China Scientists Build Nuclear Clock in Quantum Breakthrough

Chinese scientists have developed a working nuclear optical clock, a milestone in precision timekeeping that extends quantum measurement from the electron shell to the atomic nucleus. The achievement, led by researchers at Tsinghua University, was reported by the state-run Xinhua News Agency on October 8 and published online in the journal Nature the previous evening.

The device uses the energy-level transitions of thorium-229 nuclei, embedded in a calcium fluoride crystal, as its frequency reference rather than the electron transitions that drive conventional atomic clocks. According to Xinhua, the team said it established a complete technical system spanning the core light source, the clock crystal, nuclear transition spectroscopy and closed-loop operation.

A Pendulum Inside the Nucleus

Atomic clocks, the standard for global timekeeping since the 1950s, work by measuring the precise “ticking” of electrons as they shift between energy states when stimulated by a laser. A nuclear clock instead tracks transitions inside the nucleus itself.

Ding Shiqian, an associate professor in Tsinghua’s physics department and a jointly appointed researcher at the Beijing Academy of Quantum Information Sciences, offered a vivid analogy.

“An atomic clock uses the transitions of electrons between specific energy levels inside an atom as its frequency reference - like listening to the beat of electrons,” he said. “A nuclear optical clock instead puts the pendulum inside the atomic nucleus and uses transitions between different nuclear energy levels to measure time.”

Because the nucleus is thousands of times smaller than the atom and sits deep within it, it is far less vulnerable to external electromagnetic interference. Researchers say this could allow nuclear clocks to provide a more accurate and more practical time-frequency reference than today’s atomic clocks.

Overcoming Two Bottlenecks

The team had to solve two central problems. The first was the lack of a suitable laser to drive the thorium-229 transition, which requires vacuum-ultraviolet light at roughly 148 nanometers. After more than four years of work, Ding’s group produced a continuous-wave VUV laser, described as the first of its kind in the world.

The second hurdle was the extreme scarcity of thorium-229. According to Xinhua, the team and its collaborators grew a millimeter-scale, high-quality thorium-doped calcium fluoride crystal using just 1.4 micrograms of the isotope - about one two-hundredth of the amount used by a competing European team.

With the light source and crystal in place, the researchers locked the laser frequency to the nuclear transition. They reported second-level stability nearly an order of magnitude better than the contemporaneous European effort.

A Two-Race Finish

The Chinese result was not achieved in isolation. According to ScienceAlert, two independent teams - one in Europe, led by the Technical University of Vienna, and one in China, led by Tsinghua’s Beichen Huang - built the first functional nuclear clocks, both initially circulating their findings as arXiv preprints in June 2026.

The European team’s paper describes a stand-alone device that continuously stabilized a laser frequency and was compared against an established ytterbium-ion atomic clock. Its clock reached a fractional frequency instability of 3x10^-12/sqrt(t), approaching 10^-15 over a day of continuous operation, and was also used to set new constraints on models of ultralight dark matter.

The Chinese team’s paper reports a fractional frequency instability of 5x10^-13/sqrt(t). Critically, the group measured the clock transition in two independently fabricated crystals and found the frequencies agreed at the 10^-13 level - addressing the risk that the crystal environment could force per-device calibration. That result, ScienceAlert noted, suggests nuclear clocks could eventually become reproducible standards rather than one-off laboratory demonstrations.

In their abstract, the Chinese team wrote that their results “establish laser-addressed nuclei as operational clock references and provide a reproducible solid-state platform for compact nuclear clocks, nuclear quantum sensors and precision tests of fundamental physics.”

Why It Matters

Zhai Hui, director of Tsinghua’s physics department, said the nuclear clock is expected to become a new-generation time-frequency standard and called its development an important breakthrough in quantum precision measurement this century.

“The successful development and stable operation of the nuclear optical clock means humanity’s ability to manipulate quantum systems has reached the scale of the atomic nucleus,” he said, “opening up new research directions for quantum precision measurement and quantum manipulation.”

Because the thorium-229 transition is unusually sensitive to changes in fundamental constants, the clock can also serve as a precision probe for testing physical laws and searching for phenomena such as dark matter. Its solid-state design - nuclei held in a crystal at room temperature - points toward compact, engineering-friendly devices that could support satellite navigation and deep-space exploration.

What to Watch

The new devices do not yet outperform the best atomic clocks, which have a head start of some seven decades. But the simultaneous demonstrations by two teams mark the moment nuclear clocks moved from theoretical proposal - first outlined in 2003 - to working hardware.

A key point remains unresolved. Chinese state media report the Chinese result as published in Nature on October 7, while international coverage in June described both teams’ work as preprints that had not yet been peer reviewed. If the Nature publication is accurate, the October paper represents the formal, peer-reviewed version of work first circulated in June.

Thorsten Schumm, a physicist at the Technical University of Vienna and a co-author of the European paper, has predicted that nuclear clocks could outstrip today’s best atomic clocks within just a few years. An international race that formally began with a 2003 proposal now has two working entrants - and a finish line in sight.