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Advancements in Timekeeping: The Future of Nuclear Clocks

While atomic clocks currently set the standard for precision timekeeping, a new innovation in the form of nuclear clocks promises to significantly enhance our ability to measure time and explore fundamental aspects of physics.

An international research collaboration, spearheaded by the JILA institute, which is part of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder, has made substantial progress in the development of this new timekeeping technology. The concept of a nuclear clock hinges on utilizing signals from the atomic nucleus, representing a groundbreaking shift from traditional atomic clocks.

The findings have been detailed in a prominent cover story in the September 4, 2024 issue of the journal Nature.

In their experiments, the research team applied a specially engineered ultraviolet laser to accurately gauge the frequency of energy transitions in thorium nuclei situated within a solid crystal matrix. They utilized an optical frequency comb, which serves as an extraordinarily precise measure of light waves, to monitor the cycles of the ultraviolet light responsible for energy transitions. Although this demonstration does not yet constitute a complete nuclear clock, it integrates essential components for its eventual realization.

The potential implications of nuclear clocks are vast, particularly their capacity to surpass the accuracy of existing atomic clocks. Currently, atomic clocks are vital for maintaining international time standards and are integral to technologies such as GPS, internet synchronization, and secure financial transactions.

A more refined timekeeping system could lead to enhanced navigation capabilities, whether GPS-based or otherwise, accelerated internet access, improved network reliability, and bolstered security for digital communications. Additionally, nuclear clocks may enable more rigorous testing of foundational theories in physics, possibly facilitating the detection of dark matter or validating the constancy of natural constants. This could ultimately lead to significant insights in particle physics without necessitating extensive particle accelerator experiments.

The Science Behind Nuclear Clocks

Atomic clocks primarily rely on laser tuning to promote electron transitions between energy levels. In contrast, nuclear clocks would observe transitions within the atomic nucleus, which consists of protons and neutrons densely packed together.

These energy transitions resemble flipping a switch. By directing a laser with precisely the right energy at the nucleus, researchers can induce these nuclear “switches” to activate.

Nuclear clocks are expected to offer notable advancements in precision. The nucleus is generally less susceptible to interference from external factors such as stray electromagnetic fields when compared to electrons in atomic clocks. Moreover, the required laser light to induce nuclear energy transitions operates at higher frequencies than that used in atomic clocks.

This increased frequency corresponds to a higher number of time “ticks” per second, enhancing overall timekeeping accuracy.

However, crafting a functional nuclear clock poses significant challenges. Most atomic nuclei necessitate exposure to coherent X-rays, which are typically beyond the output capacity of current technology. Consequently, researchers have centered their efforts on thorium-229, an isotope whose energy transitions are uniquely lower than those of other atoms, allowing for the use of less energetic ultraviolet light.

The thorium energy jump, identified as a “nuclear transition” in 1976, saw its proposal for clock utilization in 2003, with direct observations finally recorded in 2016. Recent efforts earlier this year by two distinct research teams successfully employed lab-created ultraviolet lasers to activate the nuclear transitions and measure the corresponding light wavelength.

The recent JILA research marks the creation of all essential guidelines for a nuclear clock: capturing the thorium-229 transitions as the temporal ticks, employing lasers for precise energy shifts, and integrating frequency combs for direct tick measurements.

This work has achieved precision advancements that are one million times greater than prior measurements linked to wavelength analysis. Additionally, for the first time, the research establishes a direct frequency correlation between the nuclear transition and a leading atomic clock based on strontium. This connection is pivotal for developing a fully functional nuclear clock that can be harmonized with existing timekeeping technologies.

The research has uncovered remarkable results, including the unprecedented ability to analyze the thorium nucleus’s shape in detail—akin to identifying individual blades of grass from an aircraft.

Looking Ahead: A Nuclear Timekeeping Future

While the latest developments do not yet culminate in a fully operational nuclear clock, they represent a critical stride toward creating a device that could be both portable and extraordinarily stable. The incorporation of thorium within a solid crystal, coupled with reduced sensitivity of the nucleus to external influences, sets the stage for compact yet reliable timekeeping solutions.

“Imagine a wristwatch that wouldn’t lose a second even if you left it running for billions of years,” remarked physicist Jun Ye from NIST and JILA. “While we are not there yet, this research significantly advances our journey toward that level of accuracy.”

The research team comprised members from JILA, the Vienna Center for Quantum Science and Technology, and IMRA America, Inc.

Further Reading: Chuankun Zhang, “Frequency ratio of the 229mTh isomeric transition and the 87Sr atomic clock,” Nature, 2024. DOI: 10.1038/s41586-024-07839-6. Visit the study site here.

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phys.org