Ellen Phiddian
Cosmos science journalist
“Imagine a wristwatch that wouldn’t lose a second even if you left it running for billions of years,” says Professor Jun Ye, referring to the first “nuclear clock” which could keep time even more accurately than state-of-the-art atomic clocks.
“While we’re not quite there yet, this research brings us closer to that level of precision,” says Ye, a physicist at the US National Institute of Standards and Technology (NIST) and JILA, a joint research institute of NIST and the University of Colorado Boulder, who is part of the international team of scientists which has constructed the clock.
While at present it currently only rivals the precision of atomic clocks, the new nuclear clock could be refined to be much more accurate – allowing new barriers to be crossed in fundamental physics, and further enhancing technology like GPS and high-speed internet.
The Austrian, German and US-based researchers published their breakthrough in Nature.
Atomic clocks use lasers to make electrons jump between energy levels inside atoms.
The electrons generate ultra-regular frequencies as they jump, which allows the clock to track time very closely. Most atomic clocks use caesium atoms, but some strontium-based clocks have provided even more accurate results.
The nucleus of an atom is about 100,000 times smaller than the whole atom. Physicists have theorised for decades that tracking the energy levels of the nucleus could provide an even more accurate clock, but so far it’s been difficult to achieve.
This group of researchers focussed on thorium-229 atoms, because the nuclei of these atoms can be prompted to jump levels with a relatively low amount of energy, needing only ultraviolet (UV) light.
“Thorium nuclei have two states of very similar energy, so you can switch them with lasers,” says Professor Thorsten Schumm, a physicist at the Vienna University of Technology, Austria.
“But for this to work, you have to know the energy difference between these two states very precisely.
“For many years, research teams around the world had been searching for the exact value of this energy difference in order to be able to switch thorium nuclei in a targeted manner – we were the first to succeed.”
The Austrian team grew crystals out of calcium fluoride in which to embed the thorium atoms. They were able to measure the atoms with a UV laser developed by German researchers.
The US researchers trialled this crystal in a strontium-based atomic clock housed at JILA.
The strontium clock uses infrared light rather than UV light. The team used xenon gas to convert infrared light in their ultra-precise “frequency comb” laser into UV light, which could then hit the thorium atoms.
The resulting prototype nuclear clock is not more accurate than existing atomic clocks, but it has the potential to become so.
“With this first prototype, we have proven thorium can be used as a timekeeper for ultra-high-precision measurements. All that is left to do is technical development work, with no more major obstacles to be expected,” says Schumm.
Schumm expects nuclear clocks will overtake atomic clocks in accuracy in 2-3 years.
An accompanying Nature News & Views article, written by physicists not involved with the research, calls the “impressive feat” a “result of a truly global collaboration”.
“Zhang and colleagues’ astonishing achievement therefore promises many fascinating future finds — and caps three decades of fantastic research,” write the researchers.
