Buffer gas cooling of a trapped ion to the quantum regime

Cooling an atom-ion hybrid system and bringing it into the quantum regime is challenging owing to the unavoidable heating caused by atom-ion collisions. Here a new record low is achieved in such a system, and the quantum effect starts to manifest. 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 systems(4). 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 computer(13). Remarkably, in spite of its importance, experiments with atom-ion mixtures have remained firmly confined to the classical collision regime(14). We report a collision energy of 1.15(+/- 0.23) times the s-wave energy (or 9.9(+/- 2.0) mu K) 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 systems(17).