The World's First Nuclear Clocks Arrive — Two Teams, Two Cities, One 50-Year Dream

Researchers in Vienna and Beijing independently built operating clocks that keep time by probing the nucleus of thorium-229 instead of an atom's electrons. They do not yet beat the best atomic clocks — but they open a new window on fundamental physics.

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Researchers in Vienna and Beijing independently built operating clocks that keep time by probing the nucleus of thorium-229 instead of an atom's electrons.
They do not yet beat the best atomic clocks — but they open a new window on fundamental physics.

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Physicists have dreamed of a clock built on an atomic nucleus for nearly 50 years. Now two teams — one in Vienna, one in Beijing — have independently built working versions of it, describing their results in papers published in Nature.

Rather than counting the jumps of electrons between energy levels in an atom's shell, as caesium- and strontium-based atomic clocks do, the new devices watch protons and neutrons shift energy levels inside the nucleus of thorium-229. Because the nucleus is far smaller than the electron cloud around it, its transitions are far less disturbed by stray electric and magnetic fields — the theoretical recipe for a more accurate timekeeper.

Both teams trapped thorium-229 in solid crystals of calcium fluoride and probed the 148-nanometre nuclear transition with vacuum-ultraviolet lasers. They reached operating clocks at the same time using different experimental approaches. 'I think this is very encouraging because it shows that the concept is robust and not dependent on one particular technical implementation,' said physicist Shiqian Ding of Tsinghua University, co-leader of the Beijing effort.

The nuclear clocks do not yet outperform the best conventional atomic clocks, which can run for billions of years gaining or losing a single second. Thorsten Schumm of TU Wien, who has pursued the idea since 2008, called the current device 'far from its target performance' — but noted that the two designs are complementary: Vienna's crystals have a higher thorium concentration and better optical properties, while Beijing's laser is stronger.

The prizes are practical and fundamental. Ultraprecise clocks underpin satellite navigation, the synchronisation of data networks and metrology. A nuclear clock could be built smaller and sturdier than today's instruments. And its sensitivity makes it a tool for fundamental physics: the Vienna team used its clock in a search for dark matter. The experiment detected none, but the clock performed at the level of the best atomic clocks — a promising sign for an instrument that physicists say gives 'access to a whole new physics universe'.