Laser spectroscopy of triply charged ²²⁹Th isomer for a nuclear clock

Thorium-229 (²²⁹Th) possesses an optical nuclear transition between the ground state (^229gTh) and low-lying isomer (^229mTh). A nuclear clock based on this nuclear-transition frequency is expected to surpass existing atomic clocks owing to its insusceptibility to surrounding fields^1-5. In contrast to other charge states, triply charged ²²⁹Th (²²⁹Th³⁺) is the most suitable for highly accurate nuclear clocks because it has closed electronic transitions that enable laser cooling, laser-induced fluorescence detection and state preparation of ions^1,6-8. Although laser spectroscopic studies of ²²⁹Th³⁺ in the nuclear ground state have been performed⁸, properties of ^229mTh³⁺, including its nuclear decay lifetime that is essential to specify the intrinsic linewidth of the nuclear-clock transition, remain unknown. Here we report the trapping of ^229mTh³⁺ continuously supplied by a ²³³U source and the determination of nuclear decay half-life of the isolated ^229mTh³⁺ to be $$ through nuclear-state-selective laser spectroscopy. Furthermore, by determining the hyperfine constants of ^229mTh³⁺, we reduced the uncertainty of the sensitivity of the ²²⁹Th nuclear clock to variations in the fine-structure constant by a factor of four. These results offer key parameters for the ²²⁹Th³⁺ nuclear clock and its applications in the search for new physics.