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 $\left(\alpha \right)$. Although the recent successful demonstration of an ${\mathrm{Ar}}^{13+}$ optical clock has validated the feasibility of HCI-based systems, ${\mathrm{Ni}}^{12+}$ (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 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 ${\mathrm{Be}}^{+}$ ions. This work represents a pivotal step toward the realization of an optical clock based on ${\mathrm{Ni}}^{12+}$ ion.

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• Atomic, optical & lattice clocks
• Cooling & trapping