Nonlocal spatial correlations in an interacting atomic gas

We report measurements of the nonlocal two-body spatial correlation function g ( 2 ) ( R ) in an ultracold Bose gas with strong interactions using photoassociation of ultra-long-range Rydberg molecules (ULRRMs) as a spatially selective probe. This technique provides access to g ( 2 ) ( R ) on length scales from 60 to 180 nm, bridging the gap between short-range and long-range correlation measurements. The large s -wave scattering length of Sr 86 ( a = 811 a 0 ) enables exploration of the universal regime where g ( 2 ) ( R ) is governed solely by two-body interactions. Comparison with a weakly interacting isotope, Sr 84 , highlights interaction-induced suppression of pair correlations. The measured correlation functions agree quantitatively with calculations based on the two-body reduced density matrix, without adjustable parameters. Our results establish Rydberg molecular photoassociation as a powerful tool for probing nonlocal correlations in strongly interacting quantum gases.