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

Abstract

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.

Ultracold ion-atom experiments: cooling, chemistry, and quantum effects

“…For multipole traps, reaching very low temperatures for the ion-atom collision energy is in principle easier compared to a Paul trap as the ion can be confined in a region with little micromotion. But on the other hand, this makes the ion more prone to stray electric fields that cause excess micromotion (Trimby et al, 2022;Niranjan et al, 2021). Multipole traps have been used for studying ion-atom collisions and buffer gas cooling at K-mK temperatures (Wester, 2009;Asvany and Schlemmer, 2009;Nötzold et al, 2020).…”

“…We see that combinations with a large ion-to-atom mass ratio allow for using Paul traps Table 3: Heating and the final energy for realistic ion-atom systems in an RF-Paul trap with the static and dynamic stability parameters a, q and trap driving frequency Ω given by literature. The thermal equilibrium energy of the ion is obtained from numerical simulations using the code of (Trimby et al, 2022) taking into account an radial stray field of 0.05V/m and an atom bath of 2 µK.…”

“…The competition between heating and cooling, determines the final temperature reachable in ion-atom mixtures. Using numerical simulations based on the code published by Trimby et al (2022), we now proceed to obtain this equilibrium temperature of the ion for the various ion-atom systems discussed, in the limit of many collisions. We do this for a realistic experimental scenario.…”

“…Nonetheless the discrepancy between the ion's and atom's final temperatures has its origin mostly in the micromotion-induced heating caused by the ion-atom interaction, which is typical for ion-atom systems in Paul traps (Meir et al, 2016;DeVoe, 2009;Zipkes et al, 2011;Chen et al, 2014;Rouse and Willitsch, 2017;Höltkemeier et al, 2016). Using the same type of simulations, one could show that the final ion temperature reachable by buffer gas cooling could even be a factor of two lower, through trap parameter optimization (Trimby et al, 2022)…”

“…However, singling out these two very recent results does not do full justice to the rapid developments in the whole field. Other important examples of breakthroughs include the observation of signatures of s-wave spin dynamics far above the s-wave energy Côté and Simbotin, 2018;, rapid improvements in buffer gas cooling (Haze et al, 2018;Schmidt et al, 2020b;Hirzler et al, 2020a;Dieterle et al, 2021;Trimby et al, 2022), observations of interactions between Rydberg atoms and ions (Engel et al, 2018;Ewald et al, 2019;Haze et al, 2019;Deiß et al, 2021;Zuber et al, 2021), the accurate experimental breakdown of cold chemistry reactions paths in ion-atom mixtures (da Silva Jr et al, 2017;Ben-shlomi et al, 2020;Mohammadi et al, 2021;Ben-shlomi et al, 2021), quantum logic spectroscopy of ion-atom chemistry (Katz et al, 2022) and recently the observation of interactions between atomic Feshbach dimers and trapped ions (Hirzler et al, 2020b(Hirzler et al, , 2022, just to mention a few.…”