A graduate student from eastern China is at the centre of a major advancement in precision timekeeping, ending a decades-long search for an essential piece of the puzzle of building a nuclear clock.

Zhang Chuankun was listed as first author of a study, published in the journal Nature earlier this month, that could help pave the way for ultra-precise timekeepers.

Zhang’s co-authors include his mentor, the Chinese-American physicist Jun Ye, and a team from the US National Institute of Standards and Technology (NIST) and the University of Colorado Boulder.

Ye, a pioneer in ultra-precise timekeeping credited with building the world’s most accurate atomic clock, said Zhang was among “the very best” students he has worked with.

Zhang, a native of Shandong province, was instrumental in developing a special laser device known as a vacuum ultraviolet (VUV) frequency comb at Ye’s lab at JILA, a joint institute of NIST and CU Boulder.

“An optical frequency comb acts like a ruler for light,” Zhang said. “It allows for extremely accurate frequency measurements by comparing unknown frequencies to a known standard.”

 

His work enabled Ye’s lab, which began developing the comb in 2005, to pinpoint the precise laser frequency needed to trigger a nuclear transition and drive a nuclear clock.

Nuclear clocks promise to measure time more accurately than the best atomic clock, which gains or loses one second every 40 billion years.

Their unprecedented precision would make them a powerful tool for detecting the slightest changes in fundamental constants, such as the speed of light, to test the foundations of physics and shed light on dark matter and dark energy.

Scientists have long sought to build clocks based on energy transitions within an atom’s nucleus. Unlike atomic clocks, which rely on electron transitions, nuclear clocks are less affected by external influences, making them inherently more stable.

The laser frequency needed to excite a nucleus is much higher than that required to excite an electron, allowing for finer “ticks” in time measurement and making nuclear clocks more accurate.

However, generating and controlling these high-energy lasers has been a major challenge.

In the 1990s, scientists discovered that thorium-229 (Th-229), a by-product of Cold War nuclear weapons research, was uniquely suited for laser excitation.

Using a conventional laser to search for the exact frequency of Th-229’s nuclear transition is time-consuming, as it emits light at only a single frequency.

A frequency comb can generate light at hundreds of thousands of finely separated frequencies simultaneously, significantly speeding up the process.

Zhang’s first encounter with frequency combs came during an internship at the Swiss Federal Institute of Technology in Lausanne.

“I was immediately drawn to this magic tool because it combines ultra-short laser pulses with precise frequency control, achieving what traditional lasers cannot,” said Zhang, who won medals in national physics competitions as a high schooler and undergraduate student at Tsinghua University in Beijing.

 

He chose to pursue his PhD in Ye’s lab as it was one of the birthplaces of frequency combs and home to the world’s most accurate clocks.

“What I really like about Chuankun is his unwavering dedication to this experiment,” Ye said.

“It’s been a long six years for his PhD work. He solved one difficult technical problem after another, always with a sense of confidence and a drive to learn more.”

Zhang said some fundamental aspects of the experiment – such as how the Th-229 nuclear transition occurred – remained unclear at first.

“It could be that the nucleus is excited by the laser and then decays, emitting a photon. Alternatively, the laser’s energy might be transferred directly to an electron, causing the electron to escape from the atom,” he said. The team attempted to detect the electron in the latter scenario, but it did not work.

A breakthrough came last year when a team at CERN, Europe’s particle physics lab, confirmed the first scenario, detecting photons emitted as Th-229 decayed from its excited state to its ground state.

In May, European researchers advanced the science by successfully growing a calcium fluoride crystal embedded with hundreds of trillions of Th-229 atoms.

After receiving the sample, Zhang and the team used their state-of-the-art VUV frequency comb to scan through frequencies for two weeks, with each attempt lasting about 10 minutes.

“First, we bathed the sample in ultraviolet light for about 400 seconds. Then we turned off the laser and waited for 200 seconds to see if the photon detector picked up any signals, which would indicate that the nuclear transition had been triggered,” Zhang said.

It was nearing midnight when Zhang noticed that one of the laser comb lines had successfully triggered Th-229 nuclei, causing them to emit a distinctive flash of photons. He immediately called in his teammates to verify the result, and the lab buzzed with excitement.

During a meeting later that morning, Zhang shared a slide to show the long-sought signals and surprise Ye, who had tears in his eyes. They had champagne together to celebrate.

“It’s been almost 20 years, and we’ve finally applied this unique tool, which combines our knowledge in precision metrology, ultra-fast physics, and extreme non-linear optics, to unlock the door for a nuclear clock,” Ye said.

“One would naturally be very emotional, since it’s been such a long journey, and with several generations of PhD students working on this problem, each generation pushing the state of the art forward,” he added.

The team measured the tick rate of the Th-229 nuclear clock to be about four times faster than that of the most precise strontium-based atomic clock, which was also developed by Ye’s lab.

Looking ahead, Ye said that building a fully functional nuclear clock will require many years of painstaking work.

While atomic clocks are also advancing quickly, Ye noted that “there will be an interesting race between atomic and nuclear clocks”.

Zhang said the two types of clocks can complement each other.

He added that it will be important to downsize the lasers of nuclear clocks to make them smaller, cheaper and more practical.

Zhang, who will soon graduate from his PhD programme, said he plans to continue pursuing his passion for precision measurement.

“It’s just so cool when we can measure fundamental aspects of the natural world more and more precisely,” he said.