Atomic clocks that utilize a laser beam to excite the thorium-229 nucleus within a transparent crystal have the potential to provide unparalleled precision in measuring time and gravity, which may even lead to reevaluating certain fundamental laws of physics. However, thorium-229-doped crystals are limited in availability and are radioactive. A thin film made from a dry thorium-229 precursor exhibits similar nuclear excitation as the crystal, but its affordability, lower radioactivity, and smaller dimensions make it a more scalable option for producing compact, cost-effective, and portable atomic clocks.
Atomic clocks that utilize a laser to excite the thorium-229 nucleus embedded in a transparent crystal hold promise for achieving the most precise measurements of time and gravity, potentially leading to a reexamination of foundational physics principles. Thorium-229-doped crystals are rare and radioactive. However, a thin film constructed from a dry precursor of thorium-229 can produce the same nuclear excitation as the crystal, yet is more affordable, less radioactive, and smaller, making the production process simpler for creating portable atomic clocks.
This summer, physicists at UCLA accomplished the groundbreaking task of getting thorium-229 atoms in a transparent crystal to absorb and emit photons, similar to how atomic electrons function, which resolved long-standing uncertainties regarding the feasibility of such an achievement. By using a laser to elevate the energy state of an atom’s nucleus—known as nuclear excitation—scientists can develop the most precise atomic clocks available, leading to more accurate measurements of time and gravity. These advancements could even revise some basic principles of physics.
However, there is a challenge: the thorium-229-doped crystals are both scarce and radioactive. A recent study published in Nature by a UCLA team of chemists and physicists introduces a solution with the creation of thin films derived from a thorium-229 precursor that utilizes much less thorium-229 and has radioactivity levels comparable to that of a banana. The team demonstrated that these films can achieve the necessary laser-driven nuclear excitation crucial for constructing a nuclear clock. The production of such films could be enhanced for use in nuclear clocks and various quantum optics applications.
Instead of embedding pure thorium within a fluorine-based crystal, this innovative method involves dissolving a dry nitrate precursor of thorium-229 in ultrapure water and placing it into a crucible. The process includes adding hydrogen fluoride to yield a few micrograms of thorium-229 precipitate, which is then separated from the water and heated until it evaporates and gathers unevenly on transparent surfaces made of sapphire and magnesium fluoride.
The team directed light from a vacuum ultraviolet laser at the prepared targets to excite the nuclear state, as detailed in prior UCLA research, and collected the photons emitted from the nucleus afterward.
“One key advantage of using thorium fluoride as a precursor material is that all thorium nuclei exist within the same local atomic environment and thus experience identical electric fields,” said co-author Anastassia Alexandrova, a professor of chemistry, biochemistry, and materials science and engineering at UCLA. “This uniformity ensures consistent excitation energies across all thorium atoms, resulting in a more stable and accurate clock. This characteristic makes the material remarkable.”
Central to every clock is an oscillator. Clocks measure time by calculating how long it takes for the oscillator to complete a designated number of oscillations. For example, in a grandfather clock, a second might correspond to the pendulum swinging once; for a quartz watch, it typically counts around 32,000 vibrations of the crystal.
In a thorium nuclear clock, a second is defined as approximately 2,020,407,300,000,000 excitation and relaxation cycles of the nucleus. This significantly higher ticking rate can enhance the clock’s precision, as long as the ticking rate remains stable; any variations could lead to time miscalculations. The thin films discussed in this research provide a stable environment for the nucleus that can be easily produced and has the potential for use in microfabricated devices. This breakthrough could lead to broader applications of nuclear clocks, as it makes them more affordable and easier to manufacture.
Currently existing atomic clocks, based on electron transitions, are large, vacuum-equipped devices designed to trap atoms and manage cooling systems. Conversely, a thorium-based nuclear clock would be considerably smaller, sturdier, more portable, and remarkably precise.
Moreover, beyond commercial uses, the new nuclear spectroscopy techniques may uncover answers to some of the universe’s greatest mysteries. Advanced measurements of an atom’s nucleus provide new avenues to explore its properties and interactions with energy and the environment. Consequently, this development allows scientists to critically evaluate some of their core theories regarding matter, energy, and the very laws that govern space and time.
