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

Thorium-229 ( 229 Th) possesses an optical nuclear transition between the ground state ( 229g Th) and low-lying isomer ( 229m Th). 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 229 Th ( 229 Th 3+ ) 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 229 Th 3+ in the nuclear ground state have been performed 8 , properties of 229m Th 3+ , 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 229m Th 3+ continuously supplied by a 233 U source and the determination of nuclear decay half-life of the isolated 229m Th 3+ to be 1,400 - 300 + 600 s through nuclear-state-selective laser spectroscopy. Furthermore, by determining the hyperfine constants of 229m Th 3+ , we reduced the uncertainty of the sensitivity of the 229 Th nuclear clock to variations in the fine-structure constant by a factor of four. These results offer key parameters for the 229 Th 3+ nuclear clock and its applications in the search for new physics.