Scientists have reached a major milestone in the quest to build an entirely new kind of timekeeping device. After decades of intensive research, a collaborative team in Vienna has developed the world’s first self-stabilizing nuclear clock, an innovation that could eventually measure time with far greater precision than today’s most advanced atomic clocks.
Unlike earlier experimental prototypes, this new nuclear clock can maintain its own stability without relying on a conventional atomic clock to keep it grounded. Researchers have successfully demonstrated that the device can operate steadily for more than 24 hours without human intervention or external calibration. This achievement represents a monumental leap forward in the field of precision measurement, widely known as metrology. By harnessing atomic nuclei as exceptionally stable references, nuclear clocks could eventually allow scientists to measure time and other fundamental physical quantities with levels of accuracy that existing technologies simply cannot reach.
Why Thorium Holds the Key to Nuclear Clocks
The groundbreaking development relies on an unusual and highly specific property of thorium atomic nuclei that physicists and chemists have investigated for decades. Under normal circumstances, atomic nuclei can exist in different energy states, but moving between those discrete states typically requires enormous amounts of energy, usually in the form of high-energy gamma rays.
Thorium represents a rare and remarkable exception to this rule. Two of its nuclear energy states are separated by an unusually small energy gap, making it uniquely possible to trigger a transition between them using precisely tuned ultraviolet laser light. This specific physical property is crucial because it allows researchers to directly control the energy state of an atomic nucleus using a laser beam—a feat that is not possible in the same way for the vast majority of other atomic nuclei. Scientists can then utilize this extremely precise quantum transition as a reliable reference standard for measuring time.
A major milestone toward this capability came in April 2024, when a research team led by Prof. Thorsten Schumm at the Institute of Atomic and Subatomic Physics at TU Wien, working closely with a team led by Prof. Ekkehard Peik at PTB Braunschweig, successfully identified the long-sought nuclear transition experimentally. For the first time in scientific history, they demonstrated that laser beams could successfully excite thorium nuclei.
Further progress quickly followed in the fall of 2024. The researchers connected their thorium excitation apparatus directly to a conventional optical atomic clock, demonstrating that the excited nuclei could indeed serve as a highly stable and precise timekeeping reference. Although this pivotal experiment established the fundamental operating principle of a nuclear clock, one essential capability was still missing: the system lacked the ability to maintain its own accuracy independently over extended periods.
Scientists Build a Nuclear Clock That Stabilizes Itself
Addressing this limitation required a completely new approach to system architecture. "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 complex mechanism possible in practice, the researchers developed a sophisticated system built around a specially engineered crystal containing thorium atoms, which was manufactured directly at TU Wien. In this setup, a laser shines continuously onto the crystal, interacting directly with the embedded thorium nuclei inside. The laser’s light oscillates at a particular, highly regulated frequency, providing a steady rhythm that can be utilized to measure elapsed time. However, even the tiniest micro-changes in the surrounding laboratory environment can interfere with that rhythm and degrade performance.
"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 persistent problem by relying on the energy transitions of electrons orbiting the outer shells of atoms as a stable reference. The Vienna team has now conclusively demonstrated that the dense nuclei of thorium atoms can perform that exact same stabilization function, but with inherent advantages derived from nuclear physics.
The mechanism works because thorium nuclei absorb laser light only when the incoming light has precisely the correct, uncompromised frequency. If the laser begins to drift away from that optimal frequency due to environmental noise or internal thermal shifts, the amount of light absorbed by the thorium nuclei decreases noticeably. The surrounding system immediately detects this change and automatically adjusts the laser’s output to bring it back to the correct frequency. This continuous, closed-loop feedback keeps the entire clock operating at a consistent rate, entirely without requiring a conventional atomic clock to maintain its stability. For the first time, scientists have demonstrated a fully functional, self-regulating nuclear clock that can operate independently.
A Clock With an Error of Just One Second in 30 Million Years
The profound potential advantage of nuclear clocks stems directly from the extraordinary physical properties of atomic nuclei compared to entire atoms.
"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.
To put the scale into perspective, atomic nuclei are more than 10,000 times smaller than the atoms that house them. Because of their extremely small spatial size and different internal dynamics, they are much less affected by external electromagnetic fields, stray thermal radiation, and other environmental influences that routinely disrupt precise atomic measurements. This inherent resistance to environmental disturbances makes nuclear energy transitions exceptionally promising candidates for the next generation of ultra-precise timekeeping.
To evaluate the real-world performance of their newly built prototype, the researchers measured its operational behavior over the course of a full day. The nuclear clock achieved a relative precision of approximately 10 to the power of minus 15, which is mathematically equivalent to an operational error of roughly one second over a span of 30 million years.
While that figure describes the clock’s measured short-term precision during testing rather than an actual physical test lasting millions of years, it clearly illustrates the remarkable accuracy that the researchers have already managed to achieve with an early-stage prototype. The new device is not yet quite as precise as the world’s leading optical atomic clocks, which currently remain the undisputed gold standard for high-performance timekeeping across global scientific institutions.
"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," notes Thorsten Schumm.
The Next Step Toward Even More Precise Timekeeping
Looking ahead, the researchers are confident that their novel nuclear clock design has considerable room for technical improvement. By utilizing more powerful and stable lasers, alongside developing higher-quality thorium-doped crystals with fewer structural defects, the team hopes to substantially increase the device’s operational precision in the coming years.
These upcoming upgrades could eventually allow nuclear clocks to outperform existing atomic clocks across the board, establishing an entirely new technological standard for measuring time and frequency. The broader implications of this research extend far beyond simply building a better clock for navigation or telecommunications. More accurate timekeeping can also drastically improve the measurement of other fundamental physical quantities, providing researchers with increasingly sensitive, high-resolution tools for investigating the fundamental properties and laws of nature.
For now, the Vienna research team’s achievement demonstrates that a long-pursued theoretical scientific concept can finally operate as a practical, self-sustaining timekeeping system. With the world’s first self-stabilizing nuclear clock now successfully running independently, scientists have taken a crucial, tangible step toward a future in which time can be measured with unprecedented precision.
