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A strontium-based optical clock. The Chinese team said their clock “partially met the requirement for participating in the redefinition of the second”. Photo: Science Photo Library

Chinese scientists build ultra-precise clock they hope will help redefine the second

  • Strontium-based optical clock loses or gains one second every 7 billion years
  • China is just the second country after the US to achieve such precise timekeeping

A Chinese team has developed an extremely precise optical clock that loses or gains one second every 7 billion years, bringing scientists a step closer to redefining the second as a basic unit of time.

Using ultra-cold strontium atoms and powerful laser beams, the team at the University of Science and Technology of China created a clock with stability and uncertainty under 5 quintillionths.

The achievement makes China only the second country in the world after the US to achieve such precise timekeeping.

It also lays an important foundation for the establishment of a global optical clock network, the team, led by physicist Pan Jianwei, wrote in the peer-reviewed journal Metrologia earlier this month.

In addition, they said it opened up new pathways to test fundamental physics theories, detect gravitational waves and search for dark matter.

Physicist Pan Jianwei led the team of researchers in Hefei, Anhui province. Photo: Xinhua

The current record holder for the most precise strontium-based optical clock is hosted at the University of Colorado in Boulder, developed by a group led by Chinese-American physicist Jun Ye. It remains slightly more accurate than its Chinese competitor and its operation is more stable.

Other players in the race include the University of Tokyo and the Institute of Physical and Chemical Research in Japan, and the National Metrology Institute of Germany.

Optical clocks hold great potential for applications in critical infrastructure in the future. They could significantly enhance the precision of global navigation satellite systems, and help build highly secure communication networks based on quantum key distribution.

They could also improve the synchronisation and efficiency of power grids, and even play a vital role in national defence and security.

Today, the definition of a second is based on the microwave fountain clock, a type of atomic clock. It works by releasing cesium atoms upwards, which then fall under gravity in a fountain-like motion while they are excited with microwave pulses. Their electrons then absorb and emit light particles to jump between different energy levels.

By counting such cycles as “ticks” that mark fractions of a second, scientists can achieve high-precision timekeeping with stabilities of several quadrillionths.

But the precision of a microwave clock is limited by the microwave frequency standard. In recent years, researchers have built optical clocks which use laser light to drive electronic transitions and achieve performance that is two orders of magnitude better than their microwave counterparts.

However, if microwave clocks are to be replaced by optical ones for the future definition of time, at least three laboratories in the world will need to have an optical clock with stability below 5 quintillionths and uncertainty below 2 quintillionths.

They are the two key parameters for the performance and reliability of an optical clock. Instability measures how much the clock’s frequency fluctuates over time, while uncertainty represents the degree of confidence in the frequency measured by the clock.

The race against time to build the world’s most precise clock

In their work, the Chinese team led by Pan – who has been dubbed the “father of quantum” – first cooled down strontium-87 atoms to a temperature of a few micro-Kelvin, and trapped them in a one-dimensional lattice created with intersecting laser beams.

They then used an ultra-stable laser to interact with the trapped strontium-87 atoms and trigger the so-called clock transition, which is highly stable and precise.

The researchers also made frequency comparison measurements between two independent clocks to reveal that an individual clock’s stability was about 2.2 quintillionths.

The whole-system uncertainty was 4.4 quintillionths – equivalent to the deviation of one second every 7.2 billion years, the team concluded.

“Such performances showed that our clock has partially met the requirement for participating in the redefinition of the second,” they wrote in the paper.

The team plans to carry out comparisons between optical clocks built with different atom species, such as strontium-87 and ytterbium-171.

Their research was supported by the Ministry of Science and Technology and Anhui province, along with other funding agencies, and built on previous work on the quantum simulation of ultra-cold atoms.