Researchers have long dreamed of the secrets they could unlock by raising the energy state of an atom’s nucleus using a laser. Accomplishing this would allow the atomic clocks of today to be replaced with a nuclear clock with far greater accuracy. Doing so, however, is no easy task. The electrons surrounding the nucleus react easily with light, which means a great deal of light is needed to reach the nucleus and increase its energy state. Led by Eric Hudson, a professor of physics and astronomy at University of California, Los Angeles, a research team has successfully raised the energy state of an atomic nucleus. The work paves the way for advancements in a broad range of topics, from deep space navigation to understanding longstanding questions in physics. By embedding a thorium atom within a highly transparent crystal and bombarding it with lasers, Hudson’s group has succeeded in getting the nucleus of the thorium atom to absorb and emit photons like electrons in an atom do. The work is 15 years in the making. The group had proposed a series of experiments to stimulate thorium-229 nuclei doped into crystals with a laser. When trapped in a transparent, fluorine-rich crystal, scientists can use a laser to excite the nucleus of a thorium-229 atom. Courtesy of James Terhune/Hudson Group/UCLA. The current work saw the team embed thorium-229 with a transparent crystal rich in fluorine. Fluorine can form especially strong bonds with other atoms, suspending the atoms and exposing the nucleus like a fly in a spider web. Because the electrons bind so tightly with fluorine, the amount of energy it takes to excite them is very high. This allows the lower energy light to reach the nucleus. The thorium nuclei could 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. “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.” With the new technology, measurement of time, gravity, and other fields that are currently performed using atomic electrons can be made with orders of magnitude higher accuracy. The reason is that atomic electrons are influenced 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 within the nucleus and experience less environmental disturbance. Once the technology is deployed in a nuclear clock, scientists may be able to determine if fundamental constants, such as the fine-structure constant which sets the strength of the force that holds atoms together, vary. Hints from astronomy suggest that the fine-structure constant might not be the same everywhere in the universe or at all points in time. Precise measurement using the nuclear clock of the fine-structure constant could completely rewrite some of these 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.” According to Hudson, the technology could find uses wherever extreme precision in timekeeping is required in sensing, communications, and navigation. Existing atomic clocks based on electrons are room-sized contraptions with vacuum chambers to trap atoms and equipment associated with cooling. A thorium-based nuclear clock would be much smaller, more robust, more portable, and more accurate. The research was published in Physics Review Letters (www.doi.org/10.1103/PhysRevLett.133.013201).