Fig. 1: Quantum interference in atom-ion charge exchange reactions.

a At ultracold temperatures, where only a single partial wave contributes, resonant charge exchange between an ion and its parent neutral atom proceeds via coherent interference between two molecular channels: the symmetric (gerade) and antisymmetric (ungerade) electron configurations. The reaction acts as a two-path interferometer, with the reaction probability determined by the relative scattering phase between the two channels. For ⁸⁷Rb − ⁸⁷Rb⁺, both species carry nuclear spin I = 3/2, enabling spin-resolved identification of the reaction via hyperfine de-excitation of the neutral atom (HDRCE). E_hpf denotes the hyperfine energy and F the spin quantum number. b Molecular potential energy curves for the gerade and ungerade channels in the s-wave limit. Short-range energy differences between the two curves accumulate a relative scattering phase that determines the interference outcome and governs the reaction probability (see Eq. (1)). These phases are highly sensitive to molecular details and remain beyond current ab initio predictive capability, limiting theoretical estimates of the reaction rate. l denotes the partial wave. c At elevated temperature, many partial waves contribute to the reaction. In the classical regime, their contributions are summed incoherently, and thermal averaging tends to wash out quantum interference. If, however, the short-range phase difference between the gerade and ungerade channels changes only weakly with l and with collision energy, the interference factor remains nearly the same across many partial-wave terms. In this case, averaging over partial-wave contributions can still preserve wave-specific interference signatures beyond the s-wave limit, a phenomenon known as partial-wave phase locking⁵³. d Schematic of the experimental platform. A cloud of ultracold ⁸⁷Rb atoms is shuttled into an ion trap containing a two-ion crystal composed of ⁸⁷Rb⁺ and ⁸⁸Sr⁺. Individual resonant charge exchange events are detected via energy released into the ion crystal and read out using quantum logic techniques on the ⁸⁸Sr⁺ logic ion.