World’s First Ultra-Precise Nuclear Clock Is Within Reach After Major Breakthrough, Researchers Say

Clocks are the bedrock of modern physics and technology, as they allow scientists to assess fundamental scientific theories, and they make GPS and telecommunications possible. The current gold standard for ultra-precise timekeeping is the cesium atomic clock, where the tiny and fixed energy transitions of the atom’s electrons are used to keep track of time. Now, research described earlier this month has flung open the door to a new generation of timekeepers that promises to be even more precise: nuclear clocks.

“Nuclear physics has not been the subject of very precise measurements, just because we do not have that capability,” study senior author Jun Ye, a physicist at the University of Colorado Boulder, tells Science’s Jay Bennet. His team’s study, published in the journal Nature, signals a new chapter for physicists with the world’s first thorium clock prototype.

“This paper is an incredible technical achievement,” Hannah Williams, a physicist at Durham University in England who was not involved in the research, tells Quanta Magazine’s Joseph Howlett.

Humans have long kept time to regular phenomena, from the waxing and waning of the moon to the lulling swings of the pendulum in a grandfather clock. Higher frequency oscillatory events, such as energy transitions of a single particle, refine time measurement and bring greater precision to timekeeping.

Consider the atom, the basic building block of matter that’s made up of a nucleus surrounded by a cloud of mobile electrons. The atomic clock’s lynchpin is the electron—to tell time, researchers use a laser to coax the electrons to jump back and forth between two specific energy levels.

Now, consider the atomic nucleus—the tiny, static core of jam-packed neutrons and protons, around which the electron cloud swarms. Like the electron, the nucleus, too, has energy levels that it can toggle between. This dense heart contains nearly all the atom’s mass but takes up only about one-100,000th the space of the whole atom. Shielded by the electron cloud, the nucleus is less susceptible to noises in the environment. So, nuclear transitions could theoretically offer a much more exact way to keep time compared to the electron.

The challenge in fashioning a clock out of an atomic nucleus is that the energy required to excite it is much larger than for an electron. This energy requirement is usually in the ultra-high-frequency range of Gamma rays—beyond the reach of common lasers.

A single element on the periodic table bucks this trend: thorium-229, which harbors two similar energy levels of its nucleus—close enough to each other that the gap can be bridged with an ultraviolet beam. “In the whole chart of all the nuclei, it’s the only one,” Eric Hudson, a physicist at the University of California, Los Angeles, who wasn’t involved in the study, tells Quanta. But first, researchers needed to pinpoint the exact amount of energy capable of exciting the thorium nucleus. And that value has been elusive and fuzzy.

In the past year or so, researchers around the world made significant advances in the decades-long pursuit of narrowing down the laser energies required for spurring thorium-229, writes Science. Last year, a team in Europe measured this energy gap to be 8.4 electronvolts. A few months later, a different team in Germany winnowed down the uncertainty, declaring the prerequisite energy trigger to be 8.35574 electronvolts. Despite the improvements, scientists still needed a more precise measurement to build a nuclear clock.

In the new study, the authors further refined the search using a frequency comb, a specialized laser that acts as a measuring stick for the frequency of light. The instrument that Ye’s team used could generate 100,000 discrete frequencies of light, like the fine teeth of a comb.

The team trained the laser comb onto thorium atoms embedded in calcium fluoride grains and scanned for the telltale frequency.

Finally, close to midnight in May, Ye’s graduate student Chuankun Zhang caught the sign that the thorium-229 nucleus had crossed between the two energy states. “No one could sleep,” he tells Science. According to Quanta, several other group members joined Zhang in the lab that night to celebrate, and the team took a commemorative selfie at nearly four o’clock in the morning.

Thanks to the frequency comb, Ye’s team had boosted the measurement precision of thorium’s transition by a million times. While still not as accurate as the current record-holder for precision—an optical clock made from strontium atoms—the thorium prototype presents all the ingredients for building the world’s first nuclear clock. “This research brings us closer to that level of precision,” Ye tells Cosmos’ Ellen Phiddian.

A nuclear clock could pave the way for measuring the fundamental constants of physics to a precision that an atomic clock never could. Fundamental constants—such as the speed of light—define how our universe works according to the principles of physics, but they might not be as steadfast as their name suggests. Some researchers have proposed that these values could drift over time as the underlying phenomena that they describe shift slightly, per Quanta. The same physics also dictates thorium’s transition energies, making the isotope a handy gauge to detect and calibrate these miniscule shenanigans of our physical world.

Such a precise timekeeping device could measure the tiny ways that gravity alters time or detect gravitational waves. It could even shed light on dark matter—the mysterious, invisible substance that makes up roughly 27 percent of the universe—because interaction with dark matter would alter the frequency needed to excite thorium-229’s nucleus.

But making all that possible will require cutting down the uncertainty of thorium’s transition by at least another tenfold. “All that is left to do is technical development work,” study co-author and Vienna Center for Quantum Science and Technology physicist Thorsten Schumm tells Cosmos. He expects that nuclear clocks will overtake atomic clocks in precision within two to three years.

With no more major hurdles in sight, it’s now simply a countdown to the discoveries that come only with nuclear timekeeping.

“Now the fun starts,” Hudson tells Quanta. “We can actually do this stuff.”

Shi En Kim | | READ MORE

Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.