Ultracold Rydberg Atoms Interacting with Trapped Ions

This thesis presents our studies on controlling atom-ion interactions in the ultracold regime by coupling the atoms to Rydberg states. When a trapped ion is immersed in an atom cloud, short-range Langevin collisions cause heating of the ion due to the radio frequency drive of the ion trap. We theoretically show that this heating can be inhibited through the creation of repulsive atom-ion interaction by Rydberg dressing of the atoms on a dipole-forbidden transition in the vicinity of the trapped ion. The proposed scheme can be employed with a variety of atom-ion mixtures, and extends the prospects of buffer gas cooling to otherwise unfavourable mass ratios.
We present the experimental setup and the techniques to overlap ⁶Li atoms and Yb⁺ ions, and to ensure the ions are crystallised inside the atom cloud. A two-photon Rydberg excitation scheme was implemented and a home-made optical cavity enables tunable locking of the two lasers for spectroscopy. The atom-ion interaction is governed by the electric polarisability of the atom, which varies with the principal quantum number to the power of seven. We observe charge transfer of 24S state atoms with ions and measure the increase in interaction strength. The spectral shape of the ion loss spectra can be explained by including the ion-induced Stark shift on the Rydberg levels. In addition, we demonstrate Rydberg excitation on a dipole-forbidden transition in the strong electric field around the ion. In conclusion, Rydberg-controlled interactions are a promising tool for emerging atom-ion quantum technology.