Spin-Orbit Interactions and Quantum Spin Dynamics in Cold Ion-Atom Collisions

Observation of Feshbach resonances between a single ion and ultracold atoms

The control of physical systems and their dynamics on the level of individual quanta underpins both fundamental science and quantum technologies. Trapped atomic and molecular systems, neutral¹ and charged², are at the forefront of quantum science. Their extraordinary level of control is evidenced by numerous applications in quantum information processing^3,4 and quantum metrology^5,6. Studies of the long-range interactions between these systems when combined in a hybrid atom-ion trap^7,8 have led to landmark results^9-19. However, reaching the ultracold regime-where quantum mechanics dominates the interaction, for example, giving access to controllable scattering resonances^20,21-has so far been elusive. Here we demonstrate Feshbach resonances between ions and atoms, using magnetically tunable interactions between ¹³⁸Ba⁺ ions and ⁶Li atoms. We tune the experimental parameters to probe different interaction processes-first, enhancing three-body reactions^22,23 and the related losses to identify the resonances and then making two-body interactions dominant to investigate the ion's sympathetic cooling¹⁹ in the ultracold atomic bath. Our results provide deeper insights into atom-ion interactions, giving access to complex many-body systems^24-27 and applications in experimental quantum simulation^28-30.

Phase Locking between Different Partial Waves in Atom-Ion Spin-Exchange Collisions

We present a joint experimental and theoretical study of spin dynamics of a single ^{88}Sr^{+} ion colliding with an ultracold cloud of Rb atoms in various hyperfine states. While spin exchange between the two species occurs after 9.1(6) Langevin collisions on average, spin relaxation of the Sr^{+} ion Zeeman qubit occurs after 48(7) Langevin collisions, which is significantly slower than in previously studied systems due to a small second-order spin-orbit coupling. Furthermore, a reduction of the endothermic spin-exchange rate is observed as the magnetic field is increased. Interestingly, we find that while the phases acquired when colliding on the spin singlet and triplet potentials vary largely between different partial waves, the singlet-triplet phase difference, which determines the spin-exchange cross section, remains locked to a single value over a wide range of partial waves, which leads to quantum interference effects.

Spin-controlled atom-ion chemistry

Quantum control of chemical reactions is an important goal in chemistry and physics. Ultracold chemical reactions are often controlled by preparing the reactants in specific quantum states. Here we demonstrate spin-controlled atom–ion inelastic (spin-exchange) processes and chemical (charge-exchange) reactions in an ultracold Rb-Sr⁺ mixture. The ion’s spin state is controlled by the atomic hyperfine spin state via spin-exchange collisions, which polarize the ion’s spin parallel to the atomic spin. We achieve ~ 90% spin polarization due to the absence of strong spin-relaxation channel. Charge-exchange collisions involving electron transfer are only allowed for (RbSr)⁺ colliding in the singlet manifold. Initializing the atoms in various spin states affects the overlap of the collision wave function with the singlet molecular manifold and therefore also the reaction rate. Our observations agree with theoretical predictions.

Chemical reactions with ultracold atoms and ions are explored so far with the atom–ion interactions. Here the authors discuss spin-exchange process and show that the spin state of an ensemble of neutral Rb atoms can be used to control the final spin of an imbedded Sr⁺ ion in the collisions.

3D Sisyphus Cooling of Trapped Ions

Using a laser polarization gradient, we realize 3D Sisyphus cooling of ^{171}Yb^{+} ions confined in and near the Lamb-Dicke regime in a linear Paul trap. The cooling rate and final mean motional energy of a single ion are characterized as a function of laser intensity and compared to semiclassical and quantum simulations. Sisyphus cooling is also applied to a linear string of four ions to obtain a mean energy of 1-3 quanta for all vibrational modes, an approximately order of magnitude reduction below Doppler cooled energies. This is used to enable subsequent, efficient sideband laser cooling.