Denver, Colorado | Scientists are advancing toward the creation of a much-anticipated ‘nuclear clock’, a device that would mark time through energy shifts within atomic nuclei, potentially becoming the world’s most accurate timepiece.
Years ago, it was speculated that the thorium-229 isotope could serve this purpose, yet its unique nuclear energy transition was elusive. This breakthrough, achieved with laser technology in 2024, initiated the path toward a nuclear clock.
Eric Hudson, a physicist at the University of California, Los Angeles, believes the clock is “much closer than people think” to realization. “Nuclear-clock measurements will be seen in 2026, I’m sure,” he asserts.
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Research teams across China, Europe, Japan, and the United States are nearing the assembly of this clock’s components, including a source of the radioactive 229Th and a robust continuous-wave ultraviolet laser to stimulate the energy shift. At the American Physical Society (APS) Global Physics Summit in Denver, Colorado, this week, researchers shared updates on their progress, particularly regarding laser development.
Claire Cramer, executive director of quantum science at the University of California, Berkeley, attending the event, expressed optimism about the potential of solid-state nuclear clocks, describing them as “a really, really promising technology for commercial applications.”
Nuclear clocks could offer resilience to noise and a compact form suitable for practical use beyond laboratory settings. They may even exceed the precision of optical atomic clocks, which are currently the most accurate, losing just one second every 40 billion years.
Laser jockeying
Timekeeping, whether in a pocket watch or a laboratory, involves counting rapid, regular events—essentially the ‘ticks’ of a clock. In optical atomic clocks, these ‘ticks’ are the movements of electrons between energy states, excited by a laser in the 350- to 750-nanometre range of the electromagnetic spectrum, which allows them to ‘tick’ trillions of times per second.
In contrast, a nuclear clock would measure transitions between nuclear states of 229Th, which possess the same number of protons and neutrons but vary in energy based on nuclear particle arrangements.
For decades, the exact energy transition of 229Th was uncertain. A few years ago, independent research groups began to narrow down the possibilities, with a 2024 experiment led by Chuankun Zhang, now at the California Institute of Technology, and Jun Ye at JILA in Boulder, Colorado, using a frequency comb laser to pinpoint the transition with remarkable precision. To implement this in a working nuclear clock, scientists now require a powerful, stable continuous-wave laser with an ultraviolet wavelength of approximately 148 nanometres, which has yet to be developed.
A team at Tsinghua University in Beijing, China, has made notable progress on this front. Recently, they announced in Nature that they had achieved 100 nanowatts of power at 148.4 nm. Despite praise for this advancement, some APS meeting attendees voiced concerns about the laser’s viability, given that it necessitates heating toxic cadmium vapor to 550 ºC.
Another method involves converting an optical laser’s wavelength to 148 nm using a specialized crystal. Ye mentioned that tests with a certain crystal yielded nearly stable 40 microwatts of power. Although he withheld the material’s identity, he described it as “tremendously promising.” Ye’s group works with IPG Photonics, a laser manufacturer in Marlborough, Massachusetts, which has patented a technique for cultivating specialized strontium tetraborate crystals.
The solution remains elusive, Hudson noted. “But my opinion is, this is a technical problem that no one needed to solve before, and now we will solve it,” he added.
Searching for stability
The quest for a stable source of 229Th continues, with researchers exploring two main approaches: using trillions of 229Th ions in a solid crystal or a few ions in an ion trap.
The crystal method offers a stronger clock signal due to the large number of ions, but stability remains a challenge. A stable nuclear clock requires a narrow signal linewidth for the nuclear transition. Ye’s team has achieved a signal with a linewidth of about 30 kilohertz using a calcium fluoride crystal infused with 229Th ions, which is too wide for a stable clock.
The cause of this broad linewidth remains unknown, but some researchers suspect impurities in the calcium fluoride. Others are investigating alternative crystals and thin crystalline films, which are easier to produce and contain fewer impurities. Hudson sees potential in thorium tetrafluoride and thorium oxide, materials once popular for camera lenses.
Despite this, crystals may not provide the necessary accuracy for a nuclear clock, as they inherently broaden the clock signal’s linewidth. This is why researchers are also focusing on ion traps, where 229Th ions are cooled and suspended at ultra-low temperatures, down to microkelvin. Ye explains, “If you want to be really accurate, then you will do a trapped ion” experiment. Although no one has yet accomplished this with 229Th, researchers believe it is only a matter of time.
This article is reproduced with permission and was first published on March 20, 2026.
