Thorium 229 nuclei driven by laser in opaque solid moves optical nuclear clock research forward

by Robert Schreiber
Berlin, Germany (SPX) Dec 18, 2025
A team from the University of California Los Angeles (UCLA), Ludwig Maximilian University of Munich (LMU), and Johannes Gutenberg University Mainz (JGU) has for the first time excited the atomic nucleus of the isotope thorium 229 with laser light while the atoms are embedded in a non transparent solid host. The experiment demonstrates that nuclear laser excitation is possible even when the surrounding material is almost opaque at the excitation wavelength of 148 nanometers, opening a new class of solids for nuclear laser spectroscopy. The work is presented as a step toward quantum technologies based on nuclear transitions, including a prospective optical nuclear clock, and has been reported in the journal Nature.

Laser control of atomic electron shells has been standard since the 1960s and underpins optical atomic clocks and many quantum information platforms. Only in 2024 did researchers directly excite an atomic nucleus with laser light for the first time, marking the emergence of nuclear laser spectroscopy as a distinct field. The new study extends this capability from transparent host materials to a non transparent medium that can incorporate and stabilize thorium atoms while still allowing sufficient interaction with the 148 nanometer laser light used to drive the nuclear transition.

Until now, experimental excitation of the thorium 229 nucleus had been restricted to host crystals that transmit this vacuum ultraviolet wavelength. The new demonstration shows that even materials that are nearly opaque at 148 nanometers can still support nuclear excitation if they provide the right environment for thorium ions and allow a fraction of the laser light to reach them. This broadens the range of solid state systems available for precision nuclear spectroscopy, including materials selected for mechanical, thermal, or electronic properties rather than transparency alone.

The experiment also opens the way to laser based internal conversion (IC Mossbauer spectroscopy) in solids containing thorium 229. In IC Mossbauer spectroscopy, the de excited nucleus transfers its energy to an atomic electron, which is then emitted as a conversion electron. Detecting these electrons provides a sensitive probe of the nuclear transition in a solid state environment and enables detailed studies of how crystal fields and electronic structure influence nuclear energy levels.

"This success opens the door to a previously inaccessible area of nuclear physics," explains Dr. Lars von der Wense of the Institute of Physics at JGU, who first proposed the experiment in 2017. "The fact that we can now also perform nuclear excitation in non-transparent materials enables completely new experiments - and brings us a significant step closer to realizing an optical nuclear clock."

An optical nuclear clock based on thorium 229 is regarded as a candidate for an extremely stable frequency standard. Among other things, it could improve satellite based navigation and enable more precise applications in Earth observation, autonomous transport, and fundamental research. Particular attention also lies on questions of fundamental physics, such as searching for temporal variations in the constants of nature and the detection of dark matter.

With their recent achievement, the researchers have laid the foundation for numerous future experiments and applications and shown the potential of combining vacuum ultraviolet laser technology with nuclear physics. Their results indicate how engineered solid state environments and laser control of nuclear transitions can support the development of next generation nuclear clocks and related quantum technologies.

Research Report:Laser-based conversion electron Mossbauer spectroscopy of 229ThO2