UV Lasers, Combs Provide High-Frequency Light for Nuclear Clock

Nuclear clocks, which measure time by using the energy jumps within an atom’s nucleus, could improve timekeeping, navigation, and internet speeds, and could advance physics research. They have the potential to be more accurate than atomic clocks, and are less sensitive to external disturbances like stray magnetic fields.

An international team of JILA-led scientists demonstrated the key components of a nuclear clock, including the precise frequency measurements of an energy jump in the nucleus of a thorium-229 (Th-229) atom. While the team’s laboratory demonstration is not a fully developed nuclear clock, it contains all the key technology for one.

A powerful laser shines into a jet of gas, creating a bright plasma and generating UV 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 (Th-229) nucleus, which is the core of a future nuclear clock. Courtesy of Chuankun Zhang/JILA.

A much higher frequency of light is needed to cause energy jumps in nuclei than what is needed to induce energy jumps in atomic clocks. Most atomic nuclei need to be hit by coherent X-rays, a high-frequency form of light with energies greater than what current technologies can produce, to make energy jumps.

To measure time in a nuclear clock, the researchers used Th-229, an atom whose nucleus has a smaller energy jump than any other known atom. Th-229 nuclei exhibit a low-energy nuclear transition that is within reach of vacuum ultraviolet (VUV) laser light sources.

The researchers built a VUV laser to create precise energy jumps between the individual quantum states of Th-229 nuclei and to precisely measure the frequency of an energy jump in the nuclei, which they embedded in a solid crystal. They used an optical frequency comb to count the number of VUV wave cycles that were executed to create the energy jump.

Using the VUV frequency comb, the researchers excited a Th-229 nuclear clock transition in the solid-state host material and determined the absolute transition frequency. They stabilized the fundamental frequency comb to the JILA strontium (Sr) atomic clock and coherently upconverted the fundamental to its seventh harmonic in the VUV range by using a femtosecond enhancement cavity. The VUV comb established a frequency link between the nuclear and electronic energy levels and allowed the researchers to measure the frequency ratio of the Th-229 nuclear clock transition and compare it to the Sr atomic clock.

A nuclear clock works by using UV light to excite the nucleus of a special atom, likeTh-229. When the light hits the nucleus at just the right frequency, it causes the nucleus to change its energy state, like flipping a tiny switch. By precisely measuring and counting these energy flips, scientists can create an extremely accurate timekeeping device. Courtesy of N. Hanacek/NIST.t

By comparing the frequency of the nuclear clock transition to the optical frequency used in the JILA Sr atomic clock — one of the most accurate atomic clocks in the world — the researchers established a direct frequency link between a nuclear transition and an atomic clock. To the best of the team’s knowledge, it is the first to establish a direct frequency connection between Th-229 nuclei and an existing atomic clock.

The team, which included researchers at the Vienna Center for Quantum Science and Technology and IMRA America Inc. as well as JILA, thus created all the essential components of a nuclear clock. With these components, the team achieved a level of precision one million times higher than the previous wavelength-based measurement. The direct frequency link and increase in precision are critical to developing a nuclear clock and integrating it with existing timekeeping systems.

Atomic clocks currently provide the primary time standard used to regulate clocks internationally. They are used in GPS navigation and internet synchronization and to provide timestamping for financial transactions.

The development of nuclear clocks could lead to more precise navigation systems, faster internet speeds, more reliable network connections, and more secure digital communications.

Beyond everyday technology, nuclear clocks could improve testing of fundamental scientific theories about how the universe works, opening the door to potential new discoveries in physics. For example, nuclear clocks could help detect dark matter or help determine if the constants of nature are truly constant. They could enable theories in particle physics to be verified without the need for large-scale particle accelerator facilities.

Although a functioning nuclear clock is still in the future, the team has made valuable progress toward creating a portable, highly stable nuclear clock. The research has already yielded results, including the ability to observe details in the shape of the Th nucleus that have never been observed before. The use of Th-229 embedded in a solid crystal, combined with the Th-229 nucleus’s resilience to external disturbances, makes compact, robust nuclear timekeeping devices a possibility.

“Imagine a wristwatch that wouldn’t lose a second even if you left it running for billions of years,” Jun Ye, a physicist at JILA and the National Institute of Standards and Technology, said. “While we’re not quite there yet, this research brings us closer to that level of precision.”

The research was published in Nature (www.doi.org/10.1038/s41586-024-07839-6).