Last year, researchers led by UCLA achieved a milestone that physicists had pursued for half a century. They succeeded in making radioactive thorium nuclei absorb and release photons in a controlled way, similar to how electrons behave inside atoms. The idea was first proposed by the team in 2008, and its realization is expected to open the door to a new generation of extremely precise clocks. These advances could dramatically improve navigation systems and may even help scientists test whether some of nature’s fundamental constants change over time.
Despite the breakthrough, a serious limitation remained. The specific isotope required for nuclear clocks, thorium-229, is found only in weapons-grade uranium. As a result, scientists estimate that only about 40 grams of this material exist worldwide for clock research, making efficiency a critical challenge.
A simpler approach uses far less thorium
An international collaboration led by UCLA physicist Eric Hudson has now found a way around this bottleneck. The team discovered how to reproduce their earlier results while using only a tiny fraction of the thorium previously required. Their new method, reported in Nature, is straightforward and inexpensive, raising the possibility that nuclear clocks could one day become small and affordable enough for widespread use.
If that happens, these clocks could move beyond laboratories and replace timing systems in power grids, cell phone towers, and GPS satellites. They may even shrink enough to fit into phones or wristwatches. The technology could also enable navigation in places where GPS signals cannot reach, including deep space and underwater environments such as submarines.
Fifteen years of work replaced by a simple technique
Hudson’s team spent 15 years developing the specialized thorium-doped fluoride crystals that enabled their original success. In those experiments, thorium-229 atoms were bonded with fluorine in a carefully engineered structure. The resulting crystals stabilized the thorium while remaining transparent to the laser light needed to excite the atomic nucleus. However, the process proved extremely difficult, and producing the crystals required relatively large amounts of thorium.
“We did all the work of making the crystals because we thought the crystal had to be transparent for the laser light to reach the thorium nuclei. The crystals are really challenging to fabricate. It takes forever and the smallest amount of thorium we can use is 1 milligram, which is a lot when there’s only 40 or so grams available,” said first author and UCLA postdoctoral researcher Ricky Elwell, who received the 2025 Deborah Jin Award for Outstanding Doctoral Thesis Research in Atomic, Molecular, or Optical Physics for last year’s breakthrough.
Borrowing a method from jewelry making
In the new study, the researchers took a very different approach. They deposited an extremely thin layer of thorium onto stainless steel using electroplating, a technique commonly used in jewelry. Electroplating, developed in the early 1800s, relies on an electric current to move metal atoms through a conductive solution and coat one surface with another metal. For example, gold or silver is often electroplated onto less valuable metals.
“It took us five years to figure out how to grow the fluoride crystals and now we’ve figured out how to get the same results with one of the oldest industrial techniques and using 1,000 times less thorium. Further, the finished product is essentially a small piece of steel and much tougher than the fragile crystals,” said Hudson.
Rethinking how nuclear excitation works
The success of the new system came from realizing that a long-standing assumption was incorrect. Scientists had believed that thorium needed to be embedded in a transparent material so laser light could reach and excite the nucleus. The team found that exciting the nucleus enough to observe its energy transition was far easier than previously thought.
“Everyone 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 showed that is simply not true,” said Hudson. “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 material such as 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!”
Why nuclear clocks matter beyond the lab
Beyond improving communication networks, radar systems, and power grid synchronization, ultra-precise clocks could solve a major national security concern: navigation without GPS. If a bad actor — or even an electromagnetic storm — disrupted enough satellites, GPS-based navigation would fail. Submarines already rely on atomic clocks while submerged, but existing clocks drift over time, forcing vessels to surface after weeks to confirm their position.
Nuclear clocks are far less sensitive to environmental disturbances, making them especially valuable in situations where accuracy must be maintained for long periods without external signals.
“The UCLA team’s approach could help reduce the cost and complexity of future thorium-based nuclear clocks,” said Makan Mohageg, optical clock lead at Boeing Technology Innovation. “Innovations like these may contribute to more compact, high-stability timekeeping, relevant to several aerospace applications.”
A foundation for future space exploration
More accurate clocks are also essential for long-distance space travel, where precise timing underpins navigation and communication.
“The UCLA group led by Eric Hudson has done amazing work in teasing out a viable way to probe the nuclear transition in thorium — work extending over more than a decade. This work opens the way to a viable thorium clock,” said Eric Burt, who leads the High Performance Atomic Clock project at the NASA Jet Propulsion Laboratory and was not involved in the research. “In my opinion, thorium nuclear clocks could also revolutionize fundamental physics measurements that can be performed with clocks, such as tests of Einstein’s theory of relativity. Due to their inherent low sensitivity to environmental perturbations, future thorium clocks may also be useful in setting up a solar-system-wide time scale essential for establishing a permanent human presence on other planets.”
Research collaboration and funding
The research was supported by the National Science Foundation and involved physicists from the University of Manchester, University of Nevada Reno, Los Alamos National Laboratory, Ziegler Analytics, Johannes Gutenberg-Universität at Mainz, and Ludwig-Maximilians-Universität München.