Great advances in precision quantum measurement have been achieved with trapped ions and atomic gases at the lowest possible temperatures [1-3]. These successes have inspired ideas to merge the two systems [4]. In this way one can study the unique properties of ionic impurities inside a quantum fluid [5][6][7][8][9][10][11] or explore buffer gas cooling of the trapped ion quantum computer [12]. Remarkably, in spite of its importance, experiments with atom-ion mixtures remained firmly confined to the classical collision regime [13]. We report a collision energy of 1.15(0.23) times the s-wave energy (or 9.9(2.0) µK) for a trapped ytterbium ion in an ultracold lithium gas. We observed a deviation from classical Langevin theory by studying the spin-exchange dynamics, indicating quantum behavior in the atom-ion collisions. Our results open up numerous opportunities, such as the exploration of atom-ion Feshbach resonances [14,15], in analogy to neutral systems [16].

We theoretically study the dynamics of a trapped ion that is immersed in an ultracold gas of weakly bound atomic dimers created by a Feshbach resonance. Using quasiclassical simulations, we find a crossover from dimer dissociation to molecular ion formation depending on the binding energy of the dimers. The location of the crossover strongly depends on the collision energy and the time-dependent fields of the Paul trap. Deeply bound dimers lead to fast molecular ion formation, with rates approaching the Langevin collision rate L ≈ 4.8 × 10 −9 cm 3 s −1. The kinetic energies of the created molecular ions have a median below 1 mK, such that they will stay confined in the ion trap. We conclude that interacting ions and Feshbach molecules may provide an alternative approach towards the creation of ultracold molecular ions with applications in precision spectroscopy and quantum chemistry.

We report on the observation of interactions between ultracold Rydberg atoms and ions in a Paul trap. The rate of observed inelastic collisions, which manifest themselves as charge transfer between the Rydberg atoms and ions, exceeds that of Langevin collisions for ground state atoms by about three orders of magnitude. This indicates a huge increase in interaction strength. We study the effect of the vacant Paul trap's electric fields on the Rydberg excitation spectra. To quantitatively describe the exhibited shape of the ion loss spectra, we need to include the ion-induced Stark shift on the Rydberg atoms. Furthermore, we demonstrate Rydberg excitation on a dipole-forbidden transition with the aid of the electric field of a single trapped ion. Our results confirm that interactions between ultracold atoms and trapped ions can be controlled by laser coupling to Rydberg states. Adding dynamic Rydberg dressing may allow for the creation of spin-spin interactions between atoms and ions, and the elimination of collisional heating due to ionic micromotion in atom-ion mixtures. arXiv:1809.03987v3 [physics.atom-ph]

We describe and characterize an experimental apparatus that has been used to study interactions between ultracold lithium atoms and ytterbium ions. The preparation of ultracold clouds of Li atoms is described as well as their subsequent transport and overlap with Yb + ions trapped in a Paul trap. We show how the kinetic energy of the ion after interacting with the atoms can be obtained by laser spectroscopy. We analyze the dynamics of the buffer-gas-cooled ion after releasing the atoms, which indicates that background heating, due to electric-field noise, limits attainable buffer gas cooling temperatures. This effect can be mitigated by increasing the density of the Li gas in order to improve its cooling power. Imperfections in the Paul trap lead to so-called excess micromotion, which poses another limitation to the buffer gas cooling. We describe in detail how we measure and subsequently minimize excess micromotion in our setup. We measure the effect of excess micromotion on attainable ion temperatures after buffer gas cooling and compare this to molecular dynamics simulations, which describe the observed data very well.

Yb þ ion by isotope-selective two-photon ionization, Doppler-cool it to about 0.5 mK and prepare it in the