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

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 ($\alpha$). 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.