Reviewed by Lexie CornerSep 5 2024
An international research team led by scientists at JILA—a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder—has demonstrated key components of a nuclear clock. Their findings were published in the journal Nature.
A powerful laser shines into a jet of gas, creating a bright plasma and generating ultraviolet light. The light leaves a visible white line as it interacts with leftover gas in the vacuum chamber. This process helps scientists precisely measure the energy needed to excite the thorium-229 nucleus, which is the core of a future nuclear clock. Image Credit: Chuankun Zhang/JILA
The world currently measures time using atomic clocks, but a new development—nuclear clocks—could revolutionize timekeeping and enhance the ability to probe fundamental physics. Nuclear clocks operate by utilizing signals from an atom's nucleus, offering a new level of precision beyond current atomic clocks.
In this recent breakthrough, researchers used a specially designed ultraviolet laser to measure the frequency of an energy transition in thorium nuclei within a solid crystal. To enhance precision, they also employed an optical frequency comb. This tool acts like a highly accurate light ruler, counting the number of ultraviolet wave cycles responsible for the energy transition. Although not yet fully operational, this demonstration includes all the essential components of a nuclear clock.
Compared to atomic clocks, which underpin technologies like GPS, internet synchronization, and financial transactions, nuclear clocks could offer far greater precision. This advancement holds promise for more accurate navigation systems, faster internet, improved network reliability, and enhanced digital security.
In addition to technological applications, nuclear clocks could open new avenues for testing fundamental physics. They may help verify key theories about the universe, aid in the search for dark matter, and provide insights into whether the constants of nature remain truly constant—all without relying on large particle accelerators.
Laser Precision in Timekeeping
Atomic clocks operate by measuring the energy jumps of electrons between different energy levels when exposed to specific laser frequencies. A nuclear clock, however, would measure time by detecting energy jumps within the atom's nucleus, where protons and neutrons are tightly packed. These nuclear energy transitions, or "switches," are activated by shining laser light with an exact frequency, much like flipping a light switch.
Nuclear clocks offer the potential for vastly improved precision over atomic clocks because the nucleus is far less sensitive to external disturbances, such as stray electromagnetic fields, which often affect the electron-based measurements in atomic clocks. The laser frequencies required to induce these nuclear transitions are significantly higher than those used in atomic clocks, resulting in more frequent "ticks" per second, which enhances timekeeping accuracy.
However, developing a nuclear clock is a challenging task. Most atomic nuclei require coherent X-rays at extremely high energies to undergo these transitions, a feat not easily achievable with current technology. The thorium-229 nuclear transition, first identified in 1976, is a rare exception that could be triggered by UV light. Although proposed as a potential clock mechanism in 2003, it was not observed until 2016. Earlier this year, researchers managed to measure the exact UV wavelength required to trigger this nuclear transition.
In this new study, the JILA researchers successfully assembled the key components of a nuclear clock. These include a frequency comb for precise time measurements, a UV laser to produce energy jumps in the thorium-229 nucleus, and the nuclear transition itself to generate the clock's "ticks."
They achieved a million times more precision than previous attempts and created the first direct frequency link between a nuclear transition and an atomic clock, specifically comparing the UV frequency with the optical frequency used in highly accurate atomic clocks based on strontium atoms.
This direct coupling of nuclear and atomic clocks represents a significant step toward integrating nuclear clocks into modern timekeeping systems. Additionally, the study revealed intricate details of the thorium nucleus, offering unprecedented insights into nuclear structure, akin to seeing individual blades of grass from a high altitude.
Toward a Nuclear Future
Although this is not yet a nuclear clock that works, it is an important step toward developing one that has the potential to be extremely stable and portable. The utilization of thorium contained in a solid crystal and the nucleus's decreased susceptibility to outside perturbations open the door to the possibility of creating reliable and small timepieces.
Imagine a wristwatch that would not lose a second even if you left it running for billions of years. While we are not quite there yet, this research brings us closer to that level of precision.
Jun Ye, Physicist, National Institute of Standards and Technology
Co-authors include JILA, the Vienna Center for Quantum Science and Technology, and IMRA America Inc.
Journal Reference:
Zhang, C., et al. (2024) Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock. Nature. doi.org/10.1038/s41586-024-07839-6.