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Journal of Physics B Atomic Molecular and Optical Physics

An empirical study of sympathetic cooling using a large single-species ion cloud

Nikolaos Efthymiadis, Tudor Stefan, Stephen A. Jones

Published Jul 14, 2026• 0 citations
Publisher

Abstract

Abstract Sympathetic cooling is an important technique for achieving low motional temperatures in trapped ion systems that cannot be directly laser cooled. In this paper, we report on the sympathetic cooling of large samples (5 × 10 4 ) of Ba + ions which have been optically pumped into a dark state, using laser cooled Ba + ions in a linear radiofrequency (RF) ion trap. The trap lifetime was evaluated with different dark ion fractions as a function of the RF amplitude, the DC trapping potentials, the power and frequency of the cooling lasers, the quality of the vacuum, and the total number of ions. We found that the RF amplitude was the dominant factor contributing to the loss of ions, and that under optimal conditions and with a dark ion fraction as high as 85%, no losses were observed over 100 s long measurement periods, and fewer than 10% of the ions were lost after 1000 s.

DOI
10.1088/1361-6455/ae8a97

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Abstract Sympathetic cooling is an important technique for achieving low motional temperatures in trapped ion systems that cannot be directly laser cooled. In this paper, we report on the sympathetic cooling of large samples (5 × 10 4 ) of Ba + ions which have been optically pumped into a dark state, using laser cooled Ba + ions in a linear radiofrequency (RF) ion trap. The trap lifetime was evaluated with different dark ion fractions as a function of the RF amplitude, the DC trapping potentials, the power and frequency of the cooling lasers, the quality of the vacuum, and the total number of ions. We found that the RF amplitude was the dominant factor contributing to the loss of ions, and that under optimal conditions and with a dark ion fraction as high as 85%, no losses were observed over 100 s long measurement periods, and fewer than 10% of the ions were lost after 1000 s.
We have produced ensembles of cold 16O+2, 40Ar+, 12C16O+2, and various isotopes of barium ions (135Ba+, 136Ba+ and 137Ba+) via sympathetic cooling with laser-cooled 138Ba+ in a linear radiofrequency trap. The sympathetically cooled species were embedded in the centre of large 138Ba+ Coulomb crystals containing up to 2000 ions and were identified by motional resonance excitation. Crystals with molecular fractions exceeding 70% were obtained. The observed multi-species crystal structures agree well with results from molecular dynamics simulations. The simulations were also used to deduce an upper limit for the translational temperature of the molecular ions, as low as 20 mK.
The barium ions are trapped with the laser-cooled magnesium ions simultaneously in an rf trap and cooled sympathetically. Sympathetically cooled ions are one of the candidates of accurate optical frequency standard. The barium ions are ionized by the charge transfer with the magnesium ions for avoiding the loss due to the shallow trapping potential. The temperature of the barium ions is measured and the effect of the sympathetic cooling is observed. It is also found that the application of the ionization by the charge transfer to other ion species is possible.
A detailed investigation of ion crystals produced by laser and sympathetic cooling in a linear RF trap has been conducted.\n\nThe laser cooling methods were examined and applied to the trapped ^24Mg^(positive) ions. The crystals produced by the laser cooling were studied, including the dependence on RF voltage, end cap DC voltage, laser power and laser frequency. By manipulating the different RF voltages and endcap DC voltages, the structure phase transition of the ion crystals was observed.\n\nIn addition, the sympathetic cooling of different ion species with the laser cooled 24Mg^(positive) was carried out. In this process, the mixed Mg^(positive) and He^(positive) crystals were created andidentified, and mixed Mg^(positive) and H2^(positive) crystals were produced. The effect of an unwanted chemical reaction of Mg^(positive) and H2 was observed and minimized. After sympathetic cooling of light ion species, the sympathetic cooling of heavy molecular ions such as fullerene ions was also carried out. The efficiencies and final temperature in both cases are very different. Theoretically to interpret the results, molecular dynamics simulations of the laser cooling and sympathetic cooling were implemented. And the simulations were compared with the experimental results.\n\nIn the process of carrying out this research, the optics were rebuilt to provide reliable UV sources for the photoionization and laser cooling of Mg ions. The imaging\nsystem was reconfigured to take the images of ion crystals. New elements were added tin the ion trap to improve the ability to manipulate ions.
Radiation pressure from lasers has been used to cool and compress 9Be+ ions stored in a combination of static electric and magnetic fields (Penning trap) to tem eratures less than 10 mK and densities greater than lo7 cm-q in a magnetic field of 1.4 T. A technique called sympathetic cooling can be used to transfer this cooling and compression to other ion species. ions sympathetically cooled by laser cooled 9Be+ ions is given. source vi? sympathetic cooling is also discussed. A n example of lg8Hg+ The possibility of making an ultracold positron Ion traps have been used in low energy atomic physics experiments for a period of roughly 30 years.1*2) years, however, the technique of laser cooling and compression has greatly increased the potential use of stored ions in a number of applications. One example is time and frequency standards. With the technique of laser cooling and compression, ion temperatures less than 10 mK, densities a factor of 10 less than the Brillouin limit (defined below), and essentially indefinite confinement times have been obtained. 3,4) Unfortunately, elementary charged particles (electrons, positrons, protons, etc.) can not be directly laser During the past 10
Sympathetic cooling of two-ion system, in which one is laser-cooled and the other is sympathetically cooled and their masses are mismatched, in a linear rf trap with a double-well potential is proposed. The double-well potential consists of two wells, and there is one ion in each well. The axial frequencies of the two wells and the spatial interval between them are experimentally controllable. By theoretical analysis, the normal modes of the small oscillations around the equilibrium are derived, and a measure of the sympathetic cooling rate is obtained. As a result, it is found that the sympathetic cooling rate is fast when the frequency of the axial motion of the sympathetically cooled ion is close to that of the laser-cooled ion. In the double-well potential, the sympathetic cooling rate of the ion species whose mass is much heavier or lighter than that of the laser-cooled ion can be fast. The sympathetic cooling rate of C60{\mathrm{C}}_{60}C60​+{}^{+}+ by the laser-cooled Ba+{}^{+}+ in the double-well potential is estimated to be about 80 times faster than in the conventional setup. The double-well potential may be made by the microfabricated electrode configuration or by the optical dipole force trap.