The nuclear clock breakthrough paved the way for super-precise timekeeping

Synchronising the energy transition of thorium-229 with the tick of the nuclear clock with high-accuracy frequency lines using a frequency comb

Physicists have demonstrated the ingredients of a nuclear clock which keeps time by measuring tiny energy shifts inside an atomic nucleus. New insights into fundamental physics could be led by such a clock.

Researchers measured the frequency of light that causes nuclei of the rare isotope thorium-229 to shift to a higher energy state — the ‘tick’ of the nuclear clock — with an accuracy that is 100,000 times greater than the previous best effort. They did this by synchronising the energy transition with the tick of the world’s most accurate clock. The work was led by Jun Ye at JILA, a research institute in Boulder, Colorado, and published in Nature on 5 September1. “It’s really one of the most exciting papers in recent times,” says Marianna Safronova, an atomic physicist at the University of Delaware in Newark.

The breakthrough came from probing thorium-229 nuclei with a laser device called a frequency comb. The setup is not a clock because it hasn’t been used to measure time. The results of the project make it seem possible.

The JILA team looked at trillions of thorium-229 atoms embedded in a crystal using a system known as a frequency comb. The comb pumps out an array of laser frequency lines, with regular and even spacing. It allows researchers to illuminate the crystal with many precise frequencies at once to look for a match, rather than scanning laboriously through the range of possible options using a single-frequency laser.

Physicist Chuankun Zhang said he felt amazing observing the transition for the first time. “We spent the entire night doing all the tests to check if this is actually really the signal that we were looking for,” he says.

Finding the right kind of atomic nucleus to use and determining the frequency needed to make it shift from one energy state to another has been a half a century of work for physicists. In the 1970s, indirect evidence suggested that thorium-229 had a bizarrely low-energy nuclear transition2 — one that might eventually be triggered by tabletop lasers. But it wasn’t until last year that scientists discovered the required frequency3 — and this year, they successfully initiated shift with a laser4.

honing the laser system is needed. “Fortunately, this amazing technique has high potential,” says Olga Kocharovskaya, a physicist at Texas A&M University in College Station. It’s a “prototype of the source to be used in the future clock”, she adds.

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