Laser experiment by physicists excites atomic nucleus and could lead to new type of atomic clock

For nearly 50 years, physicists have dreamed of the secrets they might discover by using a laser to raise the energy state of an atom’s nucleus. This discovery would replace today’s atomic clocks with a nuclear clock that would be the most accurate clock ever, enabling advances such as deep-space navigation and communication. It would also allow scientists to measure precisely whether the fundamental constants of nature are really constants, or whether they appear to be constants simply because we haven’t yet measured them precisely enough.

Now, a project led by Eric Hudson, a professor of physics and astronomy at UCLA, has accomplished the seemingly impossible. By embedding a thorium atom in a highly transparent crystal and bombarding it with lasers, Hudson’s group has managed to get the thorium atom’s nucleus to absorb and emit photons just like an atom’s electrons do. The astonishing feat is described in a paper published in the journal Physical Exam Letters.

This means that measurements of time, gravity, and other fields currently made using atomic electrons can be made with much greater precision. This is because atomic electrons are influenced by many factors in their environment, which affect how they absorb and emit photons and limit their precision. Neutrons and protons, on the other hand, are bound and highly concentrated in the nucleus and experience fewer environmental disturbances.

With this new technology, scientists could determine whether fundamental constants, such as the fine-structure constant that determines the strength of the force holding atoms together, vary. Clues from astronomy suggest that the fine-structure constant may not be the same everywhere in the universe or at all times. A precise measurement of the fine-structure constant using the nuclear clock could completely rewrite some of these most fundamental laws of nature.

“The nuclear forces are so strong that they mean 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 larger 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 ‘constant variation’ to date, and it is likely that no experiment in the next 100 years will be able to rival it.”

Hudson’s group was the first to propose a series of experiments to stimulate crystal-doped thorium-229 nuclei using a laser. They have spent the last 15 years working to achieve the results they recently published. Getting neutrons in the atomic nucleus to react to laser light is challenging because they are surrounded by electrons, which react readily to light and can reduce the number of photons able to reach the nucleus. A particle that has increased its energy level, for example by absorbing a photon, is said to be in an “excited” state.

The UCLA team embedded atoms of thorium-229 in a transparent crystal rich in fluorine. Fluorine can form particularly strong bonds with other atoms, suspending them 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 needed 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, making it possible to detect and measure the excitation of the nuclei.

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 induce 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 says the new technology could be used anywhere extreme precision is required in sensing, communications and navigation. Existing electron-based atomic clocks are room-sized devices with vacuum chambers to trap atoms and associated cooling equipment. A thorium-based nuclear clock would be much smaller, more robust, more portable and more accurate.

“Nobody gets excited about clocks 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 run our cell phones and our GPS.”

Beyond commercial applications, new nuclear spectroscopy could unlock some of the universe’s greatest mysteries. Sensitively measuring the nucleus of an atom opens new avenues for understanding its properties and interactions with energy and the environment. This 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 that level.”

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

“For many decades, increasingly precise measurements of fundamental constants have helped us 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 deputy director of the NSF’s Mathematical and Physical Sciences Directorate.

“This core-based technique could one day allow scientists to measure certain fundamental constants with such precision that we may have to stop calling them ‘constants.'”