Despite their promise, nuclear clocks face steep challenges. The isotope thorium-229, essential for these clocks, is rare, radioactive, and prohibitively expensive in the required quantities.

In a recent study published in Nature, researchers led by JILA and NIST Fellow Jun Ye, in collaboration with UCLA Professor Eric Hudson's team, introduced a groundbreaking method. They developed thin films of thorium tetrafluoride (ThF4), making nuclear clocks a thousand times less radioactive and significantly reducing costs.

Thin films could revolutionize nuclear clock technology by aligning with scalable technologies like semiconductors and photonic circuits, paving the way for compact and accessible designs. "A key advantage of nuclear clocks is their portability," said Ye. "To fully unleash such an attractive potential, we need to make the systems more compact, less expensive, and more radiation-friendly to users."

Past approaches required substantial quantities of thorium-229, sourced through uranium decay, escalating radiation safety concerns and costs. "Thorium-229 by weight is more expensive than some of the custom proteins I've worked with in the past," said JILA postdoctoral researcher Jake Higgins.

The new thin-film approach utilizes micrograms of thorium-229, drastically reducing radioactive material requirements. Developed through physical vapor deposition (PVD), this method involves vaporizing thorium fluoride in a chamber, which then condenses onto substrates like sapphire and magnesium fluoride. This process creates uniform films roughly 100 nanometers thick. "If we have a substrate very close by, the vaporized thorium fluoride molecules touch the substrate and stick to it," explained graduate student Chuankun Zhang.

"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 explained, emphasizing the improved stability this approach provides.

The team envisions these advancements enabling portable nuclear clocks for telecommunications, navigation, and beyond. "Imagine something you can wear on your wrist," said JILA graduate student Tian Ooi, hinting at a distant future of miniaturized precision timekeeping.

While the journey to wearable nuclear clocks remains long, the researchers believe their work could eventually unveil new physics and revolutionize critical sectors reliant on accurate timekeeping. "If we are lucky, it might even tell us about new physics," added JILA graduate student Jack Doyle.