Collisional Cooling of Light Ions by Cotrapped Heavy Atoms

Sourav Dutta, Rahul Sawant, and S. A. Rangwala

Phys. Rev. Lett. 118, 113401 – Published 16 March 2017

Abstract

We experimentally demonstrate cooling of trapped ions by collisions with cotrapped, 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 ${}^{85}R b$ atoms in a MOT, exhibit a longer trap lifetime than without the localized ${}^{85}R b$ MOT atoms. A similar cooling of trapped ${}^{85}R b^{+}$ ions by ultracold ${}^{133}C s$ 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 the 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.

Received 6 June 2016

DOI:https://doi.org/10.1103/PhysRevLett.118.113401

Physics Subject Headings (PhySH)

Atom & ion cooling Atom & ion trapping & guiding Ultracold collisions

Atomic, Molecular & Optical

Authors & Affiliations

Sourav Dutta, Rahul Sawant, and S. A. Rangwala

Raman Research Institute, C. V. Raman Avenue, Sadashivanagar, Bangalore 560080, India

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Issue

Vol. 118, Iss. 11 — 17 March 2017

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Images

Figure 1
(a) A schematic of the experimental setup. The rf voltage $V_{rf}$ is fed to the two central wire electrodes, and the two external wire electrodes serve as the end caps. The extraction of the trapped ions to the CEM is done by biasing two of the rod extraction electrodes appropriately after the hold time. (b) Details of overlap volumes of the ${}^{85}R b$ MOT and a calculated, representative ion trajectory. (c) Illustrative diagram for a transfer of momentum from the ${{}^{39}K}^{+}$ ion to the almost stationary ${}^{85}R b$ MOT atom, resulting in a lower kinetic energy of the ion postcollision and a hot atom which usually escapes the MOT. $| u_{i} |$ ( $| u_{a} |$ ) and $| v_{i} |$ ( $| v_{a} |$ ) represent the magnitude of the ion (atom) velocity before and after a collision, respectively. (d) The timing sequence for the experiment.
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Figure 2
(a) The number of ${{}^{39}K}^{+}$ ions remaining in the ion trap for different values of the hold time either in the absence (squares) or the presence (circles) of the ${}^{85}R b$ MOT. The dotted and the solid lines are single exponential fits for the respective cases. The increase in the survival probability of trapped ${{}^{39}K}^{+}$ ions in the presence of the ${}^{85}R b$ MOT indicates the cooling of ${{}^{39}K}^{+}$ ions. The error bars represent the width (1 s.d.) of the underlying ion number distribution. (b) The ratio between the number of trapped ${{}^{39}K}^{+}$ ions in the presence and absence of the ${}^{85}R b$ MOT, for low ( $\leq 5.9 \times 10^{- 10} Torr$ ) background partial pressure of Rb (symbols) and for high ( $\geq 8.3 \times 10^{- 10} Torr$ ) background partial pressure of Rb (dotted line). The shaded region represents 1 s.d. for the measurements at high background pressure. At a high background pressure, the cooling of ${{}^{39}K}^{+}$ ions (indicated by ratios $> 1$ ) is not experimentally discernible. The solid line is a fit to an exponential for the low background pressure case. The quoted partial pressure of Rb was obtained from the loading rate of the ${}^{85}R b$ MOT [29].
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Figure 3
The number of ${}^{85}R b^{+}$ ions remaining in the linear Paul trap for different values of the hold time either in the absence (squares) or the presence (circles) of the ${}^{133}C s$ MOT. The dotted and the solid lines are single exponential fits for the respective cases. The error bars represent the width (1 s.d.) of the underlying ion number distribution. Inset: The ratio between the number of surviving ${}^{85}R b^{+}$ ions in the presence and absence of the ${}^{133}C s$ MOT.
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Figure 4
(a) The number of surviving ${{}^{39}K}^{+}$ ions in the simulation as a function of the number of collisions each ion experiences. The ions collide with both the background gas of Rb atoms and the ${}^{85}R b$ MOT atoms. At high background pressures, the ion losses with and without a MOT are the same, as seen experimentally in Fig. 2. (b) The mean kinetic energy of the ions for the above simulations. As the background gas density is reduced, cooling due to the MOT overcomes the heating due to background vapor and therefore lowers the ion kinetic energy.
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