Collisional Cooling of Light Ions by Cotrapped Heavy Atoms

Abstract

We experimentally demonstrate cooling of trapped ions by collisions with co-trapped, higher mass neutral atoms. It is shown that the lighter 39 K + ions, created by ionizing 39 K atoms in a magneto-optical trap (MOT), when trapped in an ion trap and subsequently allowed to cool by collisions with ultracold, heavier 85 Rb atoms in a MOT, exhibit a longer trap lifetime than without the localized 85 Rb MOT atoms. A similar cooling of trapped 85 Rb + ions by ultracold 133 Cs atoms in a MOT is also demonstrated in a different experimental configuration to validate this mechanism of ion cooling by localized and centered ultracold neutral atoms. Our results suggest that cooling of ions by localized cold atoms holds for any mass ratio, thereby enabling studies on a wider class of atom-ion systems irrespective of their masses. DOI:PACS numbers: 37.10.Rs, 37.10.Ty, 37.10.DeThe cooling and trapping of dilute gases of ions [1,2] and atoms [3] has led to unprecedented precision in spectroscopy [4] and the study of interacting many particle systems [5]. Co-trapping ions and atoms [6][7][8][9][10][11][12][13][14] widens the scope of inquiry to the two-particle asymptotic interaction. Among the different methods to cool trapped ions, cooling by elastic collisions with cold neutral atoms is arguably the most generic. Indeed, this has been extensively used in buffer gas cooling of relatively heavy ions by collisions with cold lower mass neutral atoms. However, the complementary phenomenon, that of cooling of low-mass ions by collisions with heavier neutral atoms, has never been demonstrated experimentally. This is partly because calculations show that trapped ions in a uniform buffer gas will heat up when the ratio of the atom mass ( ) to the ion mass ( ) exceeds a critical value [15][16][17][18]. In recent years, theoretical studies suggest a possible experimental resolution by using a localized ensemble of ultracold neutral atoms placed precisely at the centre of the ion trap [12,19].In ) we demonstrate cooling for, exceed the critical mass ratios (CMRs) [15][16][17][18] beyond which ion heating is predicted for uniform atomic gas densities. The experiment is consistent with our Monte Carlo (MC) simulations and other theoretical models [19] that consider collisions with centrally localized density (as opposed to uniform density) of cold atoms. We further demonstrate the competing ion heating mechanism due to collisions with the background gas vapour. The dependence of steady state ion temperature on the size of the cold atomic cloud is also established numerically. The present work lays the foundation for studies on a wider class of sympathetically cooled ion-atom mixtures and the resulting clarity may enable the realization of the few-partial-wave regime.Trapping and cooling ions and atoms. The conditions and mechanisms for trapping atoms and ions are different because the atom is neutral and the ion charged. Atom traps typically have small trap depths and use a combination of static magnetic and/or optical fields. Ions o...

Minimizing rf-induced excess micromotion of a trapped ion with the help of ultracold atoms

“…Because of its relevance for the developing field of cold atom-ion interactions [7][8][9], this issue has been recently investigated in a number of studies (e.g. [20,[27][28][29][30][31][32][33]). The ionic energy distribution depends in a non-trivial way on quantities such as the atom-ion mass ratio, the atomic cloud size, and the ion trap parameters.…”

Trapping Single Ions and Coulomb Crystals with Light Fields

“…While this so-called micromotion can be neglected in most applications based on the manipulation of common mode excitations within linear ion crystals, it can impose a fundamental limitation on other experiments sensitive to kinetic energy [33,34]. A prominent example is the rapidly growing field of ion-atom interactions [35][36][37][38][39][40][41][42][43][44][45] where the presence of driven electromagnetic fields typically limits the accessible collision energies to the range of 1 mK [10,34]. While some approaches exploiting the favorable kinematic properties of specific combinations of neutral atom and atomic ion species confined in conventional Paul traps exist [33,43] and seem promising for accessing a range of collision energies that could approach the quantum interaction regime, the generalization to either generic atom-ion combinations or to experiments involving more than a single ion or a linear chain remains an outstanding challenge in the field and stands to benefit from alternative ways to avoid micromotion.…”

Measurement of charge exchange between Na and Ca+ in a hybrid trap

“…Hybrid traps have been used to measure charge exchange in many heteronuclear atomic systems, including Rb+Yb + [24], Rb+Ba + [9], Rb+Sr + [25], Rb+Ca + [26], Rb + K + [27], Rb + Cs + [5], Li + Ca + [28], Li + Yb + [29], Ca+Yb + [30], Ca+Ba + [31], Na+Ca + [32], and Cs+Rb + [27]. Additionally, studies have been done in the molecular system Rb + N + 2 [33], as well as the homonuclear systems Rb + Rb + [9,24], Cs + Cs + [5], Yb + Yb + [34], and Na + Na + [35].…”