Scientists Just Made the Most Accurate Clock Even Better

UCLA physicists have developed a new thin film that uses far less of the rare thorium-229 while also being significantly less radioactive, making it a safer and more practical alternative for atomic clocks.

• Atomic clocks using thorium-229 nuclei excited by laser beams could provide the most precise measurements of time and gravity ever recorded, potentially reshaping fundamental physics.
• Traditional thorium-229-doped crystals are both rare and radioactive, limiting their practicality.
• The newly developed thin film, made from a dry precursor of thorium-229, demonstrates the same nuclear excitation as the crystal but with significant advantages.
• Its lower cost, reduced radioactivity, and smaller size make it easier to produce at scale, paving the way for smaller, more affordable, and highly portable atomic clocks.

Unlocking the Power of Thorium-229

This summer, UCLA physicists achieved a long-sought breakthrough: they successfully made the nucleus of a thorium-229 atom, embedded in a transparent crystal, absorb and emit photons — just like an atom’s electrons do. This achievement confirms what scientists had speculated for decades. By using a laser to excite the nucleus, researchers could develop atomic clocks with unprecedented accuracy, leading to more precise measurements of time and gravity. Such advancements might even challenge some of the fundamental principles of physics.

However, there’s a challenge — thorium-229-doped crystals are both rare and radioactive. To address this, a team of UCLA chemists and physicists, in a study published in Nature, developed thin films made from a thorium-229 precursor. These films use significantly less thorium-229 and emit radiation comparable to that of a banana, making them far safer and more practical.

Crucially, they demonstrated that these films still allow for the same laser-driven nuclear excitation required for a nuclear clock. With scalable production, these films could not only revolutionize atomic clocks but also enable new applications in quantum optics.

A Revolutionary Method for Thorium Thin Films

Instead of embedding a pure thorium atom in a fluorine-based crystal, the new method uses a dry nitrate parent material of thorium-229 dissolved in ultrapure water and pipetted into a crucible. The addition of hydrogen fluoride yields a few micrograms of thorium-229 precipitate that is separated from the water and heated until it evaporates and condenses unevenly 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.

The Key to a More Stable Clock

“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.”

Redefining Time with a Nuclear Oscillator

At the heart of every clock is an oscillator. The clock operates by defining time as how long it takes for the oscillator to undergo a certain number of oscillations. In a grandfather clock, a second may be defined as the time for the pendulum to go back and forth once; in the quartz oscillator of a wristwatch, it is typically about 32,0000 vibrations of the crystal.

In a thorium nuclear clock, a second is about 2,020,407,300,000,000 excitation and relaxation cycles of the nucleus. This higher tick rate can make the clock more precise, provided the tick rate is stable; if the tick rate changes, the clock will mismeasure time. The thin films described in this work provide a stable environment for the nucleus that is both easily constructed and has the potential to be harnessed to produce microfabricated devices. This could allow widespread use of nuclear clocks as it makes them cheaper and easier to produce.

The Path to a Smaller, More Accurate Clock

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.

Unraveling the Mysteries of the Universe

Above and beyond commercial applications, the new nuclear spectroscopy could pull back the curtain on some of the universe’s biggest mysteries. Sensitive measurement of an atom’s nucleus opens up a new way to learn about its properties and interactions with energy and the environment. This, in turn, will let scientists test some of their most fundamental ideas about matter, energy and the laws of space and time.

Reference: “²²⁹ThF₄ thin films for solid-state nuclear clocks” by Chuankun Zhang, Lars von der Wense, Jack F. Doyle, Jacob S. Higgins, Tian Ooi, Hans U. Friebel, Jun Ye, R. Elwell, J. E. S. Terhune, H. W. T. Morgan, A. N. Alexandrova, H. B. Tran Tan, Andrei Derevianko and Eric R. Hudson, 18 December 2024, Nature.
DOI: 10.1038/s41586-024-08256-5

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