Buffer gas cooling of a trapped ion to the quantum regime

Great advances in precision measurements in the quantum regime have been achieved with trapped ions and atomic gases at the lowest possible temperatures^1–3. These successes have inspired ideas to merge the two systems⁴. In this way, we can study the unique properties of ionic impurities inside a quantum fluid^5–12 or explore buffer gas cooling of a trapped-ion quantum computer¹³. Remarkably, in spite of its importance, experiments with atom–ion mixtures have remained firmly confined to the classical collision regime¹⁴. We report a collision energy of 1.15(±0.23) times the s-wave energy (or 9.9(±2.0) μK) for a trapped ytterbium ion in an ultracold lithium gas. We observed a deviation from classical Langevin theory by studying the spin-exchange dynamics, indicating quantum effects in the atom–ion collisions. Our results open up numerous opportunities, such as the exploration of atom–ion Feshbach resonances^15,16, in analogy to neutral systems¹⁷.