VIENNA — Two separate research teams have successfully operated clocks based on an atomic nucleus for the first time. One team, led by Professor of quantum metrology Thorsten Schumm, submitted a paper describing their nuclear clock to arXiv.org on June 3, while a second paper on the other nuclear clock was submitted to the same platform on June 7.

Both nuclear clock designs involve calcium fluoride crystals doped with thorium that are probed by a laser. Thorium-229 is the only atomic nucleus known to have an energy transition suitable for laser-based timekeeping. The researchers implemented a feedback loop to continuously readjust the laser frequency based on measurement outcomes. "In some types of measurements, we are already outperforming all of the atomic clocks," Schumm said.

The nuclear clocks do not currently exceed the timekeeping accuracy of the most precise atomic clocks. The concept of a thorium-based nuclear clock was initially proposed in 2003. Researchers utilized the nuclear clock to search for ultralight dark matter, finding no evidence of its presence. Nuclear clocks are less sensitive to stray electromagnetic fields compared to optical atomic clocks.

The nuclear clock operated continuously for 24 hours at room temperature. Continuous-wave lasers, operating at a precise ultraviolet wavelength, enabled the researchers to excite the thorium-229 nucleus and implement a functional feedback loop. "With the continuous lasers, we can measure the nucleus in absorption and get an immediate response, whether the laser is still at the right frequency and correct it back if it is not," Schumm said. Schumm's team used a calcium fluoride crystal with a higher thorium concentration, while the other team used a higher-power laser.

"This is an outstanding result," said theoretical physicist Victor Flambaum. "This is only the first step. The race for building super-accurate nuclear clocks just started." "I have always been optimistic about the success of this project," said physicist Ekkehard Peik. "I am certain that this momentum will continue and that a great deal of interesting research is only just beginning." "Essentially all modern theories beyond the standard model predict additional particles or fifth forces which can be probed with the nuclear clock in some parameter regime," Schumm said.