Buffer gas cooling of ions in radio-frequency traps using ultracold atoms

Reaching ultracold temperatures within hybrid atom–ion systems is a major limiting factor for
control and exploration of the atom–ion interaction in the quantum regime. In this work, we present results on numerical simulations of trapped ion buffer gas cooling using an ultracold
atomic gas in a large number of experimentally realistic scenarios. We explore the suppression of micromotion-induced heating effects through optimization of trap parameters for various
radio-frequency (rf) traps and rf driving schemes including linear and octupole traps, digital Paul
traps, rotating traps and hybrid optical/rf traps. We find that very similar ion energies can be
reached in all of them even when considering experimental imperfections that cause so-called
excess micromotion. Moreover we look into a quantum description of the system and show that quantum mechanics cannot save the ion from micromotion-induced heating in an atom–ion collision. The results suggest that buffer gas cooling can be used to reach close to the ion’s
groundstate of motion and is even competitive when compared to some sub-Doppler cooling
techniques such as Sisyphus cooling. Thus, buffer gas cooling is a viable alternative for ions that are not amenable to laser cooling, a result that may be of interest for studies into cold controlled quantum chemistry and charged impurity physics.