Molecular Probe of Pairing in the BEC-BCS Crossover

We have used optical molecular spectroscopy to probe the many-body state of paired ${}^6\mathrm{L}\mathrm{i}$ atoms near a broad Feshbach resonance. The optical probe projects pairs of atoms onto a vibrational level of an excited molecule. The rate of excitation enables a precise measurement of the closed-channel contribution to the paired state. This contribution is found to be quite small, supporting the concept of universality for the description of broad Feshbach resonances. The dynamics of the excitation provide clear evidence for pairing across the BEC-BCS crossover and into the weakly interacting BCS regime.