A self-adjusting nuclear clock... Could it lead to the discovery of dark matter?

The nucleus becomes a timekeeping reference

Traditional atomic clocks rely on energy transitions between electron levels within atoms, whereas the new clock is based on a rare energy transition within the nucleus of the thorium-229 isotope at a wavelength of 148 nanometers.

The importance of this choice lies in the fact that the nucleus is less affected by certain external disturbances compared to electrons, which could allow for higher levels of stability and precision in time measurement.

To operate the clock, the researchers placed large numbers of thorium nuclei inside a small calcium fluoride crystal, operating at room temperature, and then directed a deep ultraviolet laser at them.

When the laser frequency deviates from the frequency corresponding to the nuclear transition, the response of the nuclei changes, and the clock uses this change to detect the deviation and correct it automatically, thereby maintaining frequency stability.

Luca Toscani di Col, a doctoral student in physics at the Vienna University of Technology and co-author of the study, said that the main achievement lies in transitioning the thorium nuclear phenomenon from a mere measurable signal in the laboratory to an actual reference within a clock that can continuously monitor and correct its frequency.

Promising performance... but the competition is not over

To test the performance of the new system, the researchers compared the nuclear clock with another optical clock based on an ytterbium ion. The thorium clock was able to operate continuously for about 24 hours, and its frequency instability after this period reached about one part in a quadrillion.

However, this achievement does not mean that the nuclear clock has become more accurate than the best atomic clocks currently available; rather, it indicates the researchers' success in proving the possibility of using an atomic nucleus as an actual reference for adjusting a clock that operates with a continuous correction system.

The technology is still in its early stages, as the team noted that the measurement results are not repeatable at the same level when conducted on different days. The researchers suggest that microscopic differences within the crystal are responsible for slight changes in the nuclear transition frequency.

Could the clock help detect dark matter?

Interest in the new clock goes beyond the issue of time measurement, as it could become an important tool for searching for dark matter, a substance whose existence is inferred from its gravitational effects, even though its nature remains unknown.

Some theories assume that forms of ultra-light dark matter might lead to extremely small periodic changes in the fundamental constants of nature. If these changes occur, they might appear as slight vibrations in the frequency of precise clocks, allowing scientists to search for them by comparing clocks based on different atomic systems.

In the experiment, the researchers compared the thorium clock with the ytterbium clock over a period of nearly 23 hours, searching for periodic changes with periods ranging between about 20 seconds and one day.

The team did not detect a statistically significant signal indicating the presence of dark matter, but this result was not without benefit; the measurements enabled the researchers to set new constraints on the interaction strength of some hypothetical dark matter models with ordinary matter.

In some scenarios related to the strong nuclear force and quark masses, the results contributed to improving previous constraints by about 100 to 1000 times, which enhances the ability of future experiments to test these theories.

The next step: higher precision and longer monitoring

Despite the promising results, the short duration of the experiment does not allow for testing all potential signals, especially changes that might occur over time periods longer than one day.

The researchers are betting that improving laser performance and increasing the quality of the crystals used will increase the clock's stability and enhance its ability to detect more precise changes.

If these improvements succeed, nuclear clocks could in the future become new tools for testing the stability of the fundamental laws of nature, detecting subtle changes in physical constants, and helping to explore the mysteries of dark matter.

Thus, the current achievement is not in inventing a clock more accurate than all existing clocks, but in proving that the atomic nucleus can become a practical reference for timekeeping, opening the door to new experiments that may expand the boundaries of our knowledge of the universe.