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arXiv:2504.19182

Coulomb Crystallization of Highly Charged Ni^12+ Ions in a Linear Paul Trap

Shaolong Chen, Zhiqiang Zhou, Guosheng Zhang, Jun Xiao, Yao Huang, Kelin Gao, Hua Guan

Published Apr 27, 2025• 0 citations
arXivHTMLPDF

Abstract

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 ($α$). While the recent successful demonstration of an Ar$^{13+}$ optical clock has validated the feasibility of HCI-based systems, Ni$^{12+}$ -- featuring an ultranarrow clock transition linewidth -- stands out as a superior candidate for achieving HCI optical clocks with $10^{-19}$ 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 Ni$^{12+}$ ion.

DOI
10.48550/arxiv.2504.19182

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Optical clocks have garnered widespread attention due to their unparalleled precision in the fields of time-frequency standards, geodetic measurements, and fundamental physics research. Among recent advances, highly charged ion (HCI)-based optical clocks have attracted significant scientific interest owing to their remarkable insensitivity to electromagnetic perturbations and enhanced sensitivity to potential variations of the fine-structure constant $(\ensuremath{\alpha})$. Although the recent successful demonstration of an Ar13+{\mathrm{Ar}}^{13+}Ar13+ optical clock has validated the feasibility of HCI-based systems, Ni12+{\mathrm{Ni}}^{12+}Ni12+ (featuring an ultranarrow clock transition linewidth) stands out as a superior candidate for achieving HCI optical clocks with ${10}^{\ensuremath{-}19}$ level uncertainty and stability. In this work, we report the Coulomb crystallization of highly charged nickel ions (Ni-HCIs). Through a precise deceleration and sympathetic cooling protocol in a room-temperature linear Paul trap, high-energy Ni-HCI bunches were sympathetically cooled from megakelvin energies down to the hundred-millikelvin range using laser-cooled Be+{\mathrm{Be}}^{+}Be+ ions. This work represents a pivotal step toward the realization of an optical clock based on Ni12+{\mathrm{Ni}}^{12+}Ni12+ ion.
Control over the motional degrees of freedom of atoms, ions, and molecules in a field-free environment enables unrivalled measurement accuracies but has yet to be applied to highly charged ions (HCIs), which are of particular interest to future atomic clock designs and searches for physics beyond the Standard Model. Here, we report on the Coulomb crystallization of HCIs (specifically (40)Ar(13+)) produced in an electron beam ion trap and retrapped in a cryogenic linear radiofrequency trap by means of sympathetic motional cooling through Coulomb interaction with a directly laser-cooled ensemble of Be(+) ions. We also demonstrate cooling of a single Ar(13+) ion by a single Be(+) ion-the prerequisite for quantum logic spectroscopy with a potential 10(-19) accuracy level. Achieving a seven-orders-of-magnitude decrease in HCI temperature starting at megakelvin down to the millikelvin range removes the major obstacle for HCI investigation with high-precision laser spectroscopy.
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^{40}40Ca+^++ 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 Xeq+^{q+}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.
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 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.
We report on the design of a segmented linear Paul trap for optical clock applications using trapped ion Coulomb crystals. For an optical clock with an improved short-term stability and a fractional frequency uncertainty of 10−18, we propose 115In+ ions sympathetically cooled by 172Yb+. We discuss the systematic frequency shifts of such a frequency standard. In particular, we elaborate on high-precision calculations of the electric radiofrequency field of the ion trap using the finite element method. These calculations are used to find a scalable design with minimized excess micromotion of the ions at a level at which the corresponding second-order Doppler shift contributes less than 10−18 to the relative uncertainty of the frequency standard.