What if the key to unlocking new physics lies in keeping better time? Inside the quiet journey to build a nuclear clock so precise, it could redefine our understanding of reality.
Image Credit: Bruce Rolff/Shutterstock.com
Researchers are looking for new ways to improve timekeeping because even small gains in stability can help physicists discover subtle physical effects.
The thorium-229 nuclear clock is a newer venture in this effort and brings an interesting angle with its unusually low-energy nuclear transition, supporting precise optical control and strong resistance to environmental noise.
This article discusses how the nuclear clock works, the importance of thorium-229 in its design, and what recent studies indicate about its potential impact.
Get all the details: Grab your PDF here!
What Is a Nuclear Clock- and Why Thorium-229?
A nuclear clock is based on the same principle as an atomic clock, using a very stable, periodic quantum transition as a reference and counting its 'ticks' to maintain time.
The difference is that the atomic clocks use transitions of electrons orbiting nuclei, whereas nuclear clocks rely on transitions within the nucleus itself.
As the nucleus is more compact and tightly bound than the electron cloud, it tends to be less sensitive to external changes such as electromagnetic fields, which makes it a potentially more stable and accurate frequency reference. 1, 2, 3
Image Credit: Ilya Lukichev/Shutterstock.com
However, not all nuclei are fit for the job.
Most nuclear transitions are either very high energy or too broad to serve as a precise clock. This is where thorium-229 comes in, with its metastable excited nuclear state, known as the isomeric state, lying at a very low energy of around 8.4 eV, which puts it within reach of vacuum-ultraviolet (VUV) lasers.
This makes coherent laser control feasible. A thorium-229 nuclear optical clock provides a frequency standard that is more immune to environmental noise than even the most refined atomic clocks. Experts argue that such a clock might reach fractional uncertainties as low as ~10-19. 1, 2, 3
Potential Impact of the Thorium Clock
There are several reasons why physicists care so much about making ever-more-accurate clocks. Precision timekeeping is a foundational tool in metrology. A more stable and accurate clock can detect the finer effects.
A thorium-based nuclear clock could be an extremely sensitive probe for new physics.
For example, some theories beyond the Standard Model predict that fundamental constants like the fine-structure constant, α, might drift over time or vary across the universe. A clock that responds more strongly to these variations can test such theories more accurately.
With its unique characteristics and sensitivity, nuclear clocks could even be used to detect dark matter. Tiny shifts in the resonance frequency, when measured over time, might reveal interactions between dark matter fields and nuclear states. 4, 5
A nuclear clock could also help serve more mundane tasks, enhancing navigation (e.g., GPS), telecommunications, and synchronization in distributed systems. Its resilience to electromagnetic interference makes it an alternative option to atomic clocks in environments where atomic clocks may be disturbed.
Learn more about hyper-precise time keeping with atomic clocks, here.
Recent Advances
Temperature Sensitivity in a Solid-State Clock
A 2025 paper in Physical Review Letters examined how temperature affects the performance of a thorium-229 clock embedded in a crystal. The researchers doped a calcium fluoride (CaF2) crystal with thorium-229 and performed quantum-state-resolved spectroscopy of its nuclear transitions at three temperatures, including 150 K, 229 K, and 293 K. 2
They measured four of the strongest transitions and found that as temperature rises, the frequency shifts, specifically, the unsplit frequency and the electric quadrupole splittings.
Physically, this corresponds to a decrease in electron density and changes in the electric field gradient at the nucleus.
For one particular line (a transition between m = ±5/2 and ±3/2), the shift in this range was only about 62 kHz, or roughly 0.4 kHz per kelvin. 2
To reach a fractional precision on the order of 10-18, the authors estimate that the crystal's temperature would need to be stabilized to within 5 μK. That is a strict but in principle achievable requirement. The fact that one of the transitions shows such small temperature sensitivity makes it a good potential for a highly stable, solid-state nuclear clock. 2
Sensitivity to Fundamental Constants
Another development came from a paper in Nature Communications, which looked at how the thorium-229 clock transition responds to variations in the fine-structure constant, α.
Using high-precision laser spectroscopy data at the 10-12 level, the researchers extracted nuclear parameters and plugged them into a geometric nuclear model to estimate how much the clock frequency would shift if α changed. 1
Their results showed a sensitivity factor K of approximately 5,900 ± 2,300. This indicates that a tiny fractional change in α would produce a considerably amplified fractional change in the clock frequency.
This is orders of magnitude larger than the sensitivity of typical atomic clocks. The enhanced sensitivity makes the thorium-229 system a powerful probe of possible new physics.
The researchers also went deeper to examine how nuclear deformation, including quadrupole and octupole moments, contributes to this sensitivity. This indicated that previous assumptions, taken as fact, such as a constant nuclear volume during excitation, may need to be revised. 1
Looking Ahead for Nuclear Timekeeping
These recent studies show how thorium-229 could provide a practical path toward ultra-precise clocks that combine strong environmental resilience with useful sensitivity to new physical effects.
Its low-energy nuclear transition, small temperature shifts, and amplified response to changes in fundamental constants give researchers a clearer view of what a next-generation reference could accomplish.
As experimental techniques improve, the thorium-229 system will help physicists to further timekeeping research and broaden the scientific uses of precision clocks.
References
- Beeks, K. et al. Fine-structure constant sensitivity of the Th-229 nuclear clock transition; 2024. arXiv preprint arXiv. https://www.nature.com/articles/s41467-025-64191-7
- Higgins, J. S. et al. (2025). Temperature sensitivity of a thorium-229 solid-state nuclear clock. Physical Review Letters. https://doi.org/10.1103/PhysRevLett.134.113801
- Tong, X. Hua, L., Hua, X., & Liu, X. (2025). The ticking of thorium nuclear optical clocks: a developmental perspective. National Science Review. https://academic.oup.com/nsr/article/12/8/nwaf083/8051347
- Fuchs, E. et al. (2025). Searching for Dark Matter with the Th 229 Nuclear Lineshape from Laser Spectroscopy. Physical Review X. https://doi.org/10.1103/PhysRevX.15.021055
- Weizmann Institute of Science. The dark side of time: Scientists develop nuclear clock method to detect dark matter using thorium-229. Phys.org. https://phys.org/news/2025-07-dark-side-scientists-nuclear-clock.html
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.