Great advances in precision measurements in the quantum regime have been achieved with trapped ions and atomic gases at the lowest possible temperatures^1,2,3. 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^{5,6,7,8,9,10,11,12} 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.

^{在可能的最低温度1,2,3}下，捕获离子和原子气体在量子状态下的精确测量方面取得了巨大进步。这些成功激发了将这两个系统合并的想法⁴。^{通过这种方式，我们可以研究量子流体5、6、7、8、9、10、11、12}中离子杂质的独特性质，或者探索捕获离子量子计算机¹³的缓冲气体冷却。值得注意的是，尽管它很重要，但原子-离子混合物的实验仍然牢牢地局限于经典碰撞机制¹⁴。我们报告了s的 1.15(±0.23) 倍的碰撞能量超冷锂气体中捕获的镱离子的波能量（或 9.9（±2.0）μK）。我们通过研究自旋交换动力学观察到与经典朗之万理论的偏差，表明原子 - 离子碰撞中的量子效应。我们的结果开辟了许多机会，例如探索原子-离子 Feshbach 共振^15,16，类似于中性系统¹⁷。