Researchers in Vienna and Beijing report nuclear clock “milestone”

Two papers published recently in Nature report what the journal describes as "the first implementation of a nuclear clock, a milestone in the field of metrology." The research behind both papers involve the radioisotope thorium-229.

"A thorium-229 optical nuclear clock with feedback loop," by Luca Toscani de Col et al., describes research led by a team from the Vienna Center for Quantum Science and Technology in Austria. "A nuclear clock synchronized to ²²⁹Th," by Beichen Huang et al., describes research by a team from Tsinghua University and the Beijing Academy of Quantum Information Sciences in China.

Atomic and nuclear clocks: Nuclear clocks can be considered a special type of atomic clock. Regular atomic clocks, which were first demonstrated in the late 1940s, function by using lasers or microwaves to cause electrons to jump back and forth between different energy levels in the "shells" that surround atomic nuclei. By contrast, nuclear clocks work by using lasers to switch between quantum energy states of an atomic nucleus.

By tuning a laser to the "resonant frequency" of that nuclear transition and then counting the number of times that the laser light passes a given point in one second, extremely precise measurements of the nuclear resonant frequency can be made, potentially resulting in the most stable and precise timekeeping device ever created. Because the nucleus of Th-229 has an unusually low resonant transition frequency, a clock using this isotope would be exceptionally precise.

Beijing research: Physicist Shiqian Ding of Tsinghua University noted that his group and the Vienna-based research team "worked independently and reached operating thorium-229 nuclear clocks at the same time, using different experimental approaches. I think this is very encouraging because it shows that the concept is robust and not dependent on one particular technical implementation."

Vienna research: Physicist Thorsten Schumm, who was part of the team from the Vienna Center for Quantum Science and Technology, said that "a nuclear clock was something that physicists dreamt of for almost 50 years. In my team, we have been working toward this goal since 2008."

While there are similarities between the two teams' approaches, the Vienna researchers used thorium crystals at higher concentration and with higher-quality optical properties for their nuclear clock, while the Beijing team used an especially strong laser in their clock.

Dark matter experiment: Schumm and his colleagues in Vienna applied their nuclear clock technology in a physics experiment involving dark matter, an invisible form of matter that does not emit, absorb, or reflect light and that is thought to make up most of the mass of galaxies. The experiment demonstrated that the nuclear clock performed at least as well as current atomic clocks.

"We use the nuclear clock to constrain models of ultralight dark matter by searching for periodic fluctuations and slow drifts in the nuclear transition energy, on timescales between 20 s and 1 day. Benefitting from the enhanced sensitivity of the thorium-229 transition, these constraints compete with the best atomic clocks concerning dark matter coupling to photons and go beyond previous measurements regarding coupling to the strong force," the paper said.

Practical applications: The extreme precision and stability of a functional nuclear clock would have a number of practical applications, including in global navigation satellite systems, cellular and fiber-optic communications, synchronization of data transfer, and surveying and metrology. Such clocks would also be very useful in advancing fundamental physics research, as suggested by the dark matter experiment.

The researchers in both Vienna and Beijing plan to continue working to improve their nuclear clocks, because, as Schumm said, the technology remains "far from its target performance." He added that the two teams will also continue to collaborate in their research reports-as they did in their simultaneous Nature publications-and combine their findings to achieve the best possible nuclear clock.