Physicists’ laser experiment excites atom’s nucleus, could enable new kind of atomic clock

For nearly 50 years, physicists have dreamed of the secrets they could unlock by raising the energy state of an atom’s nucleus using a laser. The achievement would allow today’s atomic clocks to be replaced with a nuclear clock that would be the most accurate clock ever, allowing for advances such as navigation and communication in deep space. It would also allow scientists to measure precisely whether the fundamental constants of nature are, in fact, truly constant or merely appear to be because we have not yet measured them with sufficient precision.

Now, an effort led by Eric Hudson, professor of physics and astronomy at UCLA, has achieved the seemingly impossible. By inserting a thorium atom inside a highly transparent crystal and bombarding it with a laser, Hudson’s group succeeded in making the thorium atom’s nucleus absorb and emit photons just as electrons in an atom do. The astonishing achievement is described in a paper published in the journal Physical review papers.

This means that measurements of time, gravity and other fields that are currently made using atomic electrons can be made with higher precision. The reason is that atomic electrons are affected by many factors in their environment, which affects how they absorb and emit photons and limits their accuracy. Neutrons and protons, on the other hand, are bound and highly concentrated within the nucleus and experience less environmental disturbance.

Using the new technology, scientists may be able to determine whether fundamental constants, such as the fine structure constant, which determines the strength of the force that holds atoms together, change. Hints from astronomy suggest that the fine structure constant may not be the same everywhere in the universe or at all instants of time. Accurate measurement using the nuclear clock of the fine structure constant could completely rewrite some of these most fundamental laws of nature.

“The nuclear force is so strong that it means that the energy in the nucleus is a million times stronger than what you see in electrons, which means that if the fundamental constants of nature deviate, the resulting changes in the nucleus are much more larger and more visible, making measurement orders more sensitive,” Hudson said.

“Using a nuclear clock for these measurements will provide the most sensitive test of ‘continuous change’ to date, and it is likely that no experiment for the next 100 years will rival it.”

Hudson’s group was the first to propose a series of experiments to stimulate thorium-229 doped nuclei in crystals with a laser, and has spent the past 15 years working to achieve the newly published results. Getting neutrons in the atomic nucleus to react to laser light is challenging because they are surrounded by electrons, which react readily to the light and can reduce the number of photons actually able to reach the nucleus. A particle that has raised its energy level, such as through the absorption of a photon, is said to be in an “excited” state.

The UCLA team embedded thorium-229 atoms inside a transparent fluorine-rich crystal. Fluorine can form particularly strong bonds with other atoms, suspending the atoms and exposing the nucleus like a fly in a spider’s web. The electrons were so tightly bound to fluorine that the amount of energy needed to excite them was very high, allowing lower-energy light to reach the nucleus. The thorium nuclei can then absorb these photons and re-emit them, allowing the excitation of the nuclei to be detected and measured.

By changing the energy of the photons and monitoring the rate at which the nuclei are excited, the team was able to measure the energy of the nuclear excited state.

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

Hudson said the new technology could find uses wherever extreme accuracy is required in sensing, communication and navigation. Existing electron-based atomic clocks are room-sized contraptions with vacuum chambers to capture the atoms and associated cooling equipment. A thorium-based nuclear clock would be much smaller, stronger, more portable and more accurate.

“No one gets excited about the hours because we don’t like the idea of ​​time being limited,” he said. “But we use atomic clocks all the time every day, for example, in the technologies that make our cell phones and GPS work.”

Above and beyond commercial applications, the new nuclear spectroscopy could pull back the curtains on some of the universe’s greatest mysteries. Sensitively measuring the nucleus of an atom opens up a new way to learn about its properties and interactions with energy and the environment. This, in turn, will allow scientists to test some of their most fundamental ideas about matter, energy, and the laws of space and time.

“Humans, like most life on Earth, exist at scales either too small or too large to observe what might actually be happening in the universe,” Hudson said. “What we can observe from our limited perspective is a conglomeration of effects at different scales of size, time and energy, and the constants of nature that we have formulated seem to hold at this level.

“But if we could observe more precisely, these constants might actually change. Our work has taken a big step toward these measurements, and, in any case, I’m sure we’ll be surprised by what we learn.”

“For many decades, increasingly precise measurements of fundamental constants have allowed us to better understand the universe at all scales and then develop new technologies that grow our economy and strengthen our national security,” said Denise Caldwell, Acting of the position of NSF Assistant Director for Mathematics and Physics. Directorate of Sciences.

“This nuclear-based technique may one day allow scientists to measure some fundamental constants so precisely that we may have to stop calling them ‘constants’.”