UCLA Physicists Recently Designed Thorium Films with Ultra Precision

To achieve unparalleled precision in timekeeping, researchers have begun exploring nuclear clocks. In contrast to optical atomic clocks, which depend on electronic transitions, nuclear clocks leverage energy shifts within an atom’s nucleus. This distinction allows nuclear clocks to be less influenced by external factors, indicating that they may offer superior accuracy compared to any prior technology. 

The development of such clocks has encountered significant challenges; thorium-229, an essential isotope for nuclear clocks, is both rare and radioactive, making it exceedingly expensive to procure in the necessary quantities.

However, according to a recent study published in Nature, UCLA physicists have devised an innovative approach that dramatically reduces both the radioactivity and cost associated with nuclear clocks. Their method involves producing thin films of thorium tetrafluoride (ThF4), which renders these clocks a thousand times less radioactive and far more economical.

Advancement in Atomic Clock Technology

Last summer, physicists at UCLA achieved a significant breakthrough by embedding the nucleus of a thorium-229 atom within a transparent crystal, enabling it to absorb and emit photons in a manner similar to atomic electrons. This accomplishment has resolved decades of speculation regarding the feasibility of such an experiment. 

By utilizing a laser to elevate the energy state of an atom’s nucleus, or to excite it, researchers could pave the way for the creation of the most precise atomic clocks to date, facilitating unparalleled measurements of time and gravitational forces. This advanced atomic clock has the potential to redefine certain fundamental principles of physics.

The Need for Improved Atomic Clocks and Time Keeping

Thorium-229-doped crystals, while crucial, present two challenges: their rarity and radioactivity. However, a recent study published in Nature by a collaborative team of chemists and physicists from UCLA proposes a potential solution. They have developed thin films derived from a thorium-229 precursor, which significantly reduces the required amount of thorium-229 and exhibits radioactivity levels comparable to that of a banana. 

The team demonstrated that these films enable the same laser-driven nuclear excitation necessary for operating a nuclear clock. Furthermore, the production process for these films can be scaled up, making them suitable not only for nuclear clocks but also for various quantum optics applications.

The Development of  Thin Thorium Films

Rather than incorporating a pure thorium atom into a fluorine-based crystal, the new technique utilizes a dry nitrate source of thorium-229 that is dissolved in ultrapure water and transferred into a crucible via pipetting. Upon the addition of hydrogen fluoride, a few micrograms of thorium-229 precipitate are generated, which are then extracted from the water and heated until they evaporate, condensing in an irregular manner on transparent sapphire and magnesium fluoride surfaces.

Light from a vacuum ultraviolet laser system was directed at the targets, where it excited the nuclear state as reported in earlier UCLA research, and the subsequent photons emitted by the nucleus were collected.

“A key advantage to using a parent material — thorium fluoride — is that all the thorium nuclei are in the same local atomic environments and experience the same electric field at the nuclei,” said co-author and Charles W. Clifford Jr. professor of chemistry and biochemistry, and professor of materials science and engineering at UCLA, Anastassia Alexandrova. “This makes all thorium exhibit the same excitation energies, making for a stable and more accurate clock. In this way, the material is unique.”

Potential Applications and Advantages 

With the use of this new technology, researchers might gain the ability to investigate whether fundamental constants—such as the fine-structure constant, which governs the strength of the force binding atoms—fluctuate. Observations in astronomy have hinted that the fine-structure constant may not be uniform across the universe or consistent throughout time. By employing a nuclear clock to measure this constant with high precision, scientists could potentially challenge and redefine some of the most essential laws of nature.

Current atomic clocks that utilize electrons are large devices, often occupying entire rooms, and require vacuum chambers to confine atoms along with cooling apparatus. In contrast, a thorium-based nuclear clock would be significantly smaller, more durable, portable, and precise.

Beyond commercial uses, the innovative nuclear spectroscopy has the potential to unveil some of the universe’s greatest enigmas. By enabling sensitive measurements of an atom’s nucleus, this approach provides a novel means to investigate its characteristics and interactions with energy and the surrounding environment. Consequently, this advancement will allow scientists to evaluate some of their most fundamental theories regarding matter, energy, and the principles governing space and time.

Conclusion

In conclusion, UCLA’s research on thorium films not only addresses the limitations of current atomic clock technology but also opens new avenues for scientific exploration and technological advancement. The implications of this work could resonate across multiple fields, from quantum optics to cosmology, underscoring the importance of continued innovation in atomic measurement techniques.