Atomic clocks are the pinnacle of precise timekeeping, used to define the second and incorporated in GPS and telecommunications networks. But perhaps not for much longer. By precisely zooming in on a specific energy transition in an atomic nucleus, researchers have come closer than ever before to building a new kind of timekeeper: a nuclear clock. Such devices could surpass today’s most advanced atomic clocks in accuracy and stability, which has scientists excited to use them as probes of dark matter and other fundamental physics questions.
The new work, published today in Nature, is the first time researchers have gotten all the ingredients of a nuclear clock “working at the same time,” says study co-author Jun Ye, a physicist at JILA, a joint research center funded by the National Institute of Standards and Technology, and the University of Colorado Boulder.
“That’s really a substantial step forward that they made,” says Ekkehard Peik, a physicist at PTB, Germany’s national metrology lab. “It’s amazing how this field has accelerated and gained momentum,” says Sandro Kraemer, a physicist at the Catholic University of Leuven.
Atomic clocks rely on energy transitions in an atom’s cloud of electrons. A microwave or laser of a specific frequency is used to nudge the electrons to their higher energy state. Those electron transitions serve to stabilize the laser or microwave oscillations, which provide the “ticks” of a clock—billions or even trillions of them per second.
Nuclear clocks offer the potential for improvement, because coaxing the protons and neutrons in a nucleus to jump up in energy requires laser light of a much higher frequency, allowing time to be measured with much finer ticks. More important, atomic nuclei are not as sensitive as electrons to external electric and magnetic fields, making a nuclear clock inherently more stable. But most atomic nuclei have excitation energies far too high for any laser to attain.
Thorium, however, has an anomalously low-energy transition, and in 2003, Peik and his PTB colleague Christian Tamm proposed it as the basis of a clock. “We’re very lucky that nature gave us this nucleus,” Kraemer says.
To develop a laser that could excite the nucleus, however, the researchers needed to know exactly what thorium-229’s excitation energy was. And it wasn’t until last year that a team at CERN, Europe’s particle physics laboratory, detected the photons emitted by thorium-229 as it decayed from its excited state to its ground state and measured their energies: about 8.4 electronvolts. Earlier this year, Peik and his colleagues improved on that measurement by building an ultraviolet laser to bump thorium-229 to an excited state, then detecting the light emitted as it relaxed about 10 minutes later, further pinning down the excitation energy to 8.35574 electronvolts—and determining the associated laser frequency to be about 2020 terahertz.
That’s still not precise enough to make an accurate clock. So Ye and his colleagues decided to refine the search using a laser frequency comb, which generates light at 100,000 stable, finely separated frequencies—the “teeth” of the comb.
The JILA researchers trained the frequency comb on a crystal of calcium fluoride the size of a grain of sand, in which hundreds of trillions of thorium-229 atoms had been embedded. They slowly shifted the comb’s lines, scanning a range of frequencies. It was close to midnight one day in May when Chuankun Zhang, a JILA graduate student, saw that one of the laser comb lines had hit, triggering the thorium to fluoresce with a telltale flash of photons. He alerted his colleagues, who all rushed back into the lab. “No one could sleep,” he says.
The frequency comb helped the researchers identify the laser transition frequency about a million times more precisely than Peik’s team had. And because that frequency line was perfectly connected to others on the comb, they could connect the laser to one of the world’s most accurate atomic clocks, which is based on strontium. This link provided key data about the nuclear clock’s performance and helped the team probe the nuclear transition. In the future, the link could help integrate nuclear clocks to existing timekeeping systems. But to develop an operational clock, the laser would need to be stabilized with the nuclear transition. Zhang says doing this would be “trivial,” but would not yet make for a more accurate clock.
In theory, a clock based on thorium atoms trapped in crystal would be more stable and portable than existing atomic clocks, which use electromagnetic traps and additional laser systems to cool and hold atoms in place. Such a clock could be used to probe gravity’s effect on time—clocks separated by just 1 millimeter have been shown to tick at different rates because of tiny differences in the gravity they experience. The exquisite timekeepers could look for slight variations that might be caused by passing gravitational waves or the magma shifting underground ahead of volcanic eruptions.
Another tantalizing physics application for a nuclear clock would be to hunt for candidate particles of dark matter, the unseen stuff thought to make up 85% of the mass in the universe. Many models propose ultralight dark matter particles that would interact directly with the strong nuclear force, the attraction that binds protons and neutrons together in nuclei. If these particles were to interact with thorium nuclei, they would disrupt the transition frequency, throwing off a clock in a detectable way. “Here for the first time we have direct access to the strong sector,” says Elina Fuchs, a theoretical physicist at Leibniz University Hannover. “So far, this window was closed or only indirectly accessible, and now it’s wide open.”
The work is already revealing nuclear behavior in unprecedented detail. The JILA measurements provided evidence that atomic nuclei such as thorium unexpectedly swell and shrink as they move between the excited and ground states. “Nuclear physics has not been the subject of very precise measurements, just because we do not have that capability,” Ye says.
Now that is changing, all thanks to humble clocks. “The easiest thing to measure as a physical quantity is a frequency,” Ye says. “And that’s the reason why clocks are the best humanmade scientific instruments.”
More: https://www.science.org/content/article/breakthrough-promises-new-era-ultraprecise-nuclear-clocks
