Physicists at UCLA have devised a straightforward method to create ultra-precise nuclear clocks using minimal amounts of rare thorium, borrowing a technique from jewelry making. By electroplating thorium onto steel, the team achieved results comparable to years of complex crystal fabrication but with 1,000 times less material. This advance could enable reliable timekeeping in GPS-denied environments like deep space and submarines.
Last year, a UCLA-led team marked a half-century pursuit by successfully controlling the absorption and release of photons by radioactive thorium-229 nuclei, a milestone first proposed in 2008. This breakthrough paves the way for nuclear clocks far more accurate than atomic ones, potentially revolutionizing navigation, communications, and tests of fundamental physics constants.
However, thorium-229's scarcity—limited to about 40 grams worldwide from weapons-grade uranium—posed a major hurdle. Traditional experiments relied on thorium-doped fluoride crystals, which took 15 years to develop and required at least 1 milligram of thorium per batch. "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 UCLA postdoctoral researcher Ricky Elwell, the first author on the prior work.
In a new study published in Nature, Eric Hudson's international team overcame this by electroplating a thin thorium layer onto stainless steel, a 19th-century technique used to coat metals like gold onto base materials. This approach uses just a thousandth of the thorium and yields a durable product. "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," Hudson explained.
The key insight challenged a core assumption: thorium nuclei can be excited in opaque materials, with emissions detected as electrons via electrical current rather than photons through transparency. "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... In this work, we showed that is simply not true," Hudson noted.
Such clocks could enhance power grids, cell networks, and GPS satellites, while providing GPS-independent navigation for submarines—where current atomic clocks drift, requiring surfacing—and deep space missions. "Thorium nuclear clocks could also revolutionize fundamental physics measurements... and may also be useful in setting up a solar-system-wide time scale," said Eric Burt of NASA's Jet Propulsion Laboratory. Makan Mohageg of Boeing added that the method could reduce costs for compact, stable timekeeping in aerospace.
The research, funded by the National Science Foundation, involved collaborators from the University of Manchester, University of Nevada Reno, Los Alamos National Laboratory, and European institutions.