Imagine a clock so stable that it could keep time for the age of the universe without losing a second. That’s what JILA researchers are pursuing with thorium-229 nuclear clocks. Their latest experiments are making that dream more achievable.
Atomic clocks usually rely on electronic transitions inside atoms. But in solid materials like crystals, these transitions get disturbed. That’s why the most accurate atomic clocks must be kept in special, isolated setups, like ion traps or optical lattices, which are complex and hard to build.
But thorium-229 offers something radical: a nuclear transition that’s far less sensitive to environmental noise. In plain terms, it’s like swapping a violin string (easily disturbed) for a steel beam, much harder to shake out of tune.
And here’s the kicker: because nuclear transitions are so robust, researchers can embed orders of magnitude more emitters inside a solid crystal host. That means smaller, sturdier clocks that don’t need the elaborate setups of current atomic systems.
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A nuclear transition such as that in thorium-229 will be much less disturbed in the solid state, and its clock transition will be mostly preserved even in a crystal environment. The solid-state environment also makes it easy to achieve very high nuclear density and a large number of nuclei, which is helpful for clock performance.
The team focused on thorium-229 ions doped into calcium fluoride (CaF₂) crystals. They tested how the “beat” of this nuclear transition changes with doping concentration (how many thorium atoms are inside), temperature, and time (long-term stability).
The Th-doped CaF2 crystals were grown after many years of effort by the collaborators in Thorsten Schumm’s group at TU Wien. Importantly, CaF2 is transparent at 148 nm, which is required to detect radiative decay of the nuclear transition (via photons).
What they found is fascinating: the linewidth, the sharpness of the tick, depends on the crystal’s intrinsic properties. And at a sweet spot of 196 K (about -77 °C), the clock’s sensitivity to temperature shifts essentially vanishes. That’s like finding the perfect tuning fork pitch where the environment can’t throw it off.
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The linewidth is currently limited by imperfections of the host crystal, which shift the nuclear transition frequency. However, each imperfection affects different nuclei differently (some may be closer or farther away, for example), so each nucleus feels a different frequency shift. This leads to ‘inhomogeneous broadening’ of the nuclear transition linewidth.
At around 195 K, two differently doped crystals maintained a reproducible nuclear transition frequency within 220 Hz over 7 months. This is certainly an ideal operating temperature for a nuclear clock, as temperature-dependent frequency shifts can limit its performance, especially when temperature cannot be precisely controlled.
The fact that this ‘magic temperature’ exists in the least temperature-sensitive line and is reasonably easy to achieve is quite fortunate and very important for realizing a nuclear clock with Th: CaF2.
University of Colorado Boulder Physics professor Jun Ye noted, in an email, “220 Hz at our frequency of 2020.407 THz corresponds to a 1e-13 fractional frequency reproducibility. This would correspond to a clock losing 1 second in 10^13 seconds, or 1 second in ~300,000 years (about when humans first existed!).
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After the initial papers demonstrating high-precision spectroscopy of this nuclear transition and its temperature dependence, the next step is to characterize its reproducibility and examine its dependence on the environment (temperature, Th229 doping concentration) in more detail.
To act as a clock, this nuclear transition frequency must be stable and robust, so that the clock’s ticks are consistent. As this is a new field, there was a natural question of how the crystal environment and changes in it might disturb the clock transition.
Tian Ooi, a graduate student at JILA and the paper’s first author, wrote in an email to Tech Explorist, “It was important to us to study these effects systematically, which also helps us learn more about the physical mechanisms affecting the clock transition in the crystal.”
Unlike today’s atomic clocks, which rely on complicated setups with lasers and vacuum chambers, solid-state nuclear clocks could be much simpler. They would be compact, durable, and portable.
And because they’re so stable, scientists could use them to test some of the deepest questions in physics, like whether the fundamental constants of nature (for example, the fine-structure constant) really stay fixed, or if they slowly drift over billions of years. A clock this precise could redefine how we measure time, frequency, and even space-time itself.
The linewidth of about 20 kHz is currently limited by the crystal inhomogeneity in Th: CaF2. In the future, researchers are excited to explore other kinds of thorium-containing crystals that will not be limited by crystal imperfections and may have ~kHz or narrower linewidths.
Ooi noted, “We are also limited by our 148 nm frequency comb power, which currently is about 1 nW per comb mode. To coherently drive the nuclear transition and, for example, observe Rabi oscillations, we desire laser powers that are several orders of magnitude higher in the 1uW-1mW regime. For the driving laser, it’s important to have both high power and high coherence (narrow linewidth).”
“I would say this result represents a big step for nuclear clocks as the field transitions from focusing on direct laser excitation of the transition to focusing on evaluating its usability as a real clock. This includes measuring its frequency reproducibility over time, measuring the effect of different environmental conditions such as temperature or doping concentration, and gaining a physical understanding of how these nuclear clocks ‘tick’, he added.
Journal Reference:
Ooi, T., Doyle, J.F., Zhang, C. et al. Frequency reproducibility of solid-state thorium-229 nuclear clocks. Nature 650, 72–78 (2026). DOI: 10.1038/s41586-025-09999-5