Thorium film could replace crystals in atomic clocks of the near future
When hit by a laser beam, atomic clocks that excite thorium-229 nuclei embedded in transparent crystals can produce the most precise measurements of time and gravity ever made, even rewriting some fundamental laws of physics. Thorium-229-doped crystals are both rare and radioactive. The film using the thorium-229 dry precursor exhibits the same nuclear excitations as the crystal, but is low-cost, highly radioactive, and smaller in size, meaning its production can be more easily scaled up to make smaller, cheaper , a more portable atomic clock.
This summer, UCLA physicists succeeded in embedding thorium-229 nuclei into transparent crystals that absorb and emit photons like electrons in atoms, ending decades of speculation about whether the feat was possible. Using lasers to increase the energy state of an atomic nucleus, or to excite it, will allow the development of the most accurate atomic clocks ever built and allow the most precise measurements of time and gravity. Such atomic clocks could even rewrite some basic laws of physics.
But there’s a problem: Thorium-229-doped crystals are both rare and radioactive. In a new paper published in natureA team of chemists and physicists at UCLA may also have solved the problem by developing films made from thorium-229 precursors, which require much less thorium-229 and are about as radioactive as a banana. Using these films, they demonstrated the same laser-driven nuclear excitations required for nuclear clocks. The production of such films can be scaled up not only for nuclear clocks but also for other quantum optical applications.
Rather than embedding pure thorium atoms into fluorine-based crystals, the new method uses a dry nitrate parent material of thorium-229 dissolved in ultrapure water and moved into a crucible. Adding hydrogen fluoride creates a few micrograms of thorium-229 precipitate, which is separated from the water and heated until it evaporates and condenses uniformly on the transparent sapphire and magnesium fluoride surfaces.
Light from a vacuum ultraviolet laser system is directed to the target, where it excites the nuclear state and collects the photons subsequently emitted by the nucleus, as reported in earlier UCLA research.
“A key advantage of using the parent material thorium fluoride is that all thorium nuclei are in the same local atomic environment and experience the same electric field at the nuclei,” said co-author Charles W. Clifford, Jr. Clifford Jr. said. “This allows all thorium to exhibit the same excitation energy, resulting in a stable and more accurate clock. In this way, the material is unique.”
At the heart of every clock is an oscillator. A clock works by defining time as the time it takes for an oscillator to go through a certain number of oscillations. In a grandfather clock, one second can be defined as the time it takes for the pendulum to move back and forth once; in a watch’s quartz oscillator, the number of vibrations of the crystal is usually about 32,0000 times.
In a thorium nuclear clock, one second is approximately 2,020,407,300,000,000 excitation and relaxation cycles of the nucleus. When the tick rate is stable, a higher tick rate can make the clock more accurate; if the tick rate changes, the clock will mismeasure the time. The films described in this work provide a stable environment for atomic nuclei and are both easy to construct and have the potential to be used in the production of microfabricated devices. This could make nuclear clocks more widely available because it would make them cheaper and easier to produce.
Existing electron-based atomic clocks are room-sized devices with vacuum chambers to trap the atoms and equipment associated with cooling. Thorium-based nuclear clocks would be smaller, stronger, more portable and more accurate.
In addition to commercial applications, new nuclear spectroscopy techniques could shed light on some of the biggest mysteries in the universe. Sensitive measurements of atomic nuclei open up a new way to understand their properties and interactions with energy and the environment. In turn, this will allow scientists to test some of their most fundamental ideas about matter, energy, and the laws of space and time.
2024-12-18 18:12:55