Interactions of Ions and Ultracold Neutral Atom Ensembles in Composite Optical Dipole Traps: Developments and Perspectives

Abstract

Ion–atom interactions are a comparatively recent field of research that has drawn considerable attention due to its applications in areas including quantum chemistry and quantum simulations. In first experiments, atomic ions and neutral atoms have been successfully overlapped by devising hybrid apparatuses combining established trapping methods, Paul traps for ions and optical or magneto-optical traps for neutral atoms, respectively. Since then, the field has seen considerable progress, but the inherent presence of radiofrequency (rf) fields in such hybrid traps was found to have a limiting impact on the achievable collision energies. Recently, it was shown that suitable combinations of optical dipole traps (ODTs) can be used for trapping both atoms and atomic ions alike, allowing to carry out experiments in absence of any rf fields. Here, we show that the expected cooling in such bichromatic traps is highly sensitive to relative position fluctuations between the two optical trapping beams, suggesting that this is the dominant mechanism limiting the currently observed cooling performance. We discuss strategies for mitigating these effects by using optimized setups featuring adapted ODT configurations. This includes proposed schemes that may mitigate three-body losses expected at very low temperatures, allowing to access the quantum dominated regime of interaction.

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

“…Additionally, a second light field with an anti-confining frequency can be used to compensate for the detrimental effects of the ion optical trap. This bi-chromatic trap creates a milder potential for the atoms, while still deeply trapping the ion (Karpa, 2021). For Ba + -Rb mixture a bi-chromatic optical trap of 532 and 1064 nm was demonstrated in which both atoms and ions could be confined (Schmidt et al, 2020b).…”

“…However, extending the measurements to further cool down the ion to about the temperature of the bath was not possible due to three-body losses and the experimental stability. Especially the good relative alignment of the two beams of the optical trap and the presence of parasitic ions triggered by multi-photon ionization and three-body recombi-nation, remain ongoing challenges (Karpa, 2021). Loading the atoms not as an ensemble, but into individual lattice sites of an optical lattice, could be a possible way out.…”

“…Here, combinations of optical traps, tweezers and combinations of static fields with optical fields are all on the table, e.g. (Schaetz, 2017;Schmidt et al, 2020b;Perego et al, 2020;Karpa, 2021). Each ion-atom combination provides it own set of promises and challenges for quantum simulation.…”

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

“…However for our Li/Yb + species, this is not possible, as the only Li tune-out wavelength that is red-detuned for Yb + is too close to the D1 and D2 lines of Li [66]. Suitable combinations of bichromatic optical dipole traps can also be used for all-optical trapping of both atoms and ions [40]. We consider this as an alternative method of reducing the compression effect of the tweezer on the atom cloud in an optical-rf hybrid trap.…”

“…Therefore, the coldest atom-ion collision energies and subsequent observation of quantum effects have been reported for Li/Yb + [36] and Li/Ba + [35] systems. Attempts to avoid micromotion heating effects altogether by trapping ions in other types of trap, for example optical traps, are ongoing, yet offer different challenges [40]. For rf traps, reducing the heating effects of intrinsic micromotion (IMM) and minimizing the ion's excess micromotion (EMM) through compensation of experimental imperfections is critical to buffer gas cooling experiments [41].…”