The intrinsic complexity of modern data networks demands ever-higher throughput capacities, and multicore fiber technology has emerged as a remarkable solution by embedding multiple light-guiding cores within a single fiber strand. Each of these cores can independently carry data, drastically multiplying bandwidth capabilities compared to the traditional single-core fibers. This enhanced capacity is vital for data-intensive applications, including streaming media, financial transactions, and artificial intelligence computations, all of which hinge upon uninterrupted and rapid data flow.

Yet beyond these commercial uses lies a tantalizing scientific prospect: the ability to link ultra-precise optical atomic clocks distributed geographically by leveraging multicore fiber networks. Optical atomic clocks currently represent the pinnacle of timekeeping precision, boasting frequency stabilities necessitating representation out to 18 decimal places. They are crucial not only for redefining the international standard of the second but also for conducting sophisticated experiments in fundamental physics and improving global positioning systems.

The principal challenge in transmitting optical clock signals over fiber optic cables arises from the sensitivity of optical fibers to environmental perturbations such as temperature fluctuations and mechanical strain. These disturbances introduce detrimental noise, manifesting as frequency instability that can obscure the ultrastable signals required for high-precision applications. To overcome these hurdles, bidirectional transmission of light within fibers has traditionally been employed, enabling real-time noise sensing and adaptive correction.

However, bidirectional communication in conventional fiber optic networks engenders its own obstacles. Such networks typically operate with separate fibers for sending and receiving data to avoid noise-inducing reflections and signal degradation. Prior experiments to transfer ultrastable frequencies therefore relied heavily on "dark fibers," unused fibers devoid of conventional data traffic, or on specialized equipment capable of facilitating bidirectional transmission within a single core—both approaches presenting practical and economic limitations.

In an innovative departure from these constraints, the research team conceived a method exploiting the distinctive characteristics of multicore fibers. Since the multiple cores housed in a single fiber experience environmental disturbances almost identically, outbound optical clock signals can be routed through one core, while the returning signals traverse another core. This scheme preserves the benefits of bidirectional noise correction techniques without the drawbacks of reflections or interference that hinder single-core fibers, all while permitting the coexistence of active telecommunications data traffic.

This novel approach was rigorously tested in an operational multicore fiber testbed situated in L’Aquila, Italy. Using approximately 25 kilometers of deployed multicore fiber, the researchers meticulously quantified the fractional frequency instability of transmitted optical signals. Their measurements revealed a remarkable stability level of 3 × 10⁻¹⁹ over a near three-hour period, solidly within the stringent bounds demanded by current optical clocks. These results validate that multicore fibers do not inherently degrade signal stability as compared to traditional single-core fibers, even when carrying telecommunications traffic simultaneously.

The implications of this technology are profound. Linking precision optical clocks across metropolitan regions and potentially intercontinental distances could catalyze significant advances in fundamental physics research, such as tests of general relativity or searches for variations in fundamental constants. Moreover, establishing an ultrastable optical frequency network could precipitate a global redefinition of the second, bolstering synchronization in navigation systems and international time dissemination.

Notably, this research underscores the synergy between telecommunications infrastructure advancements and fundamental science. The continued commercial deployment of multicore fiber technology—spurred by the insatiable global appetite for data—enables scientists to repurpose existing fiber assets for next-generation time-frequency transfer applications. The collaborative effort bridging NIST, Nokia Bell Labs, Japanese industrial leader Sumitomo Electric Industries, and academic institutions in the United States and Italy embodies a successful model for multidisciplinary innovation.

While the current demonstrations are limited to medium-scale fiber lengths without optical amplification, the research team acknowledges critical challenges ahead. Fiber links exceeding around 80 kilometers require optical amplifiers to sustain signal strength, and the compatibility of amplification techniques with ultrastable frequency transfer remains an active area of investigation. Continued efforts will focus on integrating amplification without sacrificing the exceptional stability achieved thus far.

The experimental setup capitalized on the principle that environmental noise affects all cores of a multicore fiber nearly identically, allowing noise compensation algorithms to exploit this property effectively. This concept represents a significant technical advance over previous approaches necessitating dark fibers or complex bidirectional hardware, potentially accelerating the practical deployment of high-fidelity optical clock networks integrated with vast telecommunications backbones.

From a broader perspective, the demonstration of attosecond-level timing precision over deployed multicore fibers opens pathways toward unprecedented synchronization capabilities across diverse fields. Quantum networking, for instance, relies heavily on ultrastable frequency dissemination for entanglement distribution and quantum key distribution protocols. The integration of these capabilities into existing fiber networks is a decisive step toward viable quantum internet architectures.

In sum, this research marks an essential milestone: the successful convergence of ultrastable optical frequency transmission and dense telecommunications signaling via multicore fibers. By validating that standard telecommunication fiber infrastructure can support the next generation of atomic clock connectivity without sacrificing performance, the study paves the way for transformative scientific explorations and practical applications relying on exquisitely synchronized timekeeping worldwide.

As the team advances toward demonstrating long-haul transmission with optical amplification and extended multicore fiber spans, the scientific community eagerly anticipates the realization of interconnected, continent-spanning optical clock networks. Such networks promise not only to enhance measurement science but also to catalyze future technologies rooted in time synchronization and quantum communication.

Subject of Research: Ultrastable optical frequency transfer utilizing multicore fiber compatible with telecommunication data transmission.

Article Title: Ultrastable optical frequency transfer and attosecond timing in deployed multicore fiber

News Publication Date: 19-Jun-2025

Web References:
https://doi.org/10.1364/OPTICA.558821
https://opg.optica.org/optica/home.cfm

References:
N. Hoghooghi, M. Mazur, N. Fontaine, Y. Liu, D. Lee, C. McLemore, T. Nakamura, T. Hayashi, G. Di Sciullo, D. Shaji, A. Mecozzi, C. Antonelli, F. Quinlan, “Ultrastable optical frequency transfer and attosecond timing in deployed multicore fiber,” Optica, vol. 12, pp. 894-899, 2025.

Image Credits: Franklyn Quinlan, NIST

Keywords

Atomic clocks, Technology, Fibers, Physics

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.