November 1, 2023
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Nuclear clocks could break timekeeping records. Now physicists are learning how to build one.
From satellite navigation to GPS, the world runs on ultra-precise timekeeping, usually based on atomic clocks. These devices use energy sources, such as lasers tuned to specific frequencies, to excite electrons orbiting atomic nuclei. Electrons jump or “transition” to a higher energy level before falling back to a lower one at rapid, regular time intervals – the “tick” of an atomic clock.
But even atomic clocks aren’t perfect, because environmental factors can affect how electrons bounce around. As our technological tools require more and more precision, physicists are coming up with a possible solution: moving timekeeping inside the nucleus, which is isolated from such interference, exciting protons and neutrons instead of electrons. Because protons and neutrons are relatively dense, a “nuclear clock” would require much more powerful tuned lasers and a very particular type of atom. Now groundbreaking measurements of the isotope thorium 229, recently published in Naturesuggest that a practical nuclear clock could finally be within our reach.
While the best current atomic clocks move back one second every 100 million years, nuclear clocks would move back one second every 31.7 billion years (more than double the age of the universe), explains the study’s lead author, Sandro Kraemer. . This improved precision could lead to advances in time measurement, nuclear physics, and quantum sensor technology used for satellite navigation and telecommunications. “It will instantly improve nuclear physics measurements by a (factor of) trillion to quadrillion,” says José R. Crespo López-Urrutia, a scientist at Germany’s Max Planck Institute for Nuclear Physics, who was not involved in the new measurements.
In 2003, physicists first suggested that a synthetic isotope called thorium-229 could hold the key to measuring nuclear time. In theory, thorium-229 nuclear particles could transition to an excited state with an exceptionally low amount of energy, making it the only isotope that current laser technology could feasibly excite for a nuclear clock. “Most nuclear transitions (of elements) have very large energies in the range of thousands or millions of electron volts,” which is beyond the capabilities of even the most modern lasers, says Adriana Palffy, a physicist at the University from Würzburg in Germany, who was also not involved in the new work.
In the study, a team of physicists from CERN’s nuclear physics facility, ISOLDE, detected and measured the nuclear transition of thorium 229 for the first time. At 8.3 electron volts, the transition would be small enough to be activated by a specially tuned laser. Physicists are now developing lasers to run the thorium clock, says Piet Van Duppen, spokesman for the ISOLDE team and professor at the Institute of Nuclear and Radiation Physics at KU Leuven in Belgium. “Once the resonance (between thorium 229 and these new lasers) is observed,” says Van Duppen, “we will make a big leap forward.”
