The Clock That Ticks With a Nucleus

There is a mystery inside every atom: what does the nucleus actually do? For most purposes in everyday physics and chemistry, the answer is “not much.” The nucleus sits in the middle, holds the atom together, and lets the electrons do all the interesting work. Atomic clocks exploit this electron business with extraordinary precision — modern optical clocks lose or gain less than one second every 30 billion years, longer than the age of the universe.

But the nucleus? It’s been an afterthought.

A small group of physicists spent nearly fifty years trying to change that. They believed a single isotope — thorium-229 — hid a nuclear secret that could lead to a clock so precise it might actually see the fabric of spacetime shimmer. In 2024, they finally proved they were right.

The Accidental Discovery of a Nuclear Outlier

The story begins with radioactive bookkeeping. In 1976, physicists L.A. Kroger and C.W. Reich were studying the alpha decay of uranium-233, which produces thorium-229 as a daughter nucleus. When a nucleus decays, it often ends up in an excited state and quickly sheds the extra energy as a gamma ray. By carefully measuring those gamma rays, Kroger and Reich noticed something strange: the energy gap between thorium-229’s ground state and its lowest excited state was tiny — almost suspiciously small. Less than 100 electron volts. Maybe much less.

For context, most nuclear transitions require thousands or millions of electron volts. Nuclear physics is a realm of enormous energies, which is why it stays safely tucked away from everyday chemistry. But here was a nucleus with an excited state hovering just barely above the ground state, like a ball balanced at the very lip of a bowl.

For most of the following decades, the exact energy remained stubbornly uncertain. A 1994 measurement put it at 3.5 electron volts — in the optical range, potentially reachable by existing lasers. Experimenters rushed to find it. They failed, because the number was wrong. Later work pushed the estimate up to around 7.6–8.3 eV, squarely in the vacuum ultraviolet, a technically punishing region of the spectrum that ordinary glass, lenses, and even air all absorb. The experiments had to be redesigned from scratch.

Why Anyone Would Bother

In 2003, German physicists Ekkehard Peik and Christian Tamm published a paper in Europhysics Letters that changed the community’s motivation from curiosity to urgency. They calculated that this strange thorium transition, if you could ever get a laser onto it, would make a better clock than any atomic clock.

Here’s the key idea. All atomic clocks work by locking a laser to a transition frequency — when the laser is precisely on resonance, you have your standard. But every atomic clock is vulnerable to its environment. Electric fields, magnetic fields, thermal radiation: all of these slightly perturb the electron energy levels, shifting the resonance frequency and introducing error. You spend enormous effort shielding and correcting for these effects.

The nucleus is different. It’s roughly 100,000 times smaller than the electron cloud around it. Its charge and magnetic moment are correspondingly tiny. External fields that move electron energy levels barely touch the nucleus at all. A nuclear clock, in principle, would be dramatically less vulnerable to environmental noise.

Theoretical analyses since Peik and Tamm have confirmed this intuition. Recent calculations suggest enhancement factors — how much more sensitive the thorium nuclear transition is to possible new physics compared to ordinary atomic clocks — on the order of 10,000. A nuclear clock running alongside an atomic clock could detect variations in the fine-structure constant (a measure of the strength of electromagnetism) that are otherwise completely inaccessible.

A single experimental question stood between this promise and reality: could anyone actually drive the transition with a laser?

Twenty Years of Near-Misses

The hunt for the thorium isomer became a multi-decade saga of technical ingenuity and persistent frustration. The nucleus had to be ionized (neutral thorium decays by ejecting an electron from its own shell, a process called internal conversion, destroying the signal in microseconds). Getting VUV light in a controlled, tunable, narrow-bandwidth beam is genuinely hard — most VUV optics simply don’t exist, and the experiments had to run in vacuum to prevent absorption by air.

A breakthrough came in 2016, when Lars von der Wense and colleagues at Ludwig Maximilian University Munich directly detected the internal conversion electrons from the isomer decay for the first time, confirming the excited state was real and measuring its half-life. The nucleus existed. But seeing it decay is not the same as controlling it.

In 2023, Sandro Kraemer and collaborators reported observing the actual photons emitted when the isomer decayed, pinning the energy to 8.338 ± 0.024 eV (Nature 617:706, 2023). At last, experimenters had a precise enough target to aim at.

Two Shots, Both Hit

Then 2024 happened.

In April, a team led by Ekkehard Peik at Germany’s Physikalisch-Technische Bundesanstalt published in Physical Review Letters the first demonstration of direct laser excitation of the thorium-229 nucleus (Tiedau et al., PRL 132:182501, 2024). They doped thorium-229 into crystals of calcium fluoride — a material with a bandgap large enough to allow the VUV photons through — and shone a tunable tabletop laser at 148.38 nm onto them. A resonance fluorescence signal appeared. The nuclear transition lit up. As a control, the same experiment with thorium-232 (which doesn’t have the isomer) showed nothing. The nuclear resonance frequency was measured at 2,020.409 THz (wavelength 148.3821 nm), with an isomer half-life of 1,740 ± 50 seconds in vacuum.

This was already a landmark. But the work that made the physics community truly sit up came five months later.

In September, a team from JILA — the joint institute between the University of Colorado Boulder and NIST, home to some of the world’s best optical atomic clocks — published in Nature a result that moved from “we found the transition” to “we measured it against a real atomic clock” (Zhang et al., Nature 633:63–70, 2024). Led by Chuankun Zhang and Jun Ye, they built a vacuum ultraviolet frequency comb: a femtosecond laser system whose output spans a comb of sharp, evenly-spaced frequencies, coherently upconverted to the VUV range. They locked this comb to JILA’s strontium-87 optical atomic clock — one of the most accurate clocks on Earth — and used it to directly probe the thorium transition in a calcium fluoride crystal.

The result was the first-ever frequency ratio measurement between a nuclear transition and an atomic clock. The thorium nuclear frequency: 2,020,407,384,335 ± 2 kHz. The relative uncertainty of 10⁻¹² is not yet at the level of the best atomic clocks, but the measurement directly linked the nuclear world to the atomic world for the first time. The team also resolved the different nuclear quadrupole sublevels of the excited state and measured the ratio of the ground and excited state nuclear quadrupole moments, yielding new information about the thorium-229 nucleus itself.

Both papers together collected more than 280 citations within months of publication.

What a Nuclear Clock Would Actually Tell Us

Let’s step back and appreciate what makes this significant beyond the technical achievement.

Better GPS. Current GPS accuracy is already limited partly by relativistic effects requiring atomic clock precision. A 10× improvement in clock accuracy translates to better position fixes, better synchronization of telecommunications networks, and better resolution in geodesy — the science of measuring Earth’s shape.

Detecting dark matter. One theoretical class of dark matter — called topological dark matter — would pass through our solar system as extended field configurations that would shift atomic and nuclear energy levels as they went by. A nuclear clock would be sensitive to these shifts in ways atomic clocks aren’t, effectively turning a precision timekeeper into a dark matter detector.

Watching fundamental constants breathe. The fine-structure constant α describes the strength of the electromagnetic force. Every theory of quantum gravity and string theory has things to say about whether α might drift or fluctuate over cosmological time. So far, it hasn’t shifted by more than one part in 10¹⁷ per year. A nuclear clock — with its anomalously high sensitivity to α variations (enhancement factor ~10,000) — could push that limit dramatically further, testing new physics with every tick.

Relativistic geodesy. General relativity predicts that clocks run slightly faster at higher elevation (farther from Earth’s gravitational mass). This effect is already measured: a clock at sea level ticks about 10⁻¹⁶ per second slower than one a kilometer higher. With nuclear clock precision, you could map Earth’s gravitational potential field in three dimensions using nothing but timekeeping — essentially doing geology from the precision of your clock.

What Comes Next

The 2024 measurements used a solid-state setup — thorium ions embedded in a calcium fluoride crystal. This is convenient (lots of nuclei, relatively easy to work with) but the crystal environment perturbs the nuclear transition, limiting ultimate accuracy to perhaps 10⁻¹⁸. That’s still excellent, but the theoretical ceiling is higher.

The next frontier is a trap-based nuclear clock: a single thorium-229³⁺ ion held in an electromagnetic trap (a Paul trap), isolated from almost all perturbations. Theoretical analyses suggest this approach could achieve systematic uncertainties of 1.5 × 10⁻¹⁹ — a factor of ten better than the best electron-based clocks today. Multiple groups in Europe, Japan, and the United States are now racing toward this goal. In 2024, a team also announced the first trapping of triply-charged thorium-229 ions, a necessary step on that path.

The world’s supply of thorium-229 is estimated at about 40 grams — the isotope is not naturally abundant. It comes from the decay of uranium-233 stockpiles maintained at national laboratories. Enough exists for many experiments, but the scarcity adds a certain preciousness to each measurement.

A Nucleus Finally Speaks

There is something quietly wonderful about this story. A pair of physicists in 1976, doing bookkeeping on radioactive gamma rays, stumbled on an anomaly — an energy gap so small it looked almost like an error. That anomaly sat in the literature for decades, teasing theorists who saw a clock in it, tormenting experimenters who couldn’t find it.

The 2024 breakthrough required a frequency comb (Nobel Prize in Physics, 2005), ultrafast laser amplification, VUV nonlinear optics, one of the world’s best atomic clocks, calcium fluoride crystals grown with exotic dopants, and decades of accumulated knowledge about how to work in vacuum ultraviolet. Every one of those technologies had its own long development story.

And the payoff is that we can now, for the first time, put a laser on an atomic nucleus and make it ring like a bell. The nucleus, that supposedly inert passenger at the center of every atom, turns out to have something to say. We are only just beginning to listen.

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Key papers: Tiedau et al., Physical Review Letters 132, 182501 (2024), DOI: 10.1103/PhysRevLett.132.182501; Zhang et al., Nature 633, 63–70 (2024), DOI: 10.1038/s41586-024-07839-6; von der Wense et al., Nature 533, 47–51 (2016), DOI: 10.1038/nature17669; Kraemer et al., Nature 617, 706–710 (2023), DOI: 10.1038/s41586-023-05894-z; Peik & Tamm, Europhysics Letters (2003), DOI: 10.1209/epl/i2003-00210-x.