Scientists in Vienna and Beijing Build World's First Nuclear Clocks Using Thorium-229 in Timekeeping Leap

Scientists working separately in Austria and China have built the world's first operating nuclear clocks, a long-sought breakthrough that could eventually lead to timekeeping more precise than today's best atomic clocks and open a new window into the fundamental laws of physics.

The two teams, one at TU Wien in Vienna and the other at Tsinghua University in Beijing, described their devices Wednesday in separate papers published in the journal Nature. Both clocks are based on thorium-229, a rare radioactive isotope of the metal thorium, embedded in calcium fluoride crystals.

The researchers said nuclear clocks have the potential to outperform the most advanced atomic clocks, but have not yet done so.

"The creation of a nuclear clock was something that physicists dreamt of for almost 50 years. In my team, we have been working towards this goal since 2008," physicist Thorsten Schumm of TU Wien, who helped lead the Vienna team, told Reuters.

Two teams, one milestone

The teams worked independently and reached the milestone at the same time using different experimental methods.

"The two teams worked independently and reached operating thorium-229 nuclear clocks at the same time, using different experimental approaches. I think this is very encouraging because it shows that the concept is robust and not dependent on one particular technical implementation," said Shiqian Ding, a physicist at Tsinghua University who helped lead the Beijing team.

The Vienna team built its clock by locking a continuous-wave laser to the thorium nucleus' transition at a wavelength of 148 nanometers in the vacuum ultraviolet range, using a rapid feedback loop based on how much light the nuclei absorbed. The team compared its nuclear clock against a conventional ytterbium ion clock.

In both experiments, researchers shone ultraviolet laser light through a tiny crystal laced with thorium-229 and tuned the laser until the nuclei absorbed the most light. That point became the clock's "tick."

The South China Morning Post reported that the Beijing clock showed better stability than the Vienna device.

Why 'nuclear'

Despite the name, nuclear clocks have nothing to do with nuclear power, fission or fusion.

Conventional atomic clocks keep time by using lasers or microwaves to make electrons jump between energy levels in the outer shell of atoms such as cesium or strontium. The frequency of those jumps acts as an extraordinarily steady pendulum.

Nuclear clocks instead use lasers to shift protons and neutrons between energy levels inside the atom's nucleus.

Because the nucleus is about 10,000 times smaller than the atom and is well shielded by its surrounding electrons, it is far less sensitive to outside disturbances such as stray electric and magnetic fields. In theory, that could make nuclear clocks more stable and accurate than atomic clocks.

A rare isotope

For decades, the idea faced a major obstacle. Most atomic nuclei require enormous amounts of energy to switch between states, far beyond what lasers can provide.

Thorium-229 is the exception. Its nucleus has an unusually low-energy excited state that can be reached with ultraviolet light. Scientists first proposed the isotope as a candidate for a clock in the 1970s and 1990s, but spent decades trying to pin down the exact energy needed.

A major breakthrough came in 2024, when research teams in Europe and the United States used lasers to excite the thorium nucleus for the first time and precisely measured its transition frequency.

The isotope is extremely scarce. The world's supply of thorium-229 is estimated at about 40 grams. The Tsinghua team noted the scarcity leaves little room to experiment with how the crystals are made, according to the South China Morning Post.

'Far from its target performance'

The researchers stressed that the technology is still in its infancy.

Schumm said the nuclear clock is still "far from its target performance," but that combining the strengths of the two designs could quickly improve it.

"What is really nice here: the Vienna clock has slightly better thorium crystals — higher concentration, better optical properties — while the Beijing team has a stronger laser. So already by putting these components together, we can build a significantly better clock," Schumm said.

The best conventional atomic clocks lose or gain only one second over billions of years. Scientists have predicted that a fully developed nuclear clock could eventually be about 10 times more stable.

Practical uses

Ultra-precise atomic clocks already underpin modern life, keeping time for GPS and other satellite navigation systems, synchronizing financial transactions and coordinating internet, cellular and fiber-optic networks.

Schumm said he envisions nuclear clocks being used in satellite navigation, data synchronization, surveying and metrology, the science of measurement.

Because the thorium is embedded in a solid crystal rather than held in a vacuum trap, researchers believe nuclear clocks could eventually be built to be smaller, sturdier and easier to operate than the bulky, delicate atomic clocks used in laboratories today.

Hunting dark matter

The clocks may also become powerful tools for exploring unanswered questions in physics.

Because the energy of thorium-229's transition results from a delicate balance between powerful forces inside the nucleus, it is expected to be extremely sensitive to tiny changes in the fundamental constants of nature. Scientists hope such clocks could detect whether those constants drift over time or reveal signs of new physics.

The Vienna team demonstrated that potential by using its clock in a search for dark matter, the invisible substance thought to make up most of the matter in the universe but which has never been directly observed. The experiment did not detect dark matter, but the nuclear clock performed at the level of the best atomic clocks in the test.

"It gives access to a whole new physics universe," Schumm said.

A new precision race

The simultaneous results mark the start of what physicists describe as a race to improve nuclear clocks, with groups in Germany, the United States, Japan and China all working on the technology.

The first atomic clock was built in 1949. Over the following decades, steady improvements turned it into one of the most precise instruments ever created, eventually redefining the second itself.

Researchers say nuclear clocks could follow a similar path, though it may take years before they surpass the atomic clocks that now set the world's time.