Laser excitation of the Th-229 nuclear isomer in a trapped ion
In plain words
Thorium-229 nuclei have been excited by laser inside crystals, but a single thorium ion held in a trap would avoid crystal disturbances and could make a clock accurate to about one part in $10^{19}$. Laser excitation of the nucleus in a trapped ion has not yet been achieved.
Precise statement
Drive the $8.36\,\mathrm{eV}$ ($148.4\,\mathrm{nm}$) Th-229 isomer transition with a laser in a trapped, sympathetically cooled Th-229 ion (Th3+, whose single valence electron allows laser cooling and state detection, or higher charge states), detect excitation through the hyperfine structure of the electronic transitions, and determine the achievable systematic uncertainty of the resulting clock, predicted to approach $10^{-19}$ (Campbell et al, Physical Review Letters, 2012).
What would settle it
Observation of laser-driven isomer excitation in a trapped Th-229 ion, followed by a closed-loop clock operation with an evaluated systematic uncertainty budget.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Trapped-ion work up to mid-2026 is theoretical or uses isomers populated in the alpha decay of U-233; a 2026 analysis (arXiv 2604.27614) estimates that locating the line in trapped Th3+ takes about one month with existing vacuum-ultraviolet lasers.
Results
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