Scientists have made a major step towards building the world's first practical nuclear clock.
In a study published today in Nature, the team demonstrate a completely new way of probing the tiny "ticking" of the thorium-229 nucleus without needing a specialised transparent crystal - a breakthrough that could underpin a new class of timekeeping so precise it could transform navigation, communications, earthquake and volcano prediction, and deep-space exploration.
The advance builds on a landmark achievement last year, when the team succeeded in using a laser to excite the nucleus of thorium-229 inside a transparent crystal - a feat the team has been working on for the past 15 years.
Now, researchers have achieved the same results using a tiny fraction of the material and with a method so simple and inexpensive that it opens the door to real-world nuclear clock technology.
"Previously, the transparent crystals needed to hold thorium-229 were technically demanding and costly to produce, which placed real limits on any practical application," explained Dr Harry Morgan, co-author of the research and Lecturer in Computational and Theoretical Chemistry at The University of Manchester. "This new approach is a major step forward for the future of nuclear clocks and leaves little doubt that such a device is feasible and potentially much closer than anyone expected."
In the new study, the team instead excited the thorium nucleus inside a microscopic thin film of thorium oxide, made by electroplating a minute amount of thorium onto a stainless-steel disc - a process similar to gold-plating jewellery and a radical simplification of their previous method.
The thorium nuclei absorb energy from a laser and then, after a few microseconds, transfer that energy to nearby electrons so it can be measured directly as an electric current. This method, known as conversion electron Mössbauer spectroscopy, has been in use for years, but normally requires high-energy gamma rays at special facilities. This is the first time it has been demonstrated with a laser in an ordinary lab.
Crucially, it shows that thorium-229 can be studied inside far more common materials than previously thought, removing one of the biggest obstacles to building practical nuclear clocks.
The technique also offers new insight into how thorium-229 behaves and decays, which could one day inform new types of nuclear materials and future energy research.
"We had always assumed that in order to excite and then observe the nuclear transition the thorium needed to be embedded in a material that was transparent to the light used to excite the nucleus. In this work, we realized that is simply not true," said UCLA physicist Eric Hudson., who led the research. "We can still force enough light into these opaque materials to excite nuclei near the surface and then, instead of emitting photons like they do in transparent materials like the crystals, they emit electrons which can be detected simply by monitoring an electrical current - which is just about the easiest thing you can do in the lab."
Like atomic clocks, nuclear clocks rely on the natural "ticking" of single atoms. But in atomic clocks that process involves electrons, while nuclear clocks use oscillations within the nucleus itself. This makes them far less sensitive to external disturbances, giving them the potential to be orders of magnitude more accurate.
Nuclear clocks could even be used to predict earthquakes and volcanic eruptions. Because of Einstein's theory of general relativity, nuclear clocks should be sensitive to small changes in the Earth's gravity due to the movement of magma and rock deep underground. By placing nuclear clocks all over earthquake zones, like Japan, Indonesia, or Pakistan, we could watch what's going on beneath our feet in real time and predict tectonic events before they happen.
Dr Morgan added: "In the long term, this technology could revolutionise our ability to prepare for natural disasters. It's incredibly exciting to think that thorium clocks can do things we previously thought were impossible, as well as improving everything we currently use atomic clocks for."
The research was funded by the National Science Foundation, and also included physicists from the University of Nevada Reno, Los Alamos National Laboratory, Ziegler Analytics, Johannes Gutenberg-Universität at Mainz, and Ludwig-Maximilians-Universität München.
This research was published in the journal Nature
Full title: Laser-based conversion electron Mössbauer spectroscopy of 229ThO2
DOI:10.1038/s41586-025-09776-4
URL: https://www.nature.com/articles/s41586-025-09776-4 [nature.com]
