Scientists achieve world's first working nuclear clocks with potential to surpass atomic timekeeping

For decades, atomic clocks have held the crown as humanity's most reliable timekeepers. A ytterbium atomic clock is so precise it loses only one second every 23 billion years—a margin of error so small it seems almost theoretical. Yet that precision, remarkable as it is, has limits. The electrons that atomic clocks measure are vulnerable. They shift when exposed to stray electric or magnetic fields, when room temperature fluctuates, when the environment around them changes in ways both obvious and subtle. Scientists have long wondered what might happen if they could measure something more protected, something shielded from the noise of the everyday world.

That question has now found an answer. In two separate studies published in Nature, research teams from Europe and China have built and operated the world's first working nuclear clocks—devices that measure not the behavior of electrons orbiting an atom, but the vibrations of the nucleus itself, buried deep inside. The nucleus sits behind a protective cloud of electrons, insulated from the electromagnetic and thermal disturbances that plague atomic clocks. If nuclear clocks can be perfected, they promise to measure time with a precision that makes today's atomic clocks look crude by comparison.

The breakthrough hinges on a single isotope: thorium-229. Most atomic nuclei require enormous energy to excite—X-rays or gamma rays so powerful they are difficult to generate and control in a laboratory. Thorium-229 is different. Its nucleus has a transition energy so low that an ultraviolet laser can trigger it. A laser's electric field oscillates in a steady rhythm: positive, zero, negative, zero, positive again. An atomic clock counts these cycles the way a grandfather clock counts pendulum swings. In a nuclear clock, the same principle applies, but the measurement is anchored to the nucleus instead of the electron.

The Chinese teams—researchers from Beijing and Shanghai—took one approach. They embedded thorium-229 nuclei directly into calcium fluoride crystals and aimed an ultraviolet laser at them. A feedback loop continuously monitored how readily the nuclei responded to the laser's frequency. If the response weakened, it meant the laser had drifted slightly off its target frequency. A computer detected the drift and corrected it in real time. Because the entire apparatus fit inside a simple crystal, this design suggested a path toward nuclear clocks that could be compact and portable—not confined to a laboratory bench.

The European teams, based in Austria and Germany, built a similar nuclear clock but pursued a different question. They already knew that the thorium-229 nucleus is extraordinarily sensitive to changes in fundamental forces. The nucleus has low transition energy precisely because the strong nuclear force and the electromagnetic force nearly cancel each other out, leaving it exquisitely responsive to any perturbation. Some theoretical physicists have proposed that ultralight dark matter—a hypothetical form of matter that barely interacts with ordinary atoms—might cause these fundamental constants to fluctuate slightly as it passes through the Earth. If so, a nuclear clock would register those fluctuations. The European team ran their thorium-229 nuclear clock alongside a ytterbium atomic clock. The two instruments respond differently to changes in fundamental forces. Any shift in their relative tick rates would signal the presence of dark matter. The search yielded no detection, but the experiment demonstrated that nuclear clocks could be used as instruments of fundamental physics, not merely as better watches.

These first prototypes are not yet more precise than the best atomic clocks available today. The architecture is proven, but the performance still lags. Scientists expect that with improvements to laser power and crystal quality, nuclear clocks will eventually surpass their atomic predecessors. What emerges from these two papers is not a finished product but a proof of concept—evidence that the nucleus can be harnessed as a timekeeper, and that doing so opens doors to both unprecedented precision and new ways of searching for the universe's hidden constituents.