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arXiv:2505.05002 · Scientific Reports

Isotope-selective Ion trapping via sympathetic cooling using a surface-electrode trap with a hole for collimated atomic loading

Masanari Miyamoto, Takashi Higuchi, Kentaro Furusawa, Norihiko Sekine, Kazuhiro Hayasaka, U. Tanaka

Published Dec 2, 2025• 0 citations
arXivHTMLPDF

Abstract

We developed a surface-electrode ion trap with a square hole measuring [Formula: see text] for atomic loading. The hole was fabricated using anisotropic etching of a silicon substrate and was designed to minimize potential distortion in the trapping region. By introducing the atomic beam through the hole, we achieved enhanced isotope selectivity and experimentally demonstrated the selective trapping of calcium isotope ions using an atomic oven. We successfully prepared isotope ion pairs directly from the oven via sympathetic cooling at a rate comparable to that achieved using ablation loading. The sympathetic cooling process occurred on the order of a few seconds. We demonstrated the direct generation of an ion chain above the through-hole. This approach can be applied for trapping a wide range of ion species using a remarkably simple experimental setup, making it desirable for several applications such as quantum-charge-coupled-device (QCCD) architectures and precision measurements of isotope shifts.

DOI
10.1038/s41598-025-29512-2

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•Oct 3, 2013•0 citations
We developed a surface-electrode ion trap with a square hole measuring [Formula: see text] for atomic loading. The hole was fabricated using anisotropic etching of a silicon substrate and was designed to minimize potential distortion in the trapping region. By introducing the atomic beam through the hole, we achieved enhanced isotope selectivity and experimentally demonstrated the selective trapping of calcium isotope ions using an atomic oven. We successfully prepared isotope ion pairs directly from the oven via sympathetic cooling at a rate comparable to that achieved using ablation loading. The sympathetic cooling process occurred on the order of a few seconds. We demonstrated the direct generation of an ion chain above the through-hole. This approach can be applied for trapping a wide range of ion species using a remarkably simple experimental setup, making it desirable for several applications such as quantum-charge-coupled-device (QCCD) architectures and precision measurements of isotope shifts.
We present studies of resonance-enhanced photoionization for isotope-selective loading of Ca+{\mathrm{Ca}}^{+}Ca+ into a Paul trap. The ${4s}^{2}{}^{1}{S}_{0}\ensuremath{\leftrightarrow}4s4p{}^{1}{P}_{1}$ transition of neutral calcium is driven by a 423 nm laser and the atoms are photoionized by a second laser at 389 nm. Isotope selectivity is achieved by using crossed atomic and laser beams to reduce the Doppler width significantly below the isotope shifts in the 423 nm transition. The loading rate of ions into the trap is studied under a range of experimental parameters for the abundant isotope 40Ca+.{}^{40}{\mathrm{Ca}}^{+}.40Ca+. Using the fluorescence of the atomic beam at 423 nm as a measure of the Ca number density, we estimate a lower limit for the absolute photoionization cross section of 170(60) Mb. We achieve loading and laser cooling of all the naturally occurring isotopes, without the need for enriched sources. Laser heating/cooling is observed to enhance the isotope selectivity. In the case of the rare species 43Ca+{}^{43}{\mathrm{Ca}}^{+}43Ca+ and 46Ca+,{}^{46}{\mathrm{Ca}}^{+},46Ca+, which have not previously been laser cooled, the loading is not fully isotope selective, but we show that pure crystals of 43Ca+{}^{43}{\mathrm{Ca}}^{+}43Ca+ may nevertheless be obtained. We find that for loading 40Ca+{}^{40}{\mathrm{Ca}}^{+}40Ca+ the 389 nm laser may be replaced by an incoherent source.
We demonstrate loading of ions into a surface-electrode trap (SET) from a remote, laser-cooled source of neutral atoms. We first cool and load $\ensuremath{\sim}$106{10}^{6}106 neutral 88{}^{88}88Sr atoms into a magneto-optical trap from an oven that has no line of sight with the SET. The cold atoms are then pushed with a resonant laser into the trap region where they are subsequently photoionized and trapped in an SET operated at a cryogenic temperature of 4.6 K. We present studies of the loading process and show that our technique achieves ion loading into a shallow (15 meV depth) trap at rates as high as 125 ions/s while drastically reducing the amount of metal deposition on the trap surface as compared with direct loading from a hot vapor. Furthermore, we note that due to multiple stages of isotopic filtering in our loading process, this technique has the potential for enhanced isotopic selectivity over other loading methods. Rapid loading from a clean, isotopically pure, and precooled source may enable scalable quantum-information processing with trapped ions in large, low-depth surface-trap arrays that are not amenable to loading from a hot atomic beam.
We produce large numbers of low-energy ions by photoionization of laser-cooled atoms inside a surface-electrode-based Paul trap. The isotope-selective trap's loading rate of $4\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{Yb}}^{+}\phantom{\rule{0.3em}{0ex}}\mathrm{ions}∕\mathrm{s}$ exceeds that attained by photoionization (electron-impact ionization) of an atomic beam by three (six) orders of magnitude. Traps as shallow as 0.13eV0.13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}0.13eV are easily loaded with this technique. The ions are confined in the same spatial region as the laser-cooled atoms, which will allow the experimental investigation of interactions between cold ions and cold atoms or Bose-Einstein condensates.
Sympathetic cooling and Coulomb crystallisation of molecular ions above the \nsurface of an ion-trap chip were demonstrated. N2+ and CaH+ ions were confined \nin a surface-electrode radiofrequency ion trap and cooled by the interaction \nwith laser-cooled Ca+ ions to secular translational temperatures in the \nmilliKelvin range. The configuration of trapping potentials generated by the surface \nelectrodes enabled the formation of planar bicomponent Coulomb crystals \nand the spatial separation of the molecular from the atomic ions. The structural \nand thermal properties of the crystals were characterized using molecular \ndynamics simulations. The effects of trap anharmoncities on the shape and \nenergy of bicomponent crystals were theoretically investigated. It was shown \nthat the trapping potentials can also deliberately be engineered to spatially \nseparate ion species in bicomponent crystals. Furthermore, a multi-functional \nsurface-electrode radiofrequency ion-trap chip has been developed to enable experiments \nwith cold molecular ions using a monolithic device. The chip was \ndesigned to combine various tasks such as loading and preparation of ions, mass \nspectrometry, spectroscopy, reaction studies, and manipulation of ion crystals \nin a miniaturised device. This chip features carefully engineered ion channel \nintersections that enable transporting sympathetically cooled molecular ions in \nthe form of bicomponent crystals. A detailed description of the fabrication and \nsimulation of the two chips are presented. The present study extends chipbased \ntrapping techniques to Coulomb-crystallised molecular ions with potential \napplications in mass spectrometry, cold chemistry, quantum technology, and \nspectroscopy.
A range of experimental techniques for application in reaction studies between ionic and neutral atoms/molecules and high resolution spectroscopy experiments with sympathetically cooled molecular ions are presented. A novel ion trap loading scheme using the photo-ionisation of atoms generated by the pulsed laser ablation of a solid calcium target has been characterised. We have identified the range of ablation laser fluences that must be used in order to produce a flux of neutral calcium atoms, which is a prerequisite for isotope selective ion trap loading. Calcium ions are trapped and laser cooled in a linear radio-frequency ion trap. We have developed a spectroscopy scheme that allows the entire fluorescence spectrum of trapped ions to be rapidly collected with high precision while maintaining a low ion temperature and good ion localisation throughout interrogation. The scheme has been demonstrated by measuring the saturation intensity of the calcium ion 4S1/2?4P1/2 transition. We have developed a novel scheme to measure the secular motion of trapped ions and demonstrated the application of the technique to ion-neutral collision reaction experiments. Employing pulsed excitation and Doppler velocimetry, we have measured the centre-of-mass mode frequency of single ions as well as large ion crystals with a frequency precision better than 2x10-3 within an interrogation time on the order of seconds. This method has been used to measure the mass of ions and observe charge exchange collisions between trapped calcium isotopes. In particular, we have measured the 44Ca++40Ca!40Ca++44Ca reaction cross section and demonstrated the single-event resolution of the technique. Finally, we have developed a novel all-optical broadband scheme for exciting, amplifying and measuring the secular motion of ions in the trap. Oscillation induced by optical excitation has been coherently amplified to control and measure the ion's secular motion. Requiring only a single interrogation laser, the ion's oscillation amplitude can be precisely controlled. The application of this technique to non-destructive spectroscopy of trapped molecular ions is discussed.