For almost 50 years, physicists have been intrigued by the potential discoveries that could be uncovered by elevating the energy state of an atom’s nucleus with a laser. This achievement could lead to the development of a nuclear clock, surpassing the accuracy of current atomic clocks, and facilitating advancements in deep space navigation and communication.
Additionally, it could enable scientists to precisely measure the constants of nature, potentially revealing whether they are truly constant or if our previous measurements have been insufficient.
Recently, Eric Hudson and his team at UCLA successfully embedded a thorium atom within a highly transparent crystal and used lasers to stimulate the thorium nucleus to absorb and emit photons, akin to the behavior of electrons in an atom.
The improved technology allows for measurements of time, gravity, and other fields with significantly higher accuracy than current measurements using atomic electrons. This is because the influence of various environmental factors on atomic electrons affects their absorption and emission of photons, limiting their accuracy. In contrast, neutrons and protons are tightly bound within the nucleus and experience minimal environmental disturbance.
With this new technology, scientists may have the ability to investigate potential variations in fundamental constants, such as the fine-structure constant, which determines the strength of the force holding atoms together.
Astronomical clues indicate that the fine-structure constant may not be constant throughout the universe or across different points in time. The fine-structure constant’s precise measurement using the nuclear clock has the potential to fundamentally redefine some of the most basic laws of nature.
“Nuclear forces are so strong it means the energy in the nucleus is a million times stronger than what you see in the electrons, which means that if the fundamental constants of nature deviate, the resulting changes in the nucleus are much bigger 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 no experiment for the next 100 years will rival it.”
Hudson’s team put forth the initial proposal for a set of experiments aimed at exciting thorium-229 nuclei embedded in crystals using a laser. They have dedicated the past 15 years to achieving the recently published findings. The challenge lies in exciting neutrons in the atomic nucleus with laser light due to their interaction with surrounding electrons, which readily respond to light and can diminish the number of photons capable of reaching the nucleus. When a particle elevates its energy level, such as through photon absorption, it is described as being in an “excited” state.
The researchers at UCLA placed thorium-229 atoms inside a transparent crystal that is abundant in fluorine. Fluorine has a strong ability to create bonds with other atoms, trapping the atoms and revealing the nucleus like a fly caught in a spider’s web. The electrons were tightly bound to the fluorine, requiring a high amount of energy to excite them, which allowed low-energy light to reach the nucleus.
The thorium nuclei were able to absorb these photons and emit them back, enabling the detection and measurement of nucleus excitation. By altering the photon energy and observing the frequency of nucleus excitation, the team managed to determine the energy of the excited nuclear state.
“We have never been able to drive nuclear transitions like this with a laser before,” Hudson said. “If you hold the thorium in place with a transparent crystal, you can talk to it with light.”
Hudson expressed that the new technology has potential applications in fields requiring extreme precision in timekeeping, such as sensing, communications, and navigation. Current electron-based atomic clocks are large, requiring vacuum chambers to trap atoms and cooling equipment. In contrast, a thorium-based nuclear clock would be much smaller, more robust, more portable, and more accurate.
In addition to commercial uses, the new nuclear spectroscopy has the potential to unveil some of the universe’s greatest mysteries. Precise measurement of an atom’s nucleus provides a new approach to understanding its properties and interactions with energy and the environment. As a result, scientists will be able to test some of their most fundamental concepts about matter, energy, and the laws of space and time.
The research was funded by the U.S. National Science Foundation.
“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 assistant director of NSF’s Mathematical and Physical Sciences Directorate, which provided funding for the research. “This nucleus-based technique could one day allow scientists to measure some fundamental constants so precisely that we might have to stop calling them ‘constant.'”
Journal reference:
- R. Elwell, Christian Schneider, Justin Jeet, J. E. S. Terhune, H. W. T. Morgan, A. N. Alexandrova, H. B. Tran Tan, Andrei Derevianko, and Eric R. Hudson. Laser Excitation of the 229Th Nuclear Isomeric Transition in a Solid-State Host. Physical Review Letters, 2024; DOI: 10.1103/PhysRevLett.133.013201