Highly Charged ²²⁹ Th ⁶⁺ Ions as the Candidate Platform for Nuclear Clock

Abstract The 229 Th nuclear optical clocks, operating via the 8.4 eV nuclear transition, hold great promise for attaining unprecedented accuracy in frequency standards and fundamental physics tests. In this study, we propose an approach that utilizes highly charged 229 Th 6+ ions as the platform for nuclear clock, which exhibits simple electronic energy structures and enhanced nucleus–electron coupling compared to low-charge Th ions. The 3 P 2 ↔ 3 P 0 ionic clock transition in 229 Th 6+ ions has the potential to serve as a probe for nuclear structure. Moreover, we predict the existence of two excited electronic states near and slightly above the nuclear clock state, which can serve as the intermediate states in the optical repumping process. We estimate the Rabi frequencies of the electronic bridge transitions from the nuclear clock state to these intermediate states and further analyze the population dynamics of the optical repumping process, which can be completed on the millisecond timescale. Our results demonstrate the advantages of using 229 Th 6+ ions as the promising platform for nuclear clock.