LAUSR

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) 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 systems 17 . 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.