Physicists’ laser experiment excites the nucleus of an atom, may enable a new type of atomic clock

For nearly 50 years, physicists have dreamed of the secrets they could unlock by raising the energy state of an atomic nucleus with a laser. This achievement would allow the replacement of today’s atomic clocks with nuclear clocks that would be the most accurate ever and would enable advances such as deep space navigation and communication. It would also allow scientists to accurately measure whether the fundamental constants of nature are in fact constant, or merely appear to be because we haven’t yet measured them precisely enough.

Now, an effort led by Eric Hudson, a professor of physics and astronomy at UCLA, has accomplished the seemingly impossible. By inserting a thorium atom into a highly transparent crystal and bombarding it with lasers, Hudson’s group managed to get the thorium atom’s nucleus to absorb and emit photons like electrons in the atom. This amazing feat is described in an article published in the magazine Physical Review Letters.

This means that measurements of time, gravity and other fields that are currently made using atomic electrons can be made with orders of magnitude higher precision. This is because 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 in the nucleus and are less of a burden on the environment.

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, are different. Hints from astronomy suggest that the fine structure constant may not be the same everywhere in the universe or at all points in time. A precise nuclear clock measurement of the fine structure constant could completely rewrite some of these most fundamental laws of nature.

“Nuclear forces are so strong that it means 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 larger and more noticeable.” making measurements orders of magnitude more sensitive,” Hudson said.

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

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

The UCLA team embedded thorium-229 atoms in a transparent fluorine-rich crystal. Fluorine can form particularly strong bonds with other atoms, suspending 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 it would take to excite them was very high, allowing lower energy light to reach the nucleus. The thorium nuclei could then absorb these photons and re-emit them, allowing detection and measurement of the excitation of the nuclei.

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

“Never before have we been able to control nuclear transitions like this with a laser,” 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 use wherever extreme timing accuracy is required in sensing, communication and navigation. Current electron-based atomic clocks are room-sized devices with vacuum chambers to trap the atoms and associated cooling equipment. Thorium-based nuclear clocks would be much smaller, more robust, more portable and more accurate.

“No one is excited about the hours because we don’t like the idea of ​​limited time,” 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.”

Beyond commercial applications, the new nuclear spectroscopy could pull back the curtains on some of the universe’s greatest mysteries. Sensitive measurement of the atomic nucleus opens a new way to learn about its properties and interactions with energy and 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’s really going on in space,” Hudson said. “What we can observe from our limited perspective is a conglomeration of effects on different scales of size, time and energy, and the constants of nature we have formulated seem to hold at this level.

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

“For many decades, increasingly accurate measurements of fundamental constants have allowed us to better understand the universe at all scales and, in turn, develop new technologies that grow our economy and strengthen our national security,” said Denise Caldwell, acting associate director of NSF’s Mathematical and Physical Science Directorate.

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