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arXiv:2512.12266 · Physical Review A

Coulomb crystallization of xenon highly charged ions in a laser-cooled Ca+ matrix

Leonid Prokhorov, Aaron A. Smith, Mingyao Xu, Kostas Georgiou, Vera Guarrera, Lakshmi P. Kozhiparambil Sajith, Elwin A. Dijck, Christian Warnecke, Malte Wehrheim, Alexander Wilzewski, Laura Blackburn, Matthias Keller, Vincent Boyer, Thomas Pfeifer, Ullrich Schwanke, Cigdem Issever, Steven Worm, Piet O. Schmidt, José R. Crespo Lopez-Urrutia, Giovanni Barontini

physics.atom-phcond-mat.quant-gasquant-phSubmitted Dec 13, 2025• Announced Dec 16, 2025• 1 citation
v1 · Dec 13, 2025v2 · Jun 11, 2026
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Abstract

We report on the sympathetic cooling and Coulomb crystallization of xenon highly charged ions (HCIs) with laser-cooled Ca$^+$ ions. The HCIs are produced in a compact electron beam ion trap, then charge selected, decelerated, and finally injected into a cryogenic linear Paul trap. There, they are captured into $^{40}$Ca$^+$ Coulomb crystals, and co-crystallized within them, causing dark voids in their fluorescence images. Fine control over the number of trapped ions and HCIs allows us to realize mixed-species crystals with arbitrary ordering patterns. By investigating Xe$^{q+}$--Ca$^+$ strings, we confirm the HCI charge states, measure their lifetime and characterize the mixed-species motional modes. Our system effectively combines the established quantum control toolbox for Ca$^+$ with the rich set of atomic properties of Xe highly charged ions, providing a resourceful platform for optical frequency metrology, searches for signatures of new physics, and quantum information science.

Journal reference
Phys. Rev. A 113, L041102 (2026)
DOI
10.1103/lxr8-l7wq

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We report the sympathetic cooling and Coulomb crystallization of xenon highly charged ions (HCIs) with laser-cooled Ca+ ions. The HCIs are produced in a compact electron beam ion trap, then charge selected, decelerated, and finally injected into a cryogenic linear Paul trap. There, they are captured into 40 Ca+ Coulomb crystals, and cocrystallized within them, causing dark voids in their fluorescence images. Fine control over the number of trapped ions and HCIs allows us to realize mixed-species crystals with arbitrary ordering patterns. By investigating Xeq+ − Ca+ strings, we confirm the HCI charge states, measure their lifetime, and characterize the mixed-species motional modes. Our system effectively combines the established quantum control toolbox for Ca+ with the rich set of atomic properties of Xe highly charged ions, providing a resourceful platform for optical frequency metrology, searches for signatures of new physics, and quantum information science.
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Simultaneous control over both excitational and motional degrees of freedom of quantum objects facilitates measurement accuracies at a level that enables the measurement of subtle relativistic effects, tests of the Standard Model and even the search for expected effects of physics beyond it. Theoretical calculations predict enhanced sensitivities of forbidden optical transitions in highly charged ions (HCIs) to possible variations of fundamental constants, such as the fine structure constant. Within this thesis, a versatile preparation technique of cold and strongly localized HCIs has been developed and demonstrated, adding a multitude of laser accessible systems to the quantum toolbox, that have enhanced sensitivity to fundamental physics. HCIs are extracted from an electron beam ion trap (EBIT), where they are produced in the MK temperature range. Subsequently they are cooled down to the mK regime, needed for high precision laser spectroscopy, in a cryogenic linear Paul trap. Deceleration, precooling and multi-pass stopping approaches enable HCI implantation within a prestored, continuously laser-cooled Be+-Coulomb crystal. Here the HCIs are co-crystallized in mixed-species ensembles (specifically Ar13+ highly charged and Be+ coolant ions) - ranging from large 3-dimensional crystals over ion strings down to a single Ar13+ ion cooled by a single Be+ ion. The precooling, retrapping and sympathetic stopping process, as well as various Coulomb crystal configurations are characterized and compared to simulations and theoretical models. The developed experimental concept and its modular implementation enable, for the first time, the localization of HCIs in a microscopic region of a Paul trap, today's standard method for singly-charged ion frequency metrology. This is a necessary prerequisite for the development of novel ultra-precise optical clocks based on quantum logic spectroscopy on forbidden transitions in HCIs and subsequent low-energy searches for physics beyond the Standard Model.
We investigated sympathetic Coulomb crystallization of CaH+{}^{+}+ ions produced by a laser-induced reaction through an excited state according to 40Ca^{40}\mathrm{Ca}40Ca+{}^{+}+(4p2P1/24p {}^{2}{P}_{1/2}4p2P1/2​) +++ H2{\mathrm{H}}_{2}H2​ $\ensuremath{\rightarrow}$ 40Ca^{40}\mathrm{Ca}40CaH+{}^{+}+ +++ H. 40Ca^{40}\mathrm{Ca}40CaH+{}^{+}+ ions stored in a linear Paul trap were characterized by their secular-motion excitation spectrum and by the modified fluorescence images of crystallized Ca+{}^{+}+ ions. The number of two-species crystallized ions, the secular motion temperature, and the structure were determined by molecular dynamics simulations. Both the observed two-species ion crystals and simulation images demonstrate that the CaH+{}^{+}+ ions were sympathetically crystallized with their secular-motion temperature being less than 10 mK. We determined reaction rates from fluorescence images of two-species Coulomb crystals by systematically changing the reaction time. In addition a lower limit for the reaction rate coefficient $k=8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm3{}^{3}3/s was obtained.
Optical clocks have garnered widespread attention due to their unparalleled precision in time-frequency standards, geodetic measurements, and fundamental physics research. Among emerging developments, highly charged ion (HCI)-based optical clocks have attracted significant scientific interest owing to their exceptional resilience against electromagnetic perturbations and enhanced sensitivity to variations in the fine-structure constant (ααα). While the recent successful demonstration of an Ar13+^{13+}13+ optical clock has validated the feasibility of HCI-based systems, Ni12+^{12+}12+ -- featuring an ultranarrow clock transition linewidth -- stands out as a superior candidate for achieving HCI optical clocks with 10−1910^{-19}10−19 level uncertainty and stability. In this work, we report the Coulomb crystallization of nickel highly charged ions (Ni-HCIs). Through a precision deceleration and sympathetic cooling protocol in a room-temperature Paul trap, high-energy Ni-HCI bunches were sympathetically cooled from megakelvin to the 100-millikelvin range using laser-cooled Be+^{+}+ ions. This work represents a pivotal step toward the realization of an optical clock based on the Ni12+^{12+}12+ ion.