Thorium-229 Laser Excitation and the Nuclear Clock Idea

Short answer: Thorium-229 has an unusually low-energy nuclear excited state, about 8.4 eV, that can be reached with ultraviolet lasers, which makes an optical nuclear clock conceivable. Laser excitation of this state in thorium-doped crystals was reported in 2024, with measured wavelengths near 148.38 nm. Those are spectroscopy results and no working nuclear clock is reported in these abstracts.

Why thorium-229?

A 2026 conference proceeding explains the appeal. Thorium-229 has a uniquely low first excited nuclear state, with an excitation energy of 8.356 eV, so vacuum ultraviolet lasers can excite it. The authors say this opens the possibility of a clock based on nuclear energy levels, and that unlike atomic clocks, nuclear clocks could work even when embedded in solids. In the introduction they note that nuclear transitions are less susceptible to external fields than atomic transitions because nuclear moments are small, which makes them promising candidates for high-precision frequency standards. Typical nuclear excitation energies are in the keV range, where narrow-linewidth lasers are hard to build, which is what makes this isomer unusual.

What did the first laser excitation show?

A 2024 Physical Review Letters paper reports that the 8.4 eV isomer in thorium-229 was resonantly excited in thorium-doped calcium fluoride crystals using a tabletop tunable laser system. A resonance fluorescence signal appeared in two crystals with different thorium-229 dopant concentrations, and was absent in a control experiment using thorium-232. The nuclear resonance was measured at a wavelength of 148.3821(5) nm, or a frequency of 2020.409(7) THz. The fluorescence lifetime in the crystal was 630(15) seconds, corresponding, the authors say, to an isomer half-life of 1740(50) seconds for a nucleus isolated in vacuum. They conclude the results pave the way toward thorium-229 nuclear laser spectroscopy and optical nuclear clocks.

What did a second crystal show?

A second 2024 Physical Review Letters paper used crystals of lithium strontium aluminium fluoride doped with thorium-229 in a laser-based search for the transition. It reports two spectroscopic features near the nuclear transition energy: a broad excitation feature with redshifted fluorescence decaying over a few seconds, and a narrow, laser-linewidth-limited feature at 148.38219 nm with a lifetime of 568(13) seconds. The authors assign the narrow feature to excitation of the nuclear isomeric state, with an energy of 8.355733 eV in this crystal. The statistical and systematic uncertainties are given in the abstract.

What does the proceeding add?

The 2026 proceeding describes spectroscopy of thorium-doped calcium fluoride using a vacuum ultraviolet laser with a linewidth of about 30 MHz and a detector that significantly reduces radioluminescence. In solids, interactions between the nuclear electric quadrupole moment and local electric field gradients split the nuclear levels, and weak splitting transitions are hard to see against background radioluminescence. The authors report a weak transition that they say was not measured by other groups. This is a conference proceeding and its claim has not been independently checked here.

What to keep in mind

These papers establish that the transition can be excited and measured in crystals. They do not report a clock that runs continuously or has a measured clock stability, and the abstracts do not claim one. This post is based on abstracts and visible summary text and full texts were not checked. It makes no claim about products.

FAQ

What is a nuclear isomer?

A long-lived excited state of a nucleus. In thorium-229 it sits at about 8.36 eV, which the proceeding says is exceptionally low.

Why use a crystal host?

Both 2024 papers excite thorium-229 embedded in fluoride crystals. The proceeding says nuclear clocks, unlike atomic clocks, could work embedded in solids.

What wavelength was measured?

About 148.38 nm in both 2024 papers, with small differences between hosts.

Is there a working nuclear clock?

Not in these abstracts. They describe laser spectroscopy as a step toward one.