Revealing fermion pair density waves via interaction quench

The 2D Fermi-Hubbard model with attractive interactions hosts a plethora of phases from the BEC-BCS crossover to charge density wave ordering. Such a gas with balanced spin populations is fully paired at low temperatures. However, in the strongly correlated regime where the kinetic and interaction energies are comparable, the pair size is similar to the pair-pair distance. This large pair size obscures the underlying pair ordering. A quench of the interaction energy associates the long range pairs to tightly bound, local pairs without creating new ones. This reveals the underlying spin and charge ordering previously hidden by the non-local nature of pairs. This was confirmed by comparing total spatial correlations with and without this rapid ramp using our quantum gas microscope with spin and charge resolved readout. We found that the charge density wave correlations of the gas are strongly enhanced with the rapid interaction quench and they persist at weak attractive interactions where the large pair size makes it harder to observe the ordering of pairs. The number of unpaired atoms also provide a tool for directly measuring pairing in a strongly correlated gas. In the case of complete pairing all the atoms form local pairs on doubly occupied sites after the rapid ramp. This opens the door for measuring polaronic correlations in many body paired systems.