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

Now, an effort led by Eric Hudson, a professor of physics and astronomy at UCLA, has achieved the seemingly impossible. By embedding a thorium atom in a highly transparent crystal and bombarding it with lasers, Hudson’s group was able to make the nucleus of the thorium atom absorb and emit photons, just as the electrons in the atom do. The amazing achievement is described in an article published in the journal Physical examination 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. 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 in the nucleus and experience less disturbance from 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, vary. 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 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 large and more noticeable, making the measurements an order of magnitude more sensitive,” Hudson said.

“Using a nuclear clock for these measurements will provide the most sensitive test of ‘permanent variation’ 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 nuclei doped in crystals with a laser, and has spent the past 15 years working toward the newly published results. Getting neutrons in an atomic nucleus to respond to laser light is challenging because they are surrounded by electrons that react readily to light and can reduce the number of photons that can actually reach the nucleus. A particle that has raised its energy level, such as 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 the atoms and exposing the nucleus like a fly in a spider’s web. The electrons were so tightly bound to the fluorine that the amount of energy required 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.

“Never before have we been able to drive nuclear transitions like this with a laser,” Hudson said. “If you hold thorium in place with a clear crystal, you can talk to it with light.”

Hudson said the new technology could find applications anywhere extreme timing precision is required in sensors, communications and navigation. Existing electron-based atomic clocks are room-sized facilities with vacuum chambers to trap 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 watches 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.”

Beyond commercial applications, the new nuclear spectroscopy could pull back the curtains on some of the universe’s greatest mysteries. Sensitive measurement of an atom’s nucleus 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 too small or too large to observe what might really be going on in the universe,” Hudson said. “What we can observe from our limited vantage point is a conglomeration of effects on 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 vary. Our work has taken a big step toward these measurements, and, either way, 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 subsequently develop new technologies that grow our economy and strengthen our national security,” said Denise Caldwell, Acting Asst. -Director of the Mathematics and Physics Division of the NSF Science Directorate.

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