Clock reference moves into the nucleus: a one-shot success with 1.4 micrograms of thorium-229
A teaspoon of white dwarf matter, if it fell to Earth, would weigh as much as an elephant. That image opens the popular-science coverage — and sets the scale for this news.
On October 7, *Nature* published a paper titled "A nuclear clock locked to thorium-229." A team led by Ding Shiqian of Tsinghua University's Department of Physics collaborated with three institutions: the Shanghai Institute of Ceramics of the Chinese Academy of Sciences, the Shanghai Institute of Optics and Fine Mechanics, and the Beijing Academy of Quantum Information Sciences. The result is the first reported operation of a thorium-229 nuclear optical clock. A European team independently achieved this at the same time; both papers appeared in *Nature*.
Which energy level changes
Atomic optical clocks use electronic energy-level transitions as frequency references; atomic microwave clocks use the microwave portion of electronic transitions. A nuclear optical clock moves the reference to nuclear energy-level transitions inside the atomic nucleus.
| Reference | Transition level | Sensitivity to external electromagnetic environment | |---|---|---| | Atomic microwave clock | Electronic levels | Larger | | Atomic optical clock | Electronic levels | Smaller | | Nuclear optical clock | Nuclear energy levels | In principle smaller |
The nuclear scale is far smaller than the atomic scale — this is a principle-level argument, not a result measured in this experiment. In interviews, Ding said a nuclear clock could in principle provide a time-and-frequency reference more precise than atomic optical clocks.
The thorium-229 nuclear transition energy is about 8.4 electron-volts, corresponding to a 148 nm vacuum ultraviolet wavelength. Thorium-229 is too heavy to be produced by conventional fusion reactions inside stars; it forms under the extreme conditions briefly present at a star's death. It is thus a marker of stellar death.
Two bottlenecks: light source and nuclide
International peers had previously achieved direct excitation and measurement of the thorium-229 nuclear transition using pulsed lasers. What was missing was a continuous-wave laser at 148 nm — a continuous-wave source at this wavelength did not previously exist.
- Light source: The team used cadmium vapor four-wave mixing, not the mainstream nonlinear-crystal route. In February 2026, the team published this frequency-conversion method in *Nature*, with the condition that it delivers short-wavelength light without introducing significant phase noise.
- Nuclide: Thorium-229 is extremely scarce. The team grew a millimeter-scale, high-quality thorium-doped calcium fluoride crystal in a single attempt, using about 1/200 the nuclide quantity of the European team's single run. A key enabling technology is an absorption-spectroscopy technique for rapidly reading out nuclear quantum states.
The paper has four corresponding authors. Ding Shiqian is an associate professor at Tsinghua's physics department and an adjunct researcher at the Beijing Academy of Quantum Information Sciences. The other three are Su Liangbi, Hang Yin, and Bu Wenhao. Several co-first authors are doctoral students born after 2000.
Who will use this next
Ding listed satellite navigation, deep-space exploration, and high-precision time-and-frequency needs in national defense contexts. He noted that solid-state nuclear clocks have miniaturization and engineering potential — a form-factor advantage atomic optical clocks lack.
A more theoretical line: the thorium-229 nuclear transition is significantly more sensitive to variations in fundamental constants and other new-physics effects, so nuclear clocks can serve as precision platforms for testing fundamental laws. Nuclear energy levels are nature's quantum clock; whoever masters them most precisely gets the first look at the finest drift of the pendulum.
Takeaway: Moving from electronic clocks to nuclear clocks shifts the reference standard from energy levels in the atomic shell to energy levels in the nucleus. The shell is easily disturbed by the environment; the nucleus is not. This raises the ceiling on precision rather than merely improving current precision.
Sources and caveats: Facts and figures come from four outlets — Tsinghua's physics department website, Science and Technology Daily, People's Daily app, and China News Service; the paper is titled "A nuclear clock locked to thorium-229." The original paper was not directly consulted, so the following were not obtained: the clock's frequency uncertainty, exact second-scale stability figures, closed-loop operation duration, paper DOI, and the European team's leading institution and paper title. "In principle more precise" is the interviewee's theoretical judgment, not a measured result. "Second-scale stability better than the European team by nearly an order of magnitude" comes from Tsinghua's physics department and Science and Technology Daily; no side-by-side data was seen. Ding's framing: nuclear optical clocks are expected to become the next generation of time-frequency standards after atomic microwave and atomic optical clocks.
One sentence: The time standard has switched energy levels — and that shift matters more than any single precision improvement.