Last year, a UCLA-led team accomplished something scientists have been trying to do for 50 years. They made radioactive thorium nuclei absorb and emit photons like electrons in an atom do. This achievement was the realization of a dream they first proposed in 2008 and is expected to usher in a new era of high-precision timekeeping, with a dramatic impact on navigation. It could also lead to new scientific discoveries that rewrite some of the fundamental constants of nature.
But there's a catch. The isotope of thorium they need, thorium-229, can only be found in weapons-grade uranium. As such, it is estimated that only about 40 grams is currently available worldwide for use in nuclear clocks.
Now, though, an international team of researchers led by UCLA physicist Eric Hudson has found a way to use just a fraction of the thorium to achieve the same results as their earlier work with specialized crystals. The method developed by the team, and described in Nature, is so simple and inexpensive that it may pave the way to make nuclear clocks so economical and small that they could someday be in our phones or even wristwatches, in addition to replacing clocks in our power grids, cell phone towers and GPS satellites. They could also be used for navigation in GPS-denied environments, such as deep space or submarines.
A simple process improves what originally took 15 years to figure out
Hudson's group spent 15 years working to realize the specialized thorium-doped fluoride crystals that allowed their breakthrough last year. By causing the electrons of thorium-229 atoms to bond with fluorine in a special arrangement, Hudson's team, in their initial experiments, was able to create crystals that stabilize thorium-229 while remaining transparent to the laser light needed to excite the nucleus. They found that the crystals, however, were difficult to grow and required a lot 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.
In the new work, Hudson's group electroplated a minute amount of thorium onto stainless steel by slightly modifying a method used to electroplate jewelry. Electroplating, which was invented in the early 1800s, sends an electric current through an electrically conductive solution to deposit a thin layer of atoms from one metal onto another. In jewelry, for example, silver or gold is electroplated onto a less precious metal base.
"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.
The key to getting this new system working was the realization that one fundamental assumption was wrong. Stimulating the nucleus enough with a laser, or exciting it, to observe its transition to a higher energy state, was easier than anyone 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!"
Thorium-based nuclear clocks could unlock satellite-free navigation
In addition to their expected impact on everything from communication technology, power grid synchronization and radar networks, next-generation clocks have long been sought as a solution to a problem with significant national security impact: navigating without GPS. If a bad actor — or even an electromagnetic storm — disabled enough satellites, all of our GPS navigation devices would fail. Similarly, submarines that dive deep in the ocean, where satellite signals cannot reach, already use atomic clocks for navigation, but current clocks are not accurate enough and after a few weeks, the submarines must surface to verify their location. In these demanding environments, the nuclear clock, which is better protected from its environment, excels over current atomic clocks.
"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."
And, if Earthlings ever want to travel into space, we need even more improved clocks for the same reason.
"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."
The research was funded by the National Science Foundation and included 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.
