The world’s most precise clocks may soon be able to share their time across entire countries without losing the extraordinary accuracy that makes them scientifically valuable. In a new study published in Light: Science & Applications, researchers from the University of Science and Technology of China have demonstrated optical frequency dissemination across 2,067 kilometers of field-deployed fiber, achieving a fractional frequency instability of 2.9 × 10⁻²¹ after one day. The result represents a major step toward practical, long-distance networks capable of linking optical clocks, quantum systems, and precision measurement facilities.
Optical frequency transfer is the process of transmitting the highly stable oscillation of a laser from one location to another. Unlike conventional radio-frequency timing signals, optical frequencies oscillate hundreds of trillions of times per second, providing an exceptionally fine reference for measuring time and frequency. Such signals are essential for comparing the world’s most advanced optical clocks, testing whether fundamental constants change, mapping gravitational effects through relativistic geodesy, and synchronizing future quantum communication networks. Yet transporting this precision through real-world fiber is far more difficult than transmitting a signal through a controlled laboratory cable.
The challenge is caused by the environment surrounding the fiber. Temperature changes, mechanical vibration, construction work, traffic, and other disturbances continuously alter the optical path length. These fluctuations change the phase of the light, effectively scrambling the frequency information as it travels. Over thousands of kilometers, the noise accumulates and can become strong enough to interrupt the feedback systems that stabilize the link. Urban telecommunications networks are particularly demanding because they were built for data transmission rather than the preservation of ultra-stable optical phase.
The Chinese team addressed these problems with a system that combines digital phase recording, bias-free compensation, relay stations, and optical filtering. In a conventional phase-locked fiber link, the system measures disturbances and applies a correction intended to cancel them. However, long-haul networks often use frequency shifts to suppress unwanted reflections and interference. Because the forward and backward signals do not experience perfectly identical frequency conditions, these shifts can create a residual compensation bias. Even a small systematic error becomes significant when the goal is to transfer a frequency with instability at the 10⁻²¹ level.
The new approach records the radio-frequency phase digitally using time-to-digital converters. Rather than relying solely on an analog phase-locked loop, the system reconstructs the phase evolution in real time and calculates the correction needed to remove environmental noise without introducing the usual frequency-shift bias. Digital recording also gives the system a much wider phase-tracking range. This is particularly important in noisy field fibers, where sudden disturbances can produce phase changes too large for conventional control electronics to follow without losing lock.
To prevent noise from building up as the signal crosses multiple sections of fiber, the researchers installed multifunctional relay stations along the route. These stations receive the stabilized signal, process its phase information, and retransmit it to the next section. Intermediate optical filters further purify the signal by narrowing its spectral content to the hertz level. This filtering suppresses unwanted optical noise before it can be passed into the next link, allowing the full network to behave less like a single vulnerable connection and more like a chain of actively managed precision segments.
The experiment was conducted on a 2,067-kilometer network constructed from standard telecommunications fiber already deployed in the field. The route was deliberately exposed to severe environmental noise, measured at approximately 5,000 rad²/Hz/km at 1 hertz. Despite these conditions, the system continued operating stably and reached a fractional frequency instability of 2.9 × 10⁻²¹ at an averaging time of one day. The link also remained continuously locked for more than four days, an important practical achievement because long-term operation is often more difficult than reaching a record measurement during a short laboratory demonstration.
The significance of the result lies not only in the number attached to the instability, but in the setting in which it was achieved. Laboratory fiber spools can be isolated from vibration and temperature fluctuations, while field-deployed networks must share infrastructure with ordinary telecommunications services and endure changing conditions around the clock. Demonstrating extreme stability on such a network suggests that existing fiber infrastructure could be adapted for scientific applications rather than replaced by dedicated, specially protected cables. That prospect could make national and international optical timing networks faster and less expensive to build.
If expanded, systems based on this architecture could connect optical clocks separated by thousands of kilometers, enabling more precise tests of general relativity and searches for extremely small variations in fundamental constants. They could also support distributed quantum technologies, where synchronized and spectrally pure signals are needed to coordinate remote devices. In geodesy, comparisons between optical clocks can reveal changes in gravitational potential associated with tectonic movement, underground water, or sea-level variation. The researchers’ demonstration therefore moves optical frequency transfer closer to becoming an operational infrastructure technology rather than a capability confined to specialized laboratories.
The study establishes a robust strategy for overcoming one of the central barriers to global precision timing: maintaining an ultra-stable optical signal while it travels through noisy, imperfect, and heavily used fiber networks. By eliminating compensation bias, extending phase-tracking range, and purifying the signal at relay points, the system combines accuracy with resilience. The achievement over 2,067 kilometers offers a powerful glimpse of a future in which the world’s most precise clocks can operate as parts of a shared network, transforming how scientists measure time, gravity, distance, and the limits of physical law.
Subject of Research: Long-distance dissemination of ultra-stable optical frequencies through noisy, field-deployed fiber networks.
Article Title: 10⁻²¹-Level optical frequency dissemination over 2067 km of noise-loaded field-deployed fiber network
Web References: https://doi.org/10.1038/s41377-026-02299-1
References: Jiang, Hai-Feng, Qiang Zhang et al., “10⁻²¹-Level optical frequency dissemination over 2067 km of noise-loaded field-deployed fiber network,” Light: Science & Applications.
Image Credits: Jian-wei Pan et al.
Keywords
Optical frequency transfer, optical clocks, fiber-optic networks, precision timing, quantum networks, frequency stability, digital phase recording, fiber noise compensation, relativistic geodesy, metrology
