Towards the ultimate quantum sensor: theoretical aspects of developing the ²²⁹Th nuclear clock

The low-energy isomer of ²²⁹Th enables a fundamentally new class of quantum sensors: nuclear clocks with exceptional robustness and potential sensitivities far beyond existing electronic-structure–based standards. The concept was motivated in part by our proposal of a single-ion nuclear clock [PRL 108, 120802 (2012)], which demonstrated that the nuclear transition’s narrow linewidth and insensitivity to external fields could support metrology at the 10^-19 level. Realizing this promise in practice requires understanding how the nuclear excitation behaves in realistic environments—especially in emerging solid-state platforms.

I will present recent theoretical advances that establish the framework for engineering and interpreting solid-state ²²⁹Th clocks. We developed a first-principles theory of internal conversion in insulating hosts [PRL 134, 253801 (2025)], identifying the dominant electronic channels for nonradiative decay and providing predictive tools for selecting materials with suppressed quenching. Complementing this, we analyzed photo-induced quenching pathways [PRR 7, L022062 (2025)] that emerge under off-resonant laser excitation.

Building on these insights, we proposed and evaluated new host platforms—including nonlinear optical crystals [APL 126, 111101 (2025)], polyatomic-ionic materials [Dalt. Trans. (2025)], and spinless crystals engineered for long coherence [arXiv:2503.11374]. These studies establish criteria for minimizing inhomogeneous broadening, controlling local electronic structure, and approaching radiative-lifetime-limited performance. I will also highlight predictions of host-dependent frequency offsets (“clockwork shifts”) [PRL 135, 123001 (2025)], essential for connecting solid-state spectroscopy to the trapped-ion transition envisioned in our 2012 proposal.

Together with recent experimental breakthroughs, these developments outline a clear path from the original ion-clock concept to practical nuclear clocks.