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Proceedings of the ... annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting/Proceedings of a meeting - Precise Time and Time Interval (PTTI) Systems and Applications Meeting

Predicting White Rabbit Time Synchronization Error on DC-QNet Using Statistical Machine Learning Methods

Brett Martin, Douglas D. Hodson, Michael R. Grimaila, Torrey J. Wagner, Wayne McKenzie, Anne-Marie Richards

Published Feb 13, 2025• 2 citations
Publisher

Abstract

Quantum networks leverage the laws of quantum mechanics to transport quantum states over geographic distances by using the quantum teleportation and entanglement swapping protocols. These protocols necessitate successful Bell state measurements which require the distribution of high precision timing between the participating nodes. White Rabbit (WR) is a precision timing protocol developed at the European Organization for Nuclear Research (CERN) which provides the means to obtain sub-nanosecond time resolution between nodes in a terrestrial quantum optical network using optical Ethernet switches. The purpose of this research is to characterize the error in distributed high precision timing when using the White Rabbit protocol wave division multiplexed in single-mode optical fiber as a function of time of day, length of fiber, percentage of aerial fiber, and environmental conditions (e.g., temperature, humidity, barometric pressure, wind speed, cloud cover). We evaluate and compare several statistical machine learning models in terms of their ability to accurately predict distributed timing error as a function of several geographic and environmental variables. The long-term goal of this research effort is to support decision making when implementing a quantum network using existing low-cost conventional optical telecommunications infrastructure technologies.

DOI
10.33012/2025.19953

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Quantum networks leverage the laws of quantum mechanics to transport quantum states over geographic distances by using the quantum teleportation and entanglement swapping protocols. These protocols necessitate successful Bell state measurements which require the distribution of high precision timing between the participating nodes. White Rabbit (WR) is a precision timing protocol developed at the European Organization for Nuclear Research (CERN) which provides the means to obtain sub-nanosecond time resolution between nodes in a terrestrial quantum optical network using optical Ethernet switches. The purpose of this research is to characterize the error in distributed high precision timing when using the White Rabbit protocol wave division multiplexed in single-mode optical fiber as a function of time of day, length of fiber, percentage of aerial fiber, and environmental conditions (e.g., temperature, humidity, barometric pressure, wind speed, cloud cover). We evaluate and compare several statistical machine learning models in terms of their ability to accurately predict distributed timing error as a function of several geographic and environmental variables. The long-term goal of this research effort is to support decision making when implementing a quantum network using existing low-cost conventional optical telecommunications infrastructure technologies.
Quantum networking protocols relying on interference and precise time-of-flight measurements require high-precision clock synchronization. This study describes the design, implementation, and characterization of two optical time transfer methods in a metropolitan-scale quantum networking research testbed. With active electronic stabilization, sub-picosecond time deviation (TDEV) was achieved at integration times between 1 and 105 s over 53 km of deployed fiber. Over the same integration periods, 10-ps level TDEV was observed using the White Rabbit–Precision Time Protocol over 128 km. Measurement methods are described to understand the sources of environmental fluctuations on clock synchronization toward the development of in situ compensation methods. Path delay gradients, chromatic dispersion, polarization drift, and optical power variations all contributed to clock synchronization errors. The results from this study will inform future work in the development of compensation methods essential for enabling experimental research in developing practical quantum networking protocols.
Precise timing synchronization is essential for distributed quantum networking, enabling entanglement distribution, quantum teleportation, and entanglement swapping across remote nodes. Existing synchronization architectures rely on dedicated timing-distribution infrastructure, most notably White Rabbit networks, which constrain topology, scalability, and deployment in free-space and satellite environments. Here we demonstrate link-free synchronization of quantum network nodes using independently operating miniature rubidium atomic clocks and computational post-processing. We validate the approach on a deployed metropolitan-scale telecom fiber network spanning three geographically separated nodes. Following drift correction, atomic-clock-based synchronization achieves timing performance approaching that of a White Rabbit benchmark and remains stable over continuous 8-hour operation. As a stringent test of quantum-network functionality, we observe Hong-Ou-Mandel interference across spatially separated nodes with visibility exceeding 70%, statistically equivalent to that obtained using dedicated White Rabbit timing links. To the best of our knowledge, this represents the first observation of quantum interference across a deployed metropolitan-scale telecom fiber network synchronized entirely without dedicated timing-transfer infrastructure. These results establish atomic-clock-based synchronization as a scalable, topology-independent alternative to conventional timing-distribution architectures and a practical pathway toward terrestrial, airborne, and space-based quantum networks where dedicated timing links are unavailable.
White Rabbit (WR) technology provides a commercially-available off-the-shelf solution for time synchronisation with sub-nanosecond accuracy and picosecond-level precision over optical fibre links typically spanning tens of kilometres. Such high-performance time dissemination can support a variety of applications, including position, navigation and timing (PNT), financial transactions, metrology, as well as entanglement and quantum key distribution (QKD). Demonstrations of WR over significantly longer distances remain few and far between, particularly in scenarios where intermediate amplification is unavailable, such as stretches of long-haul underwater fibre. In this work, we report the longest unrepeated deployment of WR to date, achieving time synchronisation over a 300 km (51.34 dB) single-span optical fibre link, even in highly asymmetrical configurations, with 99.86% uptime, whilst maintaining picosecond-level precision and sub-nanosecond accuracy. This was achieved through careful selection and optimisation of the components deployed at the link's end points. By leveraging standard telecom fibre and off-the-shelf hardware, our results pave the way for a scalable and standardised timing backbone for large-scale quantum networks, offering a practical route toward time distribution in future heterogeneous quantum communication systems.
We demonstrate picosecond-level synchronization of two actively mode-locked Ti:Sapphire lasers via the white rabbit precision time protocol (WR-PTP), tested over 120 km of deployed optical fiber. This synchronization capability, in combination with the highly single-mode, transform-limited pulses produced by each laser, enables their use as pump lasers for indistinguishable photon sources at remote locations in a quantum network. Here, the WR-PTP serves as a scalable network synchronization protocol, and its performance is compared to traditional methods of local synchronization. We demonstrate pulse-to-pulse synchronization better than 3 ps and time deviation (TDEV) values below 4 ps for all averaging times up to 10 s. With a designed coherence time of 35 ps for single photon sources utilizing these lasers, the achievable temporal overlap corresponds to 98% Hong-Ou-Mandel (HOM) interference visibility between independent sources.