Zhiqiang Zhou

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.

Highly charged ions (HCIs) have drawn significant interest in quantum metrology and in search for new physics. Among these, Ni12+ is considered as one of the most promising candidates for the next generation of HCI optical clocks, due to its two E1-forbidden transitions M1 and E2, which occur in the visible spectral range. In this work, we used the Shanghai-Wuhan Electron Beam Ion Trap to perform a high-precision measurement of the M1 transition wavelength. Our approach involved an improved calibration scheme for the spectra, utilizing auxiliary Ar+ lines for calibration and correction. Our final measured result of the M1 transition wavelength demonstrates a five-fold improvement in accuracy compared to our previous findings, reaching the sub-picometer level accuracy. In combination with our rigorous atomic-structure calculations to capture the electron correlations and relativistic effects, the quantum electrodynamic (QED) corrections were extracted. Moreover, comparing with an estimate of the one-electron QED contributions by using the GRASP2018 package, we found that the present experimental accuracy is high enough for testing the higher-order QED corrections for such a complex system with four electrons in the p subshell.