Denver, CO | Physicists get closer to creation a much sought-after “nuclear clock”. This device would tell the time by measuring energy transitions in the nuclei of atoms and could become the most precise clock on the planet.
Decades ago, scientists predicted that the isotope thorium-229 could be used in such a clock, but they failed to pin down its unusual nuclear energy transition. This feat, made with a laser in 2024started the countdown to a nuclear clock.
Today, such a clock is “much closer than we think,” says Eric Hudson, a physicist at the University of California, Los Angeles, who is working on such a clock. “You’ll see nuclear clock measurements in 2026, I’m sure.”
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Nearly a dozen research teams, spread across China, Europe, Japan and the United States, are preparing to assemble the components of such a clock, including a source of 229Th – which is radioactive – and a powerful continuous-wave ultraviolet laser to excite the energy transition. At the American Physical Society (APS) World Physics Summit in Denver, Colorado, this week, researchers provided updates on their progress, including details on laser development.
Claire Cramer, executive director of quantum sciences at the University of California, Berkeley, who was in attendance, expressed optimism about the potential of solid-state nuclear clocks: “This is a really, really promising technology for commercial applications. »
Indeed, nuclear clocks could be noise resistant and have a compact design for use outside the laboratory. They could also surpass the precision of optical atomic clocks, the best current timekeepers in the field, which only lose one second every 40 billion years.
Laser jockey
Timekeeping, whether in a pocket watch or in a physics laboratory, comes down to counting rapid, regular events – the “ticks” of any clock. In optical atomic clocks, these events are the jump of electrons in an atom between a ground energy state and an excited energy state. A laser with a wavelength between 350 and 750 nanometers (the visible or optical part of the electromagnetic spectrum) excites this transition, which can “happen” billions of times per second.
In contrast, a nuclear clock would count the transitions between nuclear states of 229Th. These have the same number of protons and neutrons, but different energies depending on how the particles are compressed in the nucleus.
For half a century, the precise energy of 229The transition remained uncertain. Several independent research groups began moving closer to an answer a few years ago. The research culminated in an experiment conducted in 2024 by Chuankun Zhang, a physicist now at the California Institute of Technology in Pasadena, and Jun Ye, a physicist at the JILA Research Institute in Boulder, Colorado. Using a frequency comb – a laser with around 30 million frequencies that can hit a crystal simultaneously – Zhang, Ye and their colleagues identified the transition with ultra-high precision. However, to access it in a working nuclear clock, scientists now need a powerful and stable continuous-wave laser with an ultraviolet wavelength of around 148 nanometers. And no such laser has been manufactured.
A group based at Tsinghua University in Beijing, China, has made some of the most promising progress toward building such a system. Last month, the team reported in Nature that it had delivered 100 nanowatts of power at 148.4 nm. Although researchers welcomed the advance, some attendees at the APS meeting expressed hesitation about the laser’s long-term prospects because it requires heating toxic cadmium vapors to 550ºC.
Another approach converts the wavelength of an optical laser to 148 nm with a specialized crystal. Ye said preliminary tests with one particular crystal provided a nearly stable 40 microwatts of power. He did not reveal the identity of the material, instead saying it was “extremely promising.” But his group is collaborating with IPG Photonics, a laser maker based in Marlborough, Mass., which filed a patent for a method of growing specialized strontium tetraborate crystals.
The community has yet to find a solution, Hudson said. “But my opinion is that this is a technical problem that no one needed to solve before, and now we are going to solve it.”
In search of stability
The other component of a nuclear clock that researchers are looking for is a stable source of 229Th. Two general solutions emerged: using billions of 229Th ions in a solid crystal, or just a handful in an ion trap.
The crystal approach offers a much stronger clock signal due to the large number of 229Th ions are used, but they are limited by stability. A stable nuclear clock requires a narrow linewidth for the nuclear transition, that is, its signal must have a narrow frequency range. Using a calcium fluoride crystal infused with 229Ye’s group has so far obtained a signal with a linewidth of around 30 kilohertz – too large for a stable clock.
It is not yet clear what causes this large line width, but researchers at the meeting suspect the presence of impurities in the calcium fluoride. Some are exploring other types of crystals, even thin crystalline films, which are easier to make and contain fewer impurities. Hudson is particularly optimistic about thorium tetrafluoride – a once-popular radioactive coating for camera lenses – and thorium oxide.
Despite this, the use of crystals as a source of 229This might not provide enough precision for a nuclear clock, as they naturally widen the line width of the clock signal. This is why researchers are looking for ion traps, in which the ions of 229They are cooled and suspended at ultra-low temperatures, down to microkelvin. “If you want to be really precise, you’ll do an experiment with a trapped ion,” says Ye. So far, no one has achieved this with 229But researchers at the meeting said it was only a matter of time.
This article is reproduced with permission and has been published for the first time March 20, 2026.