With the development of optical frequency comb and ultra-stable laser techniques, the performance of optical frequency standards has been improved significantly over the past decades. Single charged Yb ions have an alkali-like atomic structure and have ultra-long storage time. And the ytterbium single-ion optical frequency standards have two clock transitions (E2 & E3). In this paper, we will introduce the progress in the development of a highly accurate optical frequency standard based on single ytterbium-171 ion at the National Institute of Metrology (NIM), China.
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.