizes, and correlations crucial for understanding superfluidity and related phenomena.

Contribution

It provides the first direct observation of non-local fermion pairing and associated correlations in a Hubbard lattice gas using advanced imaging techniques.

Findings

Complete fermion pairing shown by vanishing spin fluctuations.

Fermion pair size is comparable to interparticle spacing.

Polaronic correlations observed around individual spins.

Abstract

Pairing of fermions lies at the heart of superconductivity, the hierarchy of nuclear binding energies and superfluidity of neutron stars. The Hubbard model of attractively interacting fermions provides a paradigmatic setting for fermion pairing, featuring a crossover between Bose-Einstein condensation (BEC) of tightly bound pairs and Bardeen-Cooper-Schrieffer (BCS) superfluidity of long-range Cooper pairs, and a "pseudo-gap" region where pairs form already above the superfluid critical temperature. We here directly observe the non-local nature of fermion pairing in a Hubbard lattice gas, employing spin- and density-resolved imaging of $\sim$1000 fermionic ${}^{40}$K atoms under a bilayer microscope. Complete fermion pairing is revealed by the vanishing of global spin fluctuations with increasing attraction. In the strongly correlated regime, the fermion pair size is found to be on the order of the average interparticle spacing. We resolve polaronic correlations around individual spins, resulting from the interplay of non-local pair fluctuations and charge-density-wave order. Our techniques open the door toward in-situ observation of fermionic superfluids in a Hubbard lattice gas.