Chemical reactions represent a class of quantum problems that challenge both the current theoretical understanding and computational capabilities¹. Reactions that occur at ultralow temperatures provide an ideal testing ground for quantum chemistry and scattering theories, because they can be experimentally studied with unprecedented control², yet display dynamics that are highly complex³. Here we report the full product state distribution for the reaction 2KRb → K₂ + Rb₂. Ultracold preparation of the reactants allows us complete control over their initial quantum degrees of freedom, whereas state-resolved, coincident detection of both products enables the probability of scattering into each of the 57 allowed rotational state-pairs to be measured. Our results show an overall agreement with a state-counting model based on statistical theory^4,5,6, but also reveal several deviating state-pairs. In particular, we observe a strong suppression of population in the state-pair closest to the exoergicity limit as a result of the long-range potential inhibiting the escape of products. The completeness of our measurements provides a benchmark for quantum dynamics calculations beyond the current state of the art.

化学反应是一类挑战当前理论理解和计算能力的量子问题¹。在超低温下发生的反应为量子化学和散射理论提供了理想的试验场，因为它们可以通过前所未有的控制²进行实验研究，同时显示出高度复杂的动力学³。在这里，我们报告了反应 2KRb → K ₂ + Rb _{2的完整产物状态分布}. 反应物的超冷制备使我们能够完全控制它们的初始量子自由度，而两种产物的状态分辨、同时检测能够测量散射到 57 个允许旋转状态对中的每一个的概率。我们的结果显示与基于统计理论^4,5,6的状态计数模型总体一致，但也揭示了几个偏离状态对。特别是，我们观察到由于远程电位抑制产品逃逸，最接近放热极限的状态对中的人口受到强烈抑制。我们测量的完整性为超越当前技术水平的量子动力学计算提供了基准。