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USTC Researchers Achieve Breakthrough in Optical Clock Technology, Paving the Way for Redefining the Second

A research team at the University of Science and Technology of China, led by Pan Jianwei, Chen Yua, and Dai Hannin, has successfully developed a strontium atomic optical lattice clock with a stability and uncertainty both superior to 5×10-18 (equivalent to an error of not exceeding one second in billions of years).

This system is not only the best comprehensive indicator optical clock in China but also positions China as the second country, after the United States, to achieve such comprehensive indicators. The results have been published in the important international journal “Metrologia.”

Currently, the most advanced optical clocks have an accuracy more than two orders of magnitude higher than the microwave fountain clocks used internationally for defining seconds.

Based on the development of quantum precision measurement technology, the 27th International Conference on Weights and Measures has passed a resolution on the “Future Redefinition of the Second,” planning to propose a specific route for redefining the International System of Units (SI) “second” using optical clocks by 2026 and making a final decision in 2030.

To promote the new generation of second definitions based on optical clocks, it is required that the uncertainty of at least three different laboratories’ optical clocks be superior to 2×10-18, achieving a frequency ratio measurement accuracy superior to 5×10-18 through optical links or mobile optical clocks.

Building on previous work, the research team achieved laser cooling of strontium atoms (87Sr) and confined them in a long-lived one-dimensional optical lattice. They used a laser pre-locked to an ultra-stable cavity to probe the clock transition of strontium atoms and realized closed-loop operation of the optical clock.

Frequency ratio measurements were carried out with two independent strontium atomic optical lattice clocks, reaching a stability of 4×10-18 over 10,000 seconds integration time and 2.1×10-18 over 47,000 seconds. The team also assessed the systematic frequency shift factors of the Sr 1 clock item by item, resulting in a system uncertainty of 4.4×10-18, equivalent to a deviation of only 1 second in 7.2 billion years.

These performance indicators indicate that the optical clock system has partially met the requirements for the redefinition of the “second.”

The achievement lays important technical foundations for future long-distance optical clock comparisons, establishing ultra-high precision optical frequency standards, and building a global optical clock network. It holds significant value for future construction of a new generation of global time standards and provides new methods for gravitational wave detection and dark matter searches.

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