Tsinghua Team's Nuclear Clock Proves Six Times More Stable Than Vienna's

A Chinese nuclear clock developed by a team at Tsinghua University in Beijing has proven to be about six times more stable than a device built by researchers at the Vienna University of Technology, according to the first results from the world's first two working nuclear clocks, published in new papers in Nature and reported by the South China Morning Post.

Keeping time with an atomic nucleus

The Chinese and Austrian teams independently used thorium-229 nuclei embedded in crystals. Unlike conventional atomic clocks, which track transitions of electrons between energy levels, the new devices rely on the rhythm of the atomic nucleus itself. Ding Shiqiang, who heads the Chinese group, explained that a nucleus is far smaller than an atom and is less affected by external electric and magnetic fields, adding that this could eventually allow nuclear clocks to reach extremely high levels of precision.

The Chinese team also tested two separately grown crystals, and their readings matched to within about three parts in 10 trillion. The measured signal also agreed with earlier data from a group led by physicist Jun Ye at the JILA research institute in the United States, which had used different crystals and equipment.

Vienna's search for dark matter

The Vienna team used its clock in an attempt to search for dark matter, though it turned up no result. Researchers also found that the signal frequency shifted slightly depending on which part of the crystal the laser passed through, meaning the device kept a slightly different time each time it was switched on.

For both clocks to operate, ultraviolet laser light has to pass through a thorium-229 crystal, tuned to produce maximum absorption by the nuclei. The Chinese team generated that radiation by passing laser beams through cadmium vapor heated to 600 degrees Celsius. For now, both nuclear clocks remain less stable than the best existing atomic clocks, but the result is seen as a significant step in moving nuclear clocks from a laboratory concept toward practical use.