Physicists’ laser experiment excites atomic nucleus, potentially enabling new type of atomic clock

For nearly 50 years, physicists have dreamed of unlocking the secrets of increasing the energy state of the nucleus of an atom using a laser. This achievement would make it possible to replace today’s atomic clocks with a nuclear clock that would be the most accurate clock ever made, leading to 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 so because we have not yet measured them accurately enough.

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

This means that measurements of time, gravity, and other fields currently performed using atomic electrons can be performed with an order of magnitude higher accuracy. The reason for this 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 their surroundings.

Using the new technology, scientists may be able to determine whether fundamental constants, such as the fine-structure constant that determines the strength of the force holding atoms together, vary. Evidence from astronomy suggests that the fine-structure constant may not be the same everywhere in the universe or at all times. Accurately measuring 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 the energy in the nucleus is a million times stronger than what you see in the electrons. That means that when the fundamental constants of nature are off, the resulting changes in the nucleus are much larger and more dramatic, making measurements many times more sensitive,” Hudson said.

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

Hudson’s group was the first to propose a series of experiments to excite thorium-229 nuclei doped in crystals with a laser, and has been working for the past 15 years to achieve the recently published results. Getting neutrons in the nucleus to respond to laser light is a challenge because they are surrounded by electrons, which respond quickly to light and can reduce the number of photons that can actually reach the nucleus. A particle that has increased its energy level, say by absorbing a photon, is said to be in an “excited” state.

The UCLA team placed thorium-229 atoms in a transparent crystal rich in fluorine. 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 needed to excite them was very high, allowing lower-energy light to reach the nucleus. The thorium nuclei could then absorb and re-emit these photons, 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 excited nuclear state.

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

Hudson said the new technology could be used wherever extreme precision in timekeeping is needed in sensing, communications and navigation. Existing electron-based atomic clocks are room-sized devices with vacuum chambers to contain atoms and cooling devices. 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 constraints,” he said. “But we use atomic clocks all day long, for example in the technologies that make our cell phones and GPS work.”

In addition to commercial applications, the new nuclear spectroscopy could bring some of the universe’s greatest mysteries to light. Sensitive measurements of an atom’s nucleus open 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 forms on Earth, exist at scales that are far too small or far too large to observe what is really going on 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 carefully, these constants could actually vary. Our work has taken a big step toward making these measurements, and in any case, I’m sure we’ll be surprised by what we learn.”

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

“With this nucleus-based technique, scientists may one day be able to measure some fundamental constants so precisely that we may no longer need to call them ‘constants.'”