To be useful, clocks need to be consistent. Imagine two spies who synchronize their watches; they rely on them agreeing days or months later, even if one of them must take a frigid hike through arctic tundra. In many experiments, scientists similarly require that their clock is accurate to a tiny sliver of a second and that it will work the same as their colleague’s clock on the other side of the world.
Currently, when keeping time really counts, scientists and engineers turn to atomic clocks. Atomic clocks use the physics that governs the interactions between electrons and light. They can be so accurate that they could run for tens of billions of years without getting off by a second. These clocks have been used for research, such as experiments studying quantum many-body physics and relativity, and have enabled technologies, including GPS. But scientists are not satisfied. Researchers are exploring the potential of nuclear clocks to use the same principles to deliver even more precise results or to fit into an even smaller device.
JILA has been a leader in atomic clock and nuclear clock research, and in 2024 a team of researchers, led by JILA and National Institute of Standards and Technology Fellow and University of Colorado Boulder Physics professor Jun Ye, reported crucial research where they measured the first high-resolution spectrum of the nuclear transition of thorium and determined the absolute frequency of the transition. Ye and other scientists hope these transitions of thorium nuclei will be the ticking hearts of future nuclear clocks. However, there is still a lot for scientists to learn before nuclear clocks have a chance at becoming the gold standard for precision time keeping. For instance, researchers need to understand how nuclear transitions respond to things like changes in temperature, make sure that nuclear clocks can be made with a shared reproducible frequency and determine if they remain reliable over extended periods of time.
In new experiments, Ye and his colleagues have looked at crystals containing thorium to better understand how they might be used in nuclear clocks, including testing three crystal samples many times over the course of a year to check if their properties unexpectedly fluctuated over that time. In an article published in the journal Nature on January 28, 2026, they described the stability of three crystals observed over the course of multiple months, how the crystals responded to temperature changes, and how the different concentrations of thorium in each crystal affected their properties. The results revealed that the crystals have a promising stability and reproducibility and provided insights into future experiments and how similar crystals might be incorporated into high quality clocks.
“Checking frequency reproducibility, both between different host crystals and over an extended period of time, is the first step towards a systematic evaluation of the performance of the nuclear clock,” says Ye.
The group studied three crystals fabricated by Thorsten Schumm’s lab at the Technical University of Vienna. Each crystal was made of calcium fluoride but with some of the calcium atoms replaced with thorium atoms. The crystals each contained different concentrations of thorium. When the thorium atoms are in their lowest energy quantum state, Ye’s group can observe how they interact with particular frequencies of light to make their nucleus jump to higher energy states. They found that there are five transitions that they can trigger with slightly different frequencies of light. The frequencies of these transitions are critical to using thorium in a nuclear clock.
“It’s critical that Thorsten’s lab has provided three different Thorium-doped crystals, which allowed us to study the line width broadening mechanisms and the level of line center reproducibility,” says Ooi.
These interactions and frequencies follow essentially the same physics as the transitions of atoms used in atomic clocks. However, the states of the nucleus are less sensitive to fluctuations of the electric and magnetic fields around them than the states of atoms. Additionally, the nuclear states can be used even when the atoms are embedded in a crystal, unlike the states used for atomic clocks; this difference allows a nuclear clock using a crystal to have a clearer signal by using many more of the relevant atoms while perhaps also being packaged in a smaller device.
Ye’s lab previously studied how one of these crystals behaved at three different temperatures. In the new article, they continued to look at that crystal along with two others with lower concentrations of thorium.
The researchers observed that over the course of the year the properties of the first crystal were stable. The two additional crystals demonstrated the same frequency as the first and also delivered reproducible results when repeated measurements were made months apart. The fluctuations the team observed were stable to around a tenth of a trillionth of the frequency of the measured transition and are limited by the experiment’s measurement precision. These results are promising for researchers to be able to use such crystals to fabricate reliable clocks.
“We are able to show that even over the span of almost a year, we can measure the nuclear transition frequency in these crystals over and over again, and they're very consistent,” says Tian Ooi, a graduate student at JILA and first author of the paper.
The team did find some variations in the crystals’ performances based on the concentration of thorium. While the thorium all interacted with light of the same wavelength, how precisely they responded to the specific frequency varied. The state’s transition will sometimes respond to nearby frequencies and the group defines this extended range of interaction frequencies as the “line width” of the transition.
The group found that the line widths were considerably wider than theoretical calculations had predicted and that they depended on the thorium concentration with greater amounts of thorium producing broader line widths. The researchers propose that the broadening of the width may be caused by the substitution of thorium creating a subtle microstrain in the crystal’s structure that influences the nuclear transitions by making the electric field vary unevenly inside the material.
“This was an unexpected surprise,” says Ooi. “People didn’t anticipate how large this microstrain effect would be.”
Further research is needed to explain the effect and determine if it can be eliminated. Minimizing the line width is a critical factor in designing a high-performance nuclear clock, but high concentrations will also help researchers get a clear signal. So, researchers need to understand this relationship and, if possible, produce crystals with narrower line widths.
The group also continued their research into how the nuclear transition of thorium varied with temperature. They took measurements at more temperatures than they previously had, and for all three crystals, they looked at both the transition that varied the most and the transition that varied least with changes in temperature. The researchers found that the frequencies of the crystals were consistent with each other and identified the point where the material’s changes in response to temperature shift from decreasing the frequency to increasing it, which is where the impact of any temperature fluctuation is smallest. This temperature will likely be the most practical temperature to keep the crystal at when operating a nuclear clock.
The experiments also let the team map out the response of the transition that varies the most with temperature. Based on the results, the researchers suggest that in the future nuclear clocks can monitor that more sensitive frequency to record the temperature so that fluctuations to the least sensitive transition can be rapidly corrected.
Now that the group has these insights, they plan to continue studying these crystals, investigate why the line widths vary between crystals and chart a path to a future with nuclear clocks as a valuable timekeeping tool.
“I think what this paper shows is that we're moving from measuring the clock transition to really investigating how good this clock can be,” Ooi says. “There’s still interesting things to figure out, but this is one of the big steps that we have to take to show that solid-state nuclear clocks are viable.”
The authors acknowledge funding support from National Science Foundation QLCI OMA-2016244, DOE quantum center of Quantum System Accelerator, Army Research Office (W911NF2010182), Air Force Office of Scientific Research (FA9550-19-1-0148), National Science Foundation PHY-2317149, and National Institute of Standards and Technology. Part of this work has been funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 856415) and the Austrian Science Fund (FWF) [Grant DOI: 10.55776/F1004, 10.55776/J4834, 10.55776/ PIN9526523]. The project 23FUN03 HIOC [Grant DOI: 10.13039/100019599] has received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Program and by the Participating States. We thank the National Isotope Development Center of DoE and Oak Ridge National Laboratory for providing the Th-229 used in this work.