For the first time, scientists have successfully used a tabletop laser to elevate an atomic nucleus to a higher energy state. This remarkable achievement sets the stage for developing the first nuclear clock, which would mark time based on the inner workings of atomic nuclei.

Olga Kocharovskaya, a physicist at Texas A&M University in College Station who was not involved in the research, hails this as a "remarkable breakthrough." Compared to current atomic clocks—the most precise timekeepers—nuclear clocks could be simpler, more portable, and capable of testing fundamental physics theories in novel ways. With this new result, a nuclear clock seems more attainable than ever. "We know now that it’s conceptually feasible," says physicist Peter Thirolf of Ludwig-Maximilians-Universität München in Germany, who also was not part of the study.

Atomic clocks operate based on the physics of the electrons surrounding atoms. Electrons inhabit specific energy levels, and to elevate an electron to a higher level, a laser must provide the exact amount of energy, corresponding to a specific frequency of light. Scientists fine-tune a laser’s frequency to achieve this transition and use that frequency as a precise timekeeping standard.

Nuclear clocks, on the other hand, would utilize the transitions of atomic nuclei rather than electrons. Most atomic nuclei have energy levels too far apart for a laser to induce the jump. However, thorium-229 is a notable exception, with an energy jump accessible by lasers.

Until recently, the precise energy required for thorium-229's transition was unknown. In 2023, scientists measured it with unprecedented precision. This new knowledge allowed researchers to take the next crucial step: using a laser to elevate thorium-229 nuclei to a higher energy level and observing the emitted light as the nuclei returned to their original state. The experiment confirmed the transition energy as 8.35574 electron volts, consistent with the 2023 measurement but with about 800 times greater precision. For a nuclear clock, even higher precision will be necessary.

Physicist Ekkehard Peik of the National Metrology Institute of Germany in Braunschweig, who first proposed nuclear clocks with thorium-229 in 2003, expressed excitement at the success. “It was a long search,” he says.

In this experiment, thorium-229 was embedded in a calcium fluoride crystal, differing from atomic clocks that contain atoms in a vacuum chamber. The use of solid materials in nuclear clocks adds to their appeal, suggesting the possibility of simpler and more portable systems. "Potentially you could imagine building a much simpler, portable system, taking this clock out of the lab," says physicist Jun Ye of JILA in Boulder, Colo., who was not involved in the new research.