Chinese scientists develop working nuclear optical clock

(ECNS) -- Chinese scientists have developed a working nuclear clock, a milestone in improving and safeguarding the precise timekeeping vital to the digital systems that underpin modern life.

The team, led by Shiqian Ding, an associate professor at Tsinghua University's Department of Physics and a jointly appointed researcher at the Beijing Academy of Quantum Information Sciences, used a domestically developed 148 nm continuous-wave ultra-narrow-linewidth laser source and thorium-229 trapped in calcium fluoride crystals to make the breakthrough.

The work was published online in the journal Nature on Wednesday. An independent team in Vienna also reported a thorium-229 optical nuclear clock in a separate Nature paper.

A nuclear clock is a new type of timekeeping system that measures time using vacuum ultraviolet lasers to induce transitions in the thorium-229 nucleus and employing frequency comb technology to count the oscillations of the laser's electromagnetic field. In theory, its measurement precision could exceed 10⁻¹⁹, meaning it would lose or gain no more than one second over 300 billion years, according to information published on Tsinghua University's official WeChat account.

While current optical atomic clocks already achieve extremely high precision, they are susceptible to electromagnetic interference and can only maintain stable operation under controlled laboratory conditions.

Nuclear optical clocks promise even higher precision and greater resistance to environmental perturbations, but progress has been hindered by the lack of a suitable 148 nm continuous-wave laser source.

The team proposed a new approach using four-wave mixing in metal vapor and achieved continuous-wave output at 148 nm. They reduced the laser's linewidth by nearly six orders of magnitude, laying the technical foundation for ultra-stable lasers in the vacuum ultraviolet band and achieving a world first in this field.

Potential applications include satellite navigation, deep-space exploration, precision surveying and synchronization of communications and other technological systems that depend on highly accurate time and frequency references, according to the team.

(By Zhang Dongfang)