Dariusz Wiater, University of Warsaw

Spin dynamics in ultracold collisions between Yb+ ion and Li atoms in the quantum regime

Significant advances in precision measurements in the quantum regime have been achieved with trapped ions and atomic gases at the lowest possible temperatures. These successes have inspired ideas to merge the two systems [1]. Remarkably, in spite of its importance, experiments with ion-atom mixtures remained firmly confined to the classical collision regime, but recently buffer gas cooling of a single ion in a Paul trap to the quantum regime of ion-atom collisions has been realized[2]. The collision energy as small as 1.15(0.23) times the s-wave energy (or 9.9(2.0) μK) has been achieved for a trapped ytterbium ion in an ultracold lithium gas. We have observed a deviation from classical Langevin theory by studying the spin-exchange dynamics, indicating quantum effects in the collisions. Here, we present a theoretical description of the quantum ion-atom scattering used to guide and interpret the recent experiment [2]. By developing a theoretical model of measured energy-dependent spin-exchange rate constants, we have obtained singlet and triplet ion-atom scattering lengths. Next, we identify experimentally accessible Feshbach resonances in the mentioned systems and predict their properties. Control of both elastic scattering and related cooling rates, as well as inelastic spin-changing collisions, with the magnetic field is proposed and investigated to guide ongoing experimental efforts. Ion-atom Feshbach resonances in analogy to well-established techniques for neutral systems will be an important tool to manipulated ultracold ion-atom mixtures.

[1] Tomza et al, Rev. Mod. Phys. 91, 035001 (2019).
[2] Feldker et al, Nature Physics volume 16, pages 413–416(2020)