World's First Nuclear Clocks Just Started Ticking

The first working nuclear clock is real, and there are two of them. Teams in Vienna and Beijing say they have built the world's first operating nuclear clocks, according to AFP via The Straits Times, and both reported their results in the journal Nature on Oct. 7. Physicists have chased this device for almost 50 years, so the moment is a big one for anyone who cares about how we measure time.

Before you picture a reactor strapped to a watch, relax. These devices are called a nuclear clock because they track what happens inside an atom's nucleus, not because they run on fission or fusion. A high-powered laser nudges the nucleus, and the way that nucleus responds becomes the tick. The nuclear clock has no explosions, no radiation scare, and no glowing green goo.

How a nuclear clock actually ticks

Today's gold-standard atomic clocks use lasers or microwaves to make electrons jump between two energy levels in an atom's outer shell, using elements such as cesium or strontium. These machines already power GPS, cell networks and the internet. The new approach swaps the electrons for the nucleus itself, where protons and neutrons flip between two energy states.

Both clocks use thorium-229, a special isotope trapped inside a calcium fluoride crystal. Thorium-229 is weird in a useful way: its two nuclear energy states sit extremely close together. According to a TU Wien release on EurekAlert, that tiny gap is why a laser can switch the nucleus between states, while other nuclei need far more energetic radiation.

Here is the clever part. The laser's frequency can drift a little, for example because of temperature changes. The thorium nuclei only absorb light when the frequency is exactly right, so if absorption drops, a feedback loop corrects the laser automatically. That self-correcting loop is what turns a lab trick into a real clock.

Vienna and Beijing got there at the same time

The two teams worked independently and landed on working clocks at the same moment. "The two teams worked independently and reached operating thorium-229 nuclear clocks at the same time, using different experimental approaches," said physicist Shiqian Ding of Tsinghua University, who helped lead the Beijing effort, as quoted by AFP. He called it encouraging because it shows the concept is robust.

In Vienna, physicist Thorsten Schumm of TU Wien said his group has been working toward this goal since 2008. "The creation of a nuclear clock was something that physicists dreamt of for almost 50 years," he said. His team's crystal-and-laser system stayed stable for more than 24 hours without anyone touching it, according to TU Wien.

The rivalry is friendly, but it is a rivalry. The South China Morning Post reported that early results suggest the Chinese clock is about six times as stable as the Vienna one, while the Vienna crystals are higher quality. Schumm himself pointed out that Vienna has better crystals and Beijing has the stronger laser, so combining them should give a significantly better clock.

It is not a record-breaker yet

Do not expect your phone to swap its clock tomorrow. TU Wien says its prototype's precision is roughly 10 to the power of minus 15, which works out to an error of about one second in 30 million years. That sounds absurd, but the best optical atomic clocks do far better, and the researchers openly say the new devices do not beat them yet.

Schumm told AFP the nuclear clock is still "far from its target performance." The plan to close the gap is simple: stronger lasers and better thorium crystals. Atomic nuclei are more than ten thousand times smaller than atoms and react much more weakly to outside disturbances, so in theory they should make steadier timekeepers once the hardware catches up.

Why you should care

For one, size. Because the thorium sits in a solid crystal, researchers think a future nuclear clock could be less bulky and delicate than the room-sized rigs used for the best atomic clocks. Schumm sees uses in satellite navigation, data-transfer synchronization, surveying and metrology, which is the science of measurement.

For another, physics. The Vienna team used its clock for a dark matter hunt. As The Hindu explains, the thorium nucleus is extremely sensitive to tiny changes in fundamental forces, and some theories say ultralight dark matter could make those constants wobble. The experiment found no dark matter signal, but AFP reports the clock performed at the level of the best atomic clocks for that test.

"It gives access to a whole new physics universe," Schumm said. Nature's own commentary calls the result a milestone for metrology and a promising next-generation time standard. If you like following the weird frontier of research, keep an eye on our science coverage as these clocks get upgraded.

The takeaway: a century of better clocks gave us GPS and fast internet, and the next leap may come from the nucleus. The first ticks are in, they are not perfect, and two rival labs now have a clear to-do list to make them far better.