A schematic diagram of a thorium-229 nuclear clock. The nucleus is irradiated with laser light (laser 1). A second laser checks if the nucleus was successfully excited. A photomultiplier tube (PMT) checks laser 2’s output. Based on its signal, laser 1 is stabilised to the nuclear transition via a feedback loop.
| Photo Credit: LarsvdW (CC BY-SA)
Atomic clocks are the world’s gold standard for timekeeping, at the heart of everything from GPS navigation to synchronised networks. A ytterbium atomic clock is so precise it misses one second every 23 billion years.
Yet scientists are interested in more precise clocks to detect infinitesimal changes in fundamental forces and further advance navigation.
In two papers in Nature, scientists from Europe and China have now reported the world’s first working nuclear clocks.
While, in an atomic clock, an atom’s electrons are easily disturbed by electric or magnetic fields and temperature changes in a room, the nucleus is shielded by the electron cloud.
Exciting a nucleus usually requires energetic X-rays or gamma rays, which are difficult to harness. However, the thorium-229 isotope’s nucleus has a transition energy low enough to be triggered by an ultraviolet laser.
A laser’s electric field oscillates rapidly: positive, zero, negative, zero, positive, and so on. An atomic or nuclear clock counts these cycles like an old clock counts the swings of a pendulum.
The laser’s frequency can drift slightly, however. In a nuclear clock, scientists expose thorium-229 nuclei to the laser and check how readily they become excited. If it happens less often than expected, the laser’s frequency has drifted away and a computer corrects it.
In one study, teams from Beijing and Shanghai embedded thorium-229 in calcium fluoride crystals and fired an ultraviolet laser at them. A feedback loop constantly checked the laser against the thorium nuclei and corrected it. Because the clock operated within a simple crystal, it could pave the way for compact, portable clocks.
The second effort, by teams from Austria and Germany, built a thorium-229 clock and used it to look for dark matter.
The thorium-229 nucleus has such low transition energy because the strong nuclear force and the electromagnetic force nearly cancel each other out. Thus, the nuclear clock is highly sensitive to any slight changes in these fundamental forces.
Some theories suggest ultralight dark matter may cause these constants to fluctuate as it passes through earth. The team ran the nuclear clock alongside a ytterbium atomic clock. Because the two clocks react differently to fluctuating physics constants, any change in their relative tick rates could reveal the presence of dark matter. The teams didn’t find a signal, however.
These prototypes are not yet as precise as the best atomic clocks but their architecture is proven. With upgrades to laser power and crystal quality, scientists expect nuclear clocks to eventually surpass their atomic predecessors.
mukunth.v@thehindu.co.in
Published – October 08, 2026 07:00 am IST
