In the quest for ultra-precise timekeeping, scientists have turned to nuclear clocks. Unlike optical atomic clocks—which rely on electronic transitions—nuclear clocks utilize the energy transitions in the atom's nucleus, which are less affected by outside forces, meaning this type of clock could potentially keep time more accurately than any previously existing technology.
However, building such a clock has posed major challenges—thorium-229, one of the isotopes used in nuclear clocks, is rare, radioactive, and extremely costly to acquire in the substantial quantities required for this purpose.
Reported recently in a new study published in Nature, a team of researchers, led by JILA and NIST Fellow and University of Colorado Boulder Physics professor Jun Ye, in collaboration with Professor Eric Hudson's team at UCLA's Department of Physics and Astronomy, have found a way to make nuclear clocks a thousand times less radioactive and more cost-effective, thanks to a method creating thin films of thorium tetrafluoride (ThF4).
The successful use of thin films marks a potential turning point in the development of nuclear clocks. Using thin-film technology in nuclear clocks is commensurate with semiconductors and photonic integrated circuits, suggesting that future nuclear clocks could be more accessible and scalable.
"A key advantage of nuclear clocks is their portability, and to fully unleash such an attractive potential, we need to make the systems more compact, less expensive, and more radiation-friendly to users," said Ye.
The Costs of Nuclear Clockmaking
JILA has been at the forefront of atomic and optical clock research for decades, with Ye's laboratory making pioneering contributions advancing the concept, design, and implementation of optical lattice clocks, which set new standards in precision timekeeping.
Physicists have been trying to observe the energy transition of thorium-229 for nearly 50 years. In September 2024, researchers in Ye's laboratory reported the first high-resolution spectrum of the nuclear transition and determined the absolute frequency based on the JILA Sr optical lattice clock. Their result was published as a cover article in Nature .
To build their nuclear clock setup, the team worked with radioactive thorium-229 crystals, collaborating with researchers at the University of Vienna.
"The growth of that crystal is an art in itself, and our collaborators in Vienna spent many years of effort to grow a nice single crystal for this measurement," explains Chuankun Zhang, a graduate student at JILA and first author of both Nature studies.
Previous approaches using thorium-doped crystals required more radioactive material. As thorium-229 is often sourced from uranium via nuclear decay, this leads to additional radiation safety and cost considerations.
"Thorium-229 by weight is more expensive than some of the custom proteins I've worked with in the past," adds JILA postdoctoral researcher Jake Higgins, also involved in this project, "so we had to make this work with as little material as possible." The researchers collaborated closely with CU Boulder's Environmental Health & Safety department to safely build and study their nuclear clock.
As the team worked to observe the nuclear transitions in thorium-doped crystals, they simultaneously pursued methods to make the clock safer and more cost-effective by developing thin film coatings to reduce the amount of radioactive thorium needed.
Vaporizing Thorium
To produce the thin films, the researchers used a process called physical vapor deposition (PVD), which involved heating thorium fluoride in a chamber until it vaporized. The vaporized atoms then condensed on a substrate, forming a thin, even layer of thorium fluoride about 100 nanometers thick. The researchers selected sapphire and magnesium fluoride as substrates because of their transparency to the ultraviolet light used to excite the nuclear transition.
"If we have a substrate very close by, the vaporized thorium fluoride molecules touch the substrate and stick to it, so you get a nice, even thin film," Zhang says.
This method used just micrograms of thorium-229, making the product a thousand times less radioactive while producing a dense layer of active thorium nuclei. Working with the JILA Keck Metrology laboratory and JILA instrument maker Kim Hagen, the researchers reliably reproduced films that could be tested for potential nuclear transitions using a laser.
Finding Energy Transitions in Thin Film
However, the team faced a new challenge. Unlike in a crystal, where every thorium atom was situated in an ordered environment, the thin films produced variations in thorium environments, shifting their energy transitions and making them less consistent.
JILA graduate student Jack Doyle, who was also involved in this study, elaborates, "Wolfgang Pauli was rumored to have said that 'God invented the bulk and the surface is of the devil,' but he might as well have said this because the number of factors that are hard to learn about for a particular surface is immense."
After preparing the films, JILA researchers sent them to Professor Eric Hudson at UCLA, who used a high-power laser with a much greater spectral width to test the nuclear transitions. This broad-spectrum laser has all of its optical power concentrated in one spectral location instead of a frequency comb that has regularly spaced spectral lines over a larger spectral distance. This allowed the UCLA team to excite the thorium nuclei effectively, even though the observed linewidth is broader than previously seen in the previous study. When the laser's energy precisely matched the energy required for the transition, the nuclei emitted photons as they relaxed back to their original state. By detecting these emitted photons, the researchers could confirm successful nuclear excitations, verifying the thin film's potential to serve as a frequency reference for nuclear clocks.
"We made the thin film, we characterized it, and it looked pretty good," explains JILA graduate student Tian Ooi, who was also involved in this research. "It was cool to see that the nuclear decay signal was actually there."
A New Chapter of Precision Timekeeping
Based on their findings, the researchers are excited about the improvements in precision timekeeping to be gained by using thin films in nuclear clocks.
"The general advantage of using clocks in a solid state, as opposed to in a trapped-ion setting, is that the number of atoms is much, much larger," Higgins elaborates. "There are orders and orders of magnitude more atoms than one could feasibly have in an ion trap, which helps with your clock stability."
These thin films could additionally allow nuclear timekeeping to move beyond laboratory settings by making them compact and portable.
"Imagine something you can wear on your wrist," Ooi says. "You can imagine being able to miniaturize everything to that level in the far, far future."
While this level of portability is still a distant goal, it could revolutionize sectors that rely on precise timekeeping, from telecommunications to navigation.
"If we are lucky, it might even tell us about new physics," Doyle adds.
This work was supported by the Army Research Office, the Air Force Office of Scientific Research, the National Science Foundation, and the National Institute of Standards and Technology (NIST).
