Feshbach resonances between a single ion and ultracold atoms

The fields of ultracold atoms and trapped ions are important pillars of experimental quantum optics. Recently, the expertise of both fields has been combined in hybrid trapping setups [1] with the aim to prepare atom-ion mixtures at low temperatures where new quantum phenomena have been predicted. Reaching the ultracold regime however, is a challenging task, as intrinsic micromotion heating effects of conventional radio-frequency (rf) traps limit most experiments. In our setup, we follow two pathways to overcome these limitations. On the one hand, we choose an atom-ion mixture with high mass imbalance – ⁶Li atoms and ¹³⁸Ba⁺ ions – as comparably heavy ions are subject to less heating. On the other hand, we have developed optical dipole traps for ions allowing us to probe atom-ion interactions in complete absence of rf fields [2].

 

Using these techniques, we demonstrate the first observation of Feshbach resonances between atoms and ions [3]. We find 11 resonances by magnetic-field dependent ion loss spectroscopy and identify four of them as s-wave resonances. Operating with lower densities we suppress inelastic three-body recombination, allowing us to apply Feshbach resonances to control the ion’s sympathetic cooling rate -  a first step towards reaching atom-ion s-wave scattering.