Multichannel interactions of two atoms in an optical tweezer

The multichannel Na-Cs interactions are characterized by a series of measurements using two atoms in an optical tweezer, along with a multichannel quantum defect theory (MQDT) with minimal input parameters. The triplet and singlet scattering lengths are measured by performing Raman spectroscopy of the Na-Cs motional states and least-bound molecular state in the tweezer. The two-scaleMQDT improves accuracy over the single-scale model by incorporating the -C-8/r(8) potential in addition to the -C-6/r(6) potential. Magnetic Feshbach resonances are observed for only two atoms at fields which agree to within 1% of the MQDT predictions. Our tweezer-based approach, combined with an effective theory of interaction, provides a methodology for future studies of more complex interactions, such as atom-molecule and molecule-molecule interactions, and where the traditional high-phase-space-density bulk-gas techniques are technically challenging.