The world’s first nuclear clocks could redefine timekeeping

Measuring time, for much of human history, meant observing the natural world. The movement of the sun, the passage of stars, and the steady flow of water provided early ways to divide the day in civilizations from ancient Egypt and Mesopotamia to China, beginning more than 3,000 years ago. As societies became more connected, however, the demand for accuracy drove advances from mechanical clocks to the atomic devices underpinning modern navigation and communications.

In October 2026, scientists in Vienna and Beijing independently demonstrated the first fully functioning nuclear clocks, marking another milestone in that progression. Reported in two studies published in Nature, a leading international scientific journal, the devices measure time using changes within the nuclei of thorium-229 atoms rather than the electrons used by conventional atomic clocks. The Vienna clock would lose approximately one second over several million years, while the Beijing device would lose about two seconds over 65 million years.

Neither approaches the accuracy of the world's best optical atomic clocks, developed and progressively improved since the early 2000s, which remain roughly 10,000 times more reliable. The breakthrough lies instead in successfully measuring time through changes in the energy of an atom's nucleus, a previously unproven approach that could eventually offer greater accuracy and resistance to external disturbances.

From the sun to the pendulum

The history of timekeeping has closely followed the demands of human activity.By around 1500 BCE, Egyptians used sundials to divide daylight into intervals, while water clocks allowed timekeeping after sunset. Medieval European clockmakers later developed mechanical devices that operated independently of sunlight and water, although their accuracy remained limited.

The search for greater precision accelerated in 1656, when Dutch scientist Christiaan Huygens developed the pendulum clock, using its regular movements to substantially improve mechanical timekeeping and establish a foundation for advances in navigation.

At sea, even small errors in timekeeping could have serious consequences. Although sailors could determine latitude through celestial observations, calculating longitude required comparing local time with a fixed reference. In 18th-century Britain, John Harrison developed clocks accurate enough to maintain reliable time aboard moving ships, allowing sailors to navigate more precisely.

Industrialization and modern communications created new demands for synchronization. In 1927, American engineer Warren Marrison developed an early quartz clock at Bell Telephone Laboratories in New York. Quartz, a naturally occurring mineral, vibrates regularly when electricity is applied, providing a more consistent reference than mechanical movement.

The next advance moved timekeeping from the properties of minerals to those of atoms. In 1949, scientists at the US National Bureau of Standards in Washington, D.C., demonstrated the first atomic clock, using ammonia molecules to regulate time. Later versions relied on the consistent frequencies at which atoms absorb or emit energy as their electrons move between energy levels.

This new level of accuracy eventually changed the international definition of time itself. In 1967, the 13th General Conference on Weights and Measures, meeting near Paris, formally defined one second as the time it takes a cesium-133 atom to complete 9,192,631,770 cycles of a specific atomic oscillation, replacing the Earth's movement as the international standard for measuring time.

From atomic electrons to nuclear transitions

Rather than measuring energy changes involving electrons, nuclear clocks use transitions within the nucleus itself, which is more effectively shielded from environmental disturbances such as electromagnetic fields.

Most atomic nuclei require more energy to change states than conventional laboratory lasers can provide. Thorium-229, a rare radioactive isotope, is an exception because its nucleus responds to comparatively low-energy ultraviolet light.

After more than two decades of research, scientists identified the frequency needed to trigger this transition. In September 2024, researchers at JILA, a joint institute of the University of Colorado Boulder and the US National Institute of Standards and Technology, used an ultraviolet laser to trigger and precisely measure energy changes in thorium nuclei. However, they had not yet produced a functioning clock.

The October 2026 experiments brought these components together. According to the two Nature studies, researchers in Beijing and Vienna used different combinations of crystals containing thorium-229 and specialized lasers to produce the first functioning nuclear clocks.

Both teams also developed systems to detect and correct small changes in laser frequency, allowing their devices to maintain a stable reference. Their independent achievements established that nuclear transitions could be used not merely for laboratory measurements but for functioning clocks.

The next frontier in precision

Precise timekeeping underpins modern infrastructure. Satellite navigation systems calculate position using differences in signal arrival times, while telecommunications and financial networks depend on synchronized clocks.

Although existing atomic clocks already meet many of these requirements, nuclear clocks could eventually be more resistant to environmental disturbances, smaller and easier to transport. They could also help scientists investigate dark matter, which accounts for approximately 85 percent of the universe's matter, by detecting minute changes in nuclear activity that might reveal its presence. Significant obstacles remain, however, including the scarcity of thorium-229 and the specialized lasers required to operate the devices.

The breakthrough comes as international metrology institutions consider redefining the second. The International Bureau of Weights and Measures is targeting 2030 for a decision based on optical atomic clocks, although implementation could follow later.

Nuclear clocks are not yet candidates to replace the leading systems. Nevertheless, their successful demonstration introduces another physical reference against which scientists can test the consistency of time itself. Across centuries, improvements in clock technology have enabled advances in navigation, industry and communication. The newest generation may also help investigate questions about the physical universe that conventional timekeeping cannot answer.