Fig. 7: Atom-ion cold collision processes and spin structure.

a Spin level structures of the relevant species. Left: nuclear spin states of ⁸⁷Rb⁺; middle: ground-state hyperfine manifold of ⁸⁷Rb; right: electronic ground and first excited states of the spin-1/2 ion ⁸⁸Sr⁺. Prior to collisions, ⁸⁸Sr⁺ is initialized in its electronic ground state. Both ⁸⁸Sr⁺ and ⁸⁷Rb⁺ exhibit no hyperfine structure due to their zero nuclear or electron spin, respectively. b Collision processes relevant to this work. Left: resonant charge-exchange reactions between ⁸⁷Rb and ⁸⁷Rb⁺ can de-excite the atom’s hyperfine state and convert internal energy E_hpf into kinetic energy; this process is denoted HDRCE. Middle: spin-changing collisions between ⁸⁷Rb and ⁸⁸Sr⁺, primarily mediated by spin-exchange interactions, can similarly drive hyperfine de-excitation of the atom. Right: elastic collisions conserve total momentum but redistribute it between the particles. At a magnetic field of 3 G, Zeeman energy changes are negligible compared to hyperfine energy.