Scientists have reached a major milestone in the quest to build an entirely new kind of clock. After decades of research, a team in Vienna has developed the world’s first self-stabilizing nuclear clock, a device that could eventually measure time with far greater precision than today’s most advanced atomic clocks.
Unlike earlier prototypes, the new nuclear clock can maintain its own stability without depending on a conventional atomic clock. Researchers have demonstrated that it can operate steadily for more than 24 hours without intervention.
The achievement represents an important advance in precision measurement, also known as metrology. By using atomic nuclei as exceptionally stable references, nuclear clocks could eventually allow scientists to measure time and other physical quantities with levels of accuracy that existing technologies cannot reach.
Why Thorium Holds the Key to Nuclear Clocks
The breakthrough relies on an unusual property of thorium atomic nuclei that scientists have investigated for decades.
Atomic nuclei can exist in different energy states, but moving between those states typically requires enormous amounts of energy. Thorium is a rare exception. Two of its nuclear energy states are separated by an unusually small energy gap, making it possible to trigger a transition between them using laser light.
This property is crucial because it allows researchers to control the energy state of a nucleus with a laser, something that is not possible in the same way for most other atomic nuclei. Scientists can then use this extremely precise transition as a reference for measuring time.
A major breakthrough came in April 2024, when researchers led by Prof. Thorsten Schumm at the Institute of Atomic and Subatomic Physics at TU Wien, working with a team led by Prof. Ekkehard Peik at PTB Braunschweig, identified the long-sought nuclear transition experimentally. They demonstrated for the first time that laser beams could excite thorium nuclei.
Further progress followed in the fall of 2024. The researchers connected their thorium excitation apparatus to a conventional optical atomic clock, demonstrating that the nuclei could serve as a highly precise timekeeping reference.
Although this established the basic principle of a nuclear clock, one essential capability was still missing: the ability to maintain its own accuracy independently.
Scientists Build a Nuclear Clock That Stabilizes Itself
“What you really want is a self-stabilizing nuclear clock,” explains Prof. Thorsten Schumm. “The basic idea is simple: you have a laser and you have thorium. The laser changes the energy state of the thorium nuclei, and the thorium nuclei are used to stabilize the frequency of the laser.”
To make this possible, the researchers developed a system built around a special crystal containing thorium atoms, manufactured at TU Wien. A laser shines onto the crystal, interacting with the thorium nuclei inside.
The laser’s light oscillates at a particular frequency, providing a regular rhythm that can be used to measure time. However, even tiny changes in the surrounding environment can interfere with that rhythm.
“The oscillation of this laser light can be used for timekeeping, but the laser frequency can shift slightly from time to time, for example due to temperature fluctuations,” explains Thorsten Schumm. “For high-precision measurements, you therefore need a mechanism to keep the laser frequency exactly stable, so that the clock continues to tick with precisely the same rhythm.”
Traditional atomic clocks solve this problem by using the energy transitions of electrons in atoms as a stable reference. The Vienna team has now demonstrated that the nuclei of thorium atoms can perform the same function.
The mechanism works because thorium nuclei absorb laser light only when the light has precisely the correct frequency. If the laser begins to drift away from that frequency, the amount of light absorbed by the nuclei decreases.
The system detects this change and automatically adjusts the laser to bring it back to the correct frequency. This continuous feedback keeps the clock operating at a consistent rate, without requiring a conventional atomic clock to maintain its stability.
For the first time, scientists have demonstrated a self-regulating nuclear clock that can function independently.
A Clock With an Error of Just One Second in 30 Million Years
The potential advantage of nuclear clocks comes from the extraordinary properties of atomic nuclei.
“The great advantage of the new nuclear clock in Vienna is that, if you use atomic nuclei rather than atoms, much higher precision is possible in principle,” says Thorsten Schumm.
Atomic nuclei are more than 10,000 times smaller than atoms. Because of their extremely small size, they are much less affected by external influences that can disrupt precise measurements.
This resistance to environmental disturbances makes nuclear energy transitions especially promising for ultra-precise timekeeping.
To evaluate their prototype, the researchers measured its performance over the course of a day. The nuclear clock achieved a relative precision of approximately 10 to the power of minus 15, equivalent to an error of roughly one second over 30 million years.
That figure describes the clock’s measured precision rather than an actual test lasting millions of years. Nevertheless, it illustrates the remarkable accuracy the researchers have already achieved with an early prototype.
The new clock is not yet as precise as the world’s leading optical atomic clocks, which remain the benchmark for high-performance timekeeping.
“This is not yet at the level of the world’s best optical atomic clocks, but for a first prototype it is a fantastic result,” says Thorsten Schumm.
The Next Step Toward Even More Precise Timekeeping
The researchers believe their nuclear clock has considerable room for improvement. By using more powerful lasers and developing higher-quality thorium crystals, they hope to substantially increase its precision.
These upgrades could eventually allow nuclear clocks to outperform existing atomic clocks and establish a new standard for measuring time.
The implications extend beyond simply building a better clock. More accurate timekeeping can also improve the measurement of other physical quantities, providing researchers with increasingly sensitive tools for investigating the fundamental properties of nature.
For now, the Vienna team’s achievement demonstrates that a long-pursued scientific concept can operate as a self-sustaining timekeeping system. With the world’s first self-stabilizing nuclear clock now running independently, scientists have taken an important step toward a future in which time can be measured with unprecedented precision.
