Setting the Perfect Temperature for Accurate Nuclear Timekeeping

For decades, atomic clocks have represented the pinnacle of precision timekeeping technologies. They play crucial roles in a vast range of applications from GPS navigation to groundbreaking physics research and comprehensive tests of fundamental theories that govern our universe. However, researchers at JILA, under the guidance of Jun Ye—a JILA and NIST Fellow and a physics professor at the University of Colorado Boulder—are venturing beyond the standard atomic transitions. They are exploring the development of a novel type of clock based on nuclear physics that could prove to be significantly more stable and reliable than its atomic counterparts.

The basis of this potential breakthrough lies in the nuclear transitions of the thorium-229 atom. This unconventional approach to timekeeping involves a uniquely low-energy transition that occurs within the atomic nucleus, offering advantages that atomic clocks lack, such as lower sensitivity to environmental factors. According to researchers, the stability provided by the nuclear clock could enable it to perform tasks where current atomic clocks might falter, making it a promising avenue for the future of precise timekeeping technologies.

The concept of nuclear clocks is not new in the research lab led by Ye. The foundation of this innovative clock design was laid with a groundbreaking experiment that was showcased as the cover article of the prestigious journal Nature last year. In that study, the team successfully executed the first frequency-based quantum-state-resolved measurement of the thorium-229 nuclear transition embedded in a thorium-doped crystal matrix. This pivotal achievement established that the thorium nuclear transition could be measured with adequate precision to be used as a reliable reference for timekeeping applications.

Nonetheless, the journey toward creating a precise nuclear clock involves an extensive understanding of how its nuclear transitions react to external conditions, particularly temperature fluctuations. A recent investigation, which has gained recognition as an “Editor’s Choice” paper published in Physical Review Letters, delved deep into analyzing energy shifts within the thorium nuclei as the hosting crystal was subjected to various temperatures. This study marks a crucial step toward characterizing the systematic behaviors of this innovative nuclear clock.

JILA postdoctoral researcher Dr. Jacob Higgins, who led the study, emphasized the importance of their findings, stating, “This is the first step toward characterizing the systematics of the nuclear clock.” The researchers discovered a transition that exhibits a remarkable insensitivity to temperature variations—an essential characteristic for any device intended for precision timekeeping. This insight provides a solid foundation for further advancements in the development of a reliable nuclear clock.

Jun Ye expressed optimism about the potential of solid-state nuclear clocks, noting, “A solid-state nuclear clock has a great potential to become a robust and portable timing device that is highly precise.” The ongoing research is focused on identifying the precise parameter space needed for a compact nuclear clock capable of maintaining extraordinary stability, with a fractional frequency stability target of 10^-18 for continuous operation.

The intrinsic properties of the nucleus contribute to its resilience against environmental disturbances compared to the more volatile electrons surrounding it. As a result, nuclear clocks have the potential to maintain accuracy under operational conditions that could disrupt atomic clocks. Among various atomic nuclei, thorium-229 has emerged as an especially favorable candidate due to its nuclear transition exhibiting unusually low energy. This characteristic allows researchers to probe the nuclear transition using ultraviolet laser light rather than relying on high-energy gamma rays, which are typically more difficult to handle.

Rather than employing traditional ion-trapping techniques to measure thorium, the team has opted for a novel approach by embedding thorium-229 within a solid-state host—a calcium fluoride (CaF₂) crystal. This innovative method, designed in collaboration with the Technical University of Vienna, permits a much denser packing of thorium nuclei within the host material. The result is a stronger signal and increased stability for accurately measuring the nuclear transitions, thereby enhancing the precision needed for the development of a nuclear clock.

To deepen their understanding of temperature’s effect on the nuclear transition, the researchers manipulated the thermal state of the thorium-doped crystal, bringing it to three different temperatures—150K (approximately -123°C) using liquid nitrogen, 229K (around -44°C) via a dry ice-methanol mixture, and 293K (approximately room temperature). Harnessing the capabilities of a frequency comb laser, the team meticulously assessed how shifting temperatures influenced the frequency of the nuclear transitions, unveiling two competing physical phenomena at play within the crystal structure.

As the crystal’s temperature increased, it expanded, subtly altering the atomic lattice and resulting in changes to the electric field gradients experienced by the thorium nuclei. This transformation caused the thorium nuclear transition to subdivide into multiple spectral lines, which exhibited distinct shifts depending on the temperature fluctuations. In conjunction, lattice expansion influenced the electron charge density within the crystal, modifying interactions between the electrons and the nucleus, thus contributing to the movement of spectral lines in a unified direction.

Despite these competing effects, the researchers identified one specific transition that displayed remarkable robustness against temperature changes, primarily due to the way the two phenomena mitigated each other’s impact. Within the entire temperature range examined, this transition transitioned by only 62 kilohertz—a shift that stands at least 30 times smaller than those observed in other transitions. JILA graduate student Chuankun Zhang emphasized the potential of this particular transition for clock applications, stating, “This transition is behaving in a way that’s really promising for clock applications.”

The next phase of the research entails identifying a temperature ‘sweet spot’ where the nuclear transition’s frequency remains largely independent of temperature fluctuations. Preliminary data suggests a possible optimal range exists between 150K and 229K, where the transition frequency could achieve a greater degree of temperature stability, paving the way for ideal conditions for a future operational nuclear clock.

The development of an entirely new category of clock technology necessitates an array of bespoke equipment that often does not exist in commercially available forms. The JILA team’s collaboration with their in-house instrument shop proved invaluable, providing access to skilled machinists and engineers who were instrumental in crafting critical components to support the experimental setup.

Dr. Higgins acknowledged the significant contributions of the instrument shop, stating, “Kim Hagan and the whole instrument shop have been super helpful throughout this process.” They created a meticulously designed crystal mount to hold the thorium-doped crystal and engineered components for the cold trap system that facilitated precise temperature control. The ability to customize components allowed researchers to adapt their experimental setup quickly and confidently.

The implications of this research extend far beyond the realm of timekeeping itself. The sensitivity of the thorium nuclear transition to environmental perturbations is vastly lower than that of atomic clocks, yet it remains finely attuned to variations in fundamental forces. Any unexpected shifts in its frequency could serve as a precursor for uncovering new physics phenomena, including the pursuit of dark matter and beyond.

Dr. Jacob Higgins articulated the broader significance of their findings, observing, “The nuclear transition’s sensitivity could allow us to probe new physics.” Even though the primary objective of their research revolves around the development of a more stable nuclear clock, it may also unlock innovative methodologies for exploring the cosmos and provide unprecedented insights into the fundamental workings of the universe.

This extensive research endeavor has received comprehensive support from various prestigious organizations, including the Army Research Office, the Air Force Office of Scientific Research, the National Science Foundation, the Quantum System Accelerator, and the National Institute of Standards and Technology (NIST), reflecting the versatility and importance of nuclear clocks in both practical applications and theoretical physics.

Subject of Research: Thorium-229 Nuclear Clocks
Article Title: Temperature Sensitivity of a Thorium-229 Solid-State Nuclear Clock
News Publication Date: [Date Not Provided]
Web References: [Links Not Provided]
References: [Links Not Provided]
Image Credits: Steven Burrows/JILA

Keywords

Precision Timekeeping, Atomic Clocks, Nuclear Transitions, Thorium-229, Solid-State Physics, Environmental Sensitivity, Quantum Measurements, Fundamental Forces, Dark Matter, Advanced Instrumentation, Laboratory Techniques.

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