Determination of the fermion pair size in a resonantly interacting superfluid.

Fermionic superfluidity requires the formation of particle pairs, the size of which varies depending on the system. Many properties of the superfluid depend on the pair size relative to the inter-particle spacing. For example, conventional superconductors comprise a superfluid of loosely bound, large Cooper pairs of electrons, while Bose-Einstein condensates contain tightly bound molecules. The microscopic properties of the fermion pairs can be probed with radio-frequency spectroscopy. However, previous results have been difficult to interpret due to strong final-state interactions that were not well understood. Schunck et al. realize a superfluid spin mixture in an ultracold gas of lithium atoms in which such interactions have negligible influence. They find that the spectroscopic pair size is smaller than the inter-particle spacing. These are the smallest pairs yet observed for fermionic superfluids. In a related experiment, Jin et al use a technique called photoemission spectroscopy to study the excitations in a strongly interacting gas of ultracold potassium atoms. Such studies are of interest because the physics is related to that of the high transition-temperature superconductors, which are not fully understood. Microscopic properties of fermion pairs can be probed with radio-frequency spectroscopy. But previous work was difficult to interpret due to strong final-state interactions, which were not well understood. A superfluid spin mixture in an ultracold atomic gas in which such interactions have negligible influence is realized, and it is found that the spectroscopic pair size is smaller than the inter-particle spacing. These are the smallest pairs yet observed for fermionic superfluids. Fermionic superfluidity requires the formation of particle pairs, the size of which varies from the femtometre scale in neutron stars and nuclei to the micrometre scale in conventional superconductors. Many properties of the superfluid depend on the pair size relative to the interparticle spacing. This is expressed in ‘BCS–BEC crossover’ theories1,2,3, describing the crossover from a Bardeen–Cooper–Schrieffer (BCS)-type superfluid of loosely bound, large Cooper pairs to Bose–Einstein condensates (BECs) of tightly bound molecules. Such a crossover superfluid has been realized in ultracold atomic gases where high-temperature superfluidity has been observed4,5. The microscopic properties of the fermion pairs can be probed using radio-frequency spectroscopy. However, previous work6,7,8 was difficult to interpret owing to strong final-state interactions that were not well understood. Here we realize a superfluid spin mixture in which such interactions have negligible influence and present fermion pair dissociation spectra that reveal the underlying pairing correlations. This allows us to determine that the spectroscopic pair size in the resonantly interacting gas is 20 per cent smaller than the interparticle spacing. These are the smallest pairs so far observed in fermionic superfluids, highlighting the importance of small fermion pairs for superfluidity at high critical temperatures9. We have also identified transitions from fermion pairs to bound molecular states and to many-body bound states in the case of strong final-state interactions.