A 2024 measurement of a thorium 229 transition, the first ever pinned to a strontium optical clock, opens a new class of precision instrument that could eventually hunt for dark matter, drifting constants, and a hypothetical fifth force of nature.
For the first time, a clock is keeping time by counting ticks that come from inside an atom's nucleus, not from the swarm of electrons around it. The device, built around a rare transition in thorium-229, turns a fifteen-year-old theoretical idea into working hardware. It is also more than a curiosity. Because nuclear transitions are far more tightly bound against environmental disturbance than electron-shell transitions, the new clock could eventually become a precision instrument sensitive to things today's best atomic clocks cannot see, from dark matter passing through the laboratory to a slow drift in the laws of physics themselves.
The result was reported in a 2024 Nature paper by Zhang and colleagues, which describes the first direct frequency measurement of the thorium-229 nuclear transition referenced to a strontium-87 optical atomic clock. That measurement is the load-bearing step. It shows the nuclear transition exists, sits at a usable frequency, and can be read out with the same instrumentation that drives the world's most precise atomic clocks.
The standard atomic clock, the kind that keeps Coordinated Universal Time and steers the GPS satellites, works by measuring the energy jump an electron makes when it is hit with a laser tuned to exactly the right color. The thorium nuclear clock, by contrast, would watch a much narrower jump inside the nucleus of thorium-229. That jump was a long-standing puzzle. Theorists proposed it in 2003 as a candidate for an ultraprecise clock, but for two decades the energy of the transition was unknown within orders of magnitude, and the laser needed to drive it was missing.
That changed in 2023, when a team at the Physikalisch-Technische Bundesanstalt in Germany first directly excited the transition with a laser. The 2024 frequency-ratio measurement, also reported in Nature, pinned the transition down and compared it to a strontium atomic clock. The two ticks, one nuclear and one atomic-electron, agreed to a precision that confirms the nuclear clock concept works as a measurement device.
Thorsten Schumm of TU Vienna, who has worked on thorium clocks since the early 2010s, has described the result as a "wild idea made real," according to Live Science. A summary in Live Science framed the demonstration as a proof of concept for a new class of clock, rather than a finished instrument.
The payoff is not better timekeeping, at least not yet. Today's best optical atomic clocks, built around atoms like strontium or ytterbium, are extraordinarily precise. The thorium nuclear clock is currently a demonstration, not yet a more accurate timekeeper than those devices. Its real value is different. It would compare two fundamentally different kinds of tick, one nuclear and one atomic-electron, and watch for a slow mismatch between them.
If a mismatch ever appeared, it would be a signal. Two possibilities have driven interest in the device for years. The first is a slow change in the fine-structure constant, alpha, the number that sets the strength of the electromagnetic force. Some theories predict alpha should drift on cosmological timescales, but atomic clocks on their own can only test electron-shell transitions, which are all sensitive to alpha in similar ways. A nuclear clock tests a different corner of physics, and any divergence between nuclear and atomic ticks would be hard to explain with conventional physics. The second is a hypothetical fifth fundamental force, a coupling to neutrons, protons, or baryon number that the four known forces (gravity, electromagnetism, the strong nuclear force, and the weak nuclear force) do not describe. Such a force could ride along with dark matter, and a tabletop clock is one of the few probes that could see it.
The leading groups, Jun Ye's collaboration at JILA in Colorado and a PTB-led effort in Germany, are now working to push the thorium device from demonstration to a continuously running clock that can take real measurements. Until then, the dark-matter and fifth-force payoffs are goals the teams are working toward, not results delivered today.
What the field has now, for the first time, is a new category of precision ruler. Every other clock in existence, atomic, quartz, mechanical, derives its tick from electron-shell physics. The thorium clock derives its tick from the nucleus. That distinction is the basis for a genuinely different kind of measurement tool, and what the first users do with it will define a new chapter in tabletop precision physics.