Breakthrough on the path to optical nuclear clocks: laser-based excitation of Thorium-229 in non-transparent material

Researchers from Los Angeles, Munich, and Mainz open new avenues for nucleus-based quantum technologies

A research team from the University of California Los Angeles (UCLA), Ludwig Maximilian University of Munich (LMU), and Johannes Gutenberg University Mainz (JGU) has succeeded in exciting the atomic nucleus of the isotope thorium-229 using laser light in a non-transparent host material. This achievement opens up an entirely new class of materials for nuclear laser spectroscopy – a decisive step toward novel quantum technologies such as the highly promising optical nuclear clock. The results of the study have now been published in the renowned scientific journal Nature.

From controlling the atomic shell to manipulating the atomic nucleus

Since the 1960s, scientists have used lasers to precisely manipulate the atomic shell – an approach that has led to technological developments such as optical atomic clocks and quantum computers. Targeted manipulation of atomic nuclei using laser light, however, remains a young field of research: it was not until 2024 that researchers succeeded for the first time in directly exciting an atomic nucleus using laser light.

Expanding the range of usable materials

Until now, experimental excitation of the thorium-229 nucleus had only been successful in host materials transparent to the 148-nm laser light required for excitation. The newly achieved demonstration in a non-transparent material – one that incorporates and stabilizes the thorium atoms while remaining nearly opaque to the laser light – drastically broadens the range of substances that can be used. At the same time, the work opens up the field of laser-based IC Mössbauer spectroscopy, an entirely new tool for investigating atomic nuclei in solid-state environments.

“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.”

Such a nuclear clock is considered to be potentially the most stable time standard ever. Among other things, it could revolutionize 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 demonstrated the vast potential that lies in combining state-of-the-art laser technology with nuclear physics.