West LA Times

Physicists have long sought to unlock the secrets of raising the energy state of an atom’s nucleus using a laser. This breakthrough could enable the development of nuclear clocks, which would be more accurate than current atomic clocks and facilitate advances in deep space navigation and communication. It would also allow scientists to measure whether fundamental constants of nature are indeed constant or simply appear so due to imprecise measurements.

Eric Hudson, a professor of physics and astronomy at UCLA, led an effort that has achieved this goal. By embedding thorium atoms within a highly transparent crystal and bombarding them with lasers, Hudson’s team succeeded in getting the nucleus of thorium atoms to absorb and emit photons similarly to how electrons do. This achievement is detailed in a paper published in Physical Review Letters.

Hudson explained that this development means measurements of time, gravity, and other fields currently performed using atomic electrons can now be made with significantly higher accuracy. Unlike atomic electrons, which are influenced by environmental factors affecting their absorption and emission of photons, neutrons and protons within the nucleus experience less disturbance.

The new technology may help determine if fundamental constants such as the fine-structure constant vary across different parts of the universe or over time. Precise measurement using nuclear clocks could lead to revisions of some basic laws of nature.

“Nuclear forces are so strong it means the energy in the nucleus is a million times stronger than what you see in the electrons,” Hudson said. “Using a nuclear clock for these measurements will provide the most sensitive test of ‘constant variation’ to date.”

Hudson's group proposed experiments to stimulate thorium-229 nuclei doped into crystals with lasers 15 years ago. Achieving this required overcoming challenges posed by electrons surrounding neutrons in an atomic nucleus, which readily react to light and reduce photon reach. The team embedded thorium-229 atoms within fluorine-rich transparent crystals; fluorine forms strong bonds that suspend atoms and expose their nuclei like flies caught in spider webs.

“We have never been able to drive nuclear transitions like this with a laser before,” Hudson stated. “If you hold the thorium in place with a transparent crystal, you can talk to it with light.”

The new technology promises applications requiring extreme precision in timekeeping for sensing, communications, and navigation. Current room-sized atomic clocks based on electrons could be replaced by smaller, more robust, portable thorium-based nuclear clocks.

“Nobody gets excited about clocks because we don’t like the idea of time being limited,” Hudson remarked. “But we use atomic clocks all the time every day, for example, in technologies that make our cell phones and GPS work.”

Beyond commercial uses, this new nuclear spectroscopy could reveal mysteries about our universe through sensitive measurements of an atom’s nucleus properties and interactions with energy and environment.

“Humans exist at scales either far too small or far too large to observe what might really be going on in the universe,” Hudson noted. “What we can observe from our limited perspective is a conglomeration of effects at different scales... If we could observe more precisely, these constants might actually vary!”

The research was funded by the U.S. National Science Foundation (NSF). Denise Caldwell from NSF highlighted its significance: “This nucleus-based technique could one day allow scientists to measure some fundamental constants so precisely that we might have to stop calling them ‘constant.’”