A nuclear clock prototype hints at ultraprecise timekeeping 

Scientific clockmakers have crafted a prototype of a nuclear clock, hinting at future possibilities for using atomic nuclei to perform precise measurements of time and make new tests of fundamental physics theories.

While the definition of a “clock” is scientifically hazy, the prototype is not yet used to measure time. So it technically should be called a “frequency standard,” physicist Jun Ye says. But the work brings scientists closer to a nuclear clock than ever before. “For the first time, all essential ingredients for a working nuclear clock are contained in this work,” says Ye, of JILA in Boulder, Colo. 

Whereas atomic clocks measure time based on electrons jumping between energy levels in atoms, nuclear clocks’ timekeeping would depend on the energy levels of atomic nuclei. A certain frequency of laser light is needed for an atom or an atomic nucleus to make such a jump. The wiggling of that light’s electromagnetic waves can be used to mark time. 

Nuclear clocks would keep time using a variety of the element thorium, called thorium-229. Most atomic nuclei make energy leaps that are too large to be triggered by a tabletop laser. But thorium-229 has two energy levels that are close enough to each other that the transition between those two levels could serve as a clock. 

Now, researchers have precisely determined the frequency of the light needed to set off that jump. It’s 2,020,407,384,335 kilohertz, Ye and colleagues report in the Sept. 5 Nature.

Importantly, the measurement has an uncertainty of 2 kilohertz. That’s more than a million times the precision of the best previous measurement. And it’s more than a billion times the precision to which that frequency was known just over a year ago, highlighting multiple back-to-back developments. 

The improvement hinged on a component called a frequency comb (SN: 10/5/18). A crucial component of many atomic clocks, a frequency comb creates an array of discrete frequencies of light. Using a frequency comb with thorium-229 has been a major research goal, for some scientists (SN: 6/4/21). In the new work, Ye and colleagues compared the nuclear clock transition with that of an atomic clock with a known frequency. 

“This is something that will be important as a scientific application for tests of fundamental physics,” says physicist Ekkehard Peik of the National Metrology Institute in Braunschweig, Germany, who was not involved with the new research. 

In the future, such comparisons could be used to search for strange physics effects, such as drifting of the values of fundamental constants (SN: 11/2/16). These are numbers that — as the name implies — are believed to be eternally unwavering.