For decades, the world’s most accurate clocks have relied on electrons transitioning between different energy levels within atoms. Now, an international team of researchers has demonstrated a working clock based on an energy transition inside an atomic nucleus, marking a step towards a new generation of timekeeping instruments that could help test fundamental laws of nature and search for dark matter.
In a study published in Nature, the researchers reported operating an optical nuclear clock based on the thorium-229 isotope. They achieved this by continuously locking a laser’s frequency to an energy transition within the thorium nucleus. When the laser frequency begins to drift from the transition’s resonant frequency, the clock detects the deviation and automatically corrects it, allowing the nucleus to serve as a reference for maintaining frequency stability.
Luca Toscani De Col, a doctoral student in physics at the Vienna University of Technology (TU Wien) and co-first author of the study, says the key advance was turning the thorium nuclear transition from a phenomenon that could be measured in a laboratory into a functioning reference for a clock capable of continuously monitoring and correcting its own frequency.
The Atomic Nucleus Takes Control of the Clock
Toscani De Col explains that the principle is similar to that of conventional atomic clocks. The crucial difference is that traditional atomic clocks rely on transitions involving electrons, whereas the new system uses a rare transition inside the thorium-229 nucleus, excited by laser light at a wavelength of 148 nanometres.
The researchers selected this isotope because its nuclear transition has an exceptionally low energy that can be accessed using a laser. The nucleus is also less sensitive to certain external disturbances than electrons, potentially offering advantages for precision timekeeping.
The design allows large numbers of thorium nuclei to be embedded in a small crystal, rather than requiring researchers to work with a single atom or ion.
The team embedded thorium nuclei in a calcium fluoride crystal maintained at room temperature and directed a deep-ultraviolet laser through it.
When the laser frequency moves away from the nuclear transition’s resonant frequency, the response of the nuclei changes. The clock detects this change and uses it as a feedback signal to bring the laser back towards the correct frequency.
To assess the system’s performance, the researchers compared the nuclear clock with another optical clock based on a ytterbium ion.
The thorium clock operated continuously for approximately 24 hours, achieving a fractional frequency instability of around one part in a quadrillion over that period.
“This does not mean the new clock is more accurate than the best atomic clocks currently available,” Toscani De Col explains. Rather, the achievement represents an important step because the nuclear transition has become a functioning reference in a nuclear clock equipped with a continuous feedback system.
Searching for Dark Matter
One of the clock’s most promising potential applications is the search for dark matter, the invisible substance believed to account for a substantial proportion of the matter in the universe.
According to Toscani De Col, some theories suggest that ultralight particles or fields associated with dark matter could produce extremely small, periodic variations in the fundamental constants of nature. Such changes might appear as tiny oscillations in a clock’s frequency.
To investigate this possibility, the researchers compared the thorium clock with the ytterbium clock for nearly 23 hours, searching for periodic variations over timescales ranging from approximately 20 seconds to one day.
They found no statistically significant signal indicating the presence of dark matter. However, the absence of a detectable signal enabled them to establish new limits on the strength of possible interactions between certain hypothetical forms of dark matter and ordinary matter.
The results demonstrated that the thorium clock could compete with some of the most advanced atomic clocks in testing possible dark matter interactions with ordinary matter and light.
For certain scenarios involving the strong nuclear force and quark masses, the measurements improved previous constraints by approximately 100 to 1,000 times.
These findings illustrate how nuclear clocks could contribute to fundamental physics research, even before they surpass conventional atomic clocks in overall timekeeping performance.
However, the technology remains at an early stage of development.
The researchers found that measurements taken on different days did not reproduce the same frequency with equal precision. They suspect that microscopic differences within the crystal may slightly shift the nuclear transition frequency, limiting the system’s long-term reproducibility.
The dark matter search also lasted less than a day, leaving the clock unable to test potential variations occurring over longer periods.
The researchers expect that improvements in laser technology and crystal quality could enhance the clock’s stability and precision, enabling longer observation periods and more sensitive tests of fundamental physics.