Quantum-state control of reactive systems has enabled microscopic probes of underlying interaction potentials and the alteration of reaction rates using quantum statistics. However, extending such control to the quantum states of reaction outcomes remains challenging. Here, we realize this goal by utilizing the conservation of nuclear spins throughout the reaction. Using resonance-enhanced multiphoton ionization spectroscopy to investigate the products formed in bimolecular reactions between ultracold KRb molecules we find that the system retains a near-perfect memory of the reactants’ nuclear spins, manifested as a strong parity preference for the rotational states of the products. We leverage this effect to alter the occupation of these product states by changing the coherent superposition of initial nuclear spin states with an external magnetic field. In this way, we are able to control both the inputs and outputs of a reaction with quantum-state resolution. The techniques demonstrated here open up the possibilities to study quantum entanglement between reaction products and ultracold reaction dynamics at the state-to-state level.

反应系统的量子态控制已启用了使用量子统计的潜在相互作用电位和反应速率变化的微观探针。然而，将这样的控制扩展到反应结果的量子态仍然具有挑战性。在这里，我们通过在整个反应过程中利用核自旋的守恒来实现这一目标。使用共振增强多光子电离光谱研究超冷KRb分子之间的双分子反应中形成的产物，我们发现该系统保留了反应物核自旋的近乎完美的记忆，表现为对产物旋转状态的强烈奇偶性偏好。通过利用外部磁场改变初始核自旋态的相干叠加，我们利用这种效应来改变这些产物态的占有率。通过这种方式，我们能够以量子态分辨率控制反应的输入和输出。这里展示的技术为研究状态之间的反应产物和超冷反应动力学之间的量子纠缠开辟了可能性。