We have used optical molecular spectroscopy to probe the many-body state of paired 6 Li 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.

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We consider the Bose-Hubbard model of atoms in an optical lattice potential when the atomatom interactions are attractive. If the lowest energy lattice sites are degenerate (such as in the homogeneous case), then, at a critical value of the interaction strength, a phase-coherent condensate becomes unstable to a quantum superposition such that the number distribution of each of the degenerate sites becomes double peaked. In the limit when the interaction dominates, the superposition becomes macroscopic and has the form |ψ ∝ j e iφ jb †N j |vac , where N is the total number of atoms and the sum ranges over the energy-degenerate sites.

An unstable condensate of diatomic molecules will coherently disassociate into correlated pairs of atoms. This dissociation process exhibits very rich quantum dynamics depending on the quantum statistics of the constituent atoms. We show that in the case of bosonic atoms Bose-enhancement can lead to stimulated dissociation, whereas, in the case of fermions Pauli-blocking of the available states and a build-up of coherence between molecules and atom pairs can give rise to incomplete dissociation of the molecules and transient association-dissociation oscillations.Comment: 5 pages, 3 figures. to appear in PR