Clock synchronization characterization of the Washington DC metropolitan quantum network (DC-QNet)
Wayne McKenzie, Anne Marie Richards, Shirali Patel, Thomas Gerrits, Thomas G. Akin, Steven Peil, Adam T. Black, David A. Tulchinsky, Alexander S. Hastings, Ya-Shian Li-Baboud, Anouar Rahmouni, Ivan A. Burenkov, Alan Mink, Matthew Diaz, Nijil Lal, Yicheng Shi, Paulina S. Kuo, Pranish Shrestha, Mheni Merzouki, Alejandro Rodriguez Perez, Eleanya E. Onuma, Daniel E. Jones, Atiyya A. Davis, Thomas A. Searles, J. D. Whalen, Qudsia Sara Quraishi, K. S. Collins, La Vida Cooper, Harry C. Shaw, Bruce Crabill, Oliver Slattery, Abdella Battou
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.
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.
Quantum networking protocols relying on interference and precise time-of-flight measurements require highprecision clock synchronization. This study describes the design, implementation, and characterization of two optical time transfer methods in a metropolitan-scale quantum networking research testbed. The first technique called active electronic stabilization (ELSTAB), was able to achieve sub-picosecond time deviation (TDEV) at integration times between 1 s and 105 s over multiple different fiber lengths of both underground and aerial deployed fiber. The second technique, White Rabbit-Precision Time Protocol (WR-PTP), was able to achieve 10-picosecond TDEV’s over similar fiber lengths at comparable integration times. Environmental fluctuations affected the stability of clock synchronization over deployed fiber. These fluctuations resulted in path delay gradients, chromatic dispersion, polarization drift, and optical power variations. 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.
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.
Time synchronization is a fundamental requirement in entanglement-based quantum networks, where the indistinguishability of photons in the time domain is essential for enabling Hong-Ou-Mandel interference and entanglement swapping. In addition to precise temporal alignment, it is equally crucial to ensure the stability of the reference clock over time, as even small fluctuations can degrade the overall performance of the network. In this work, we investigate the stability of clock synchronization for entanglement distribution based on entangled-photon sources operating in the telecommunication C-band. Temporal correlations between photon detection events are analyzed using time-tagged coincidence measurements, enabling the extraction of synchronization peaks and their long-term stability. Experimental results demonstrate that, once locked, the sources exhibit stable temporal correlations over an 8-hour acquisition period, with a maximum observed drift of approximately 120 ps, primarily associated with long fiber links. The width of the correlation peak remains consistent with detector jitter, indicating negligible additional system-induced temporal broadening. A central result of this work is the experimental synchronization between two entanglement-photon sources in a realistic metropolitan deployment, where entanglement distribution is performed over existing telecommunication infrastructure characterized by non-negligible losses and background noise. In this scenario, despite the presence of significant imperfections introduced by the metropolitan-scale fiber network, the observed correlations remain clearly detectable and are consistently well-approximated by Gaussian statistics. This confirms that the two sources can be reliably synchronized not only in controlled laboratory conditions but also under real-world operating constraints.
The development of quantum networks marks a transformative advance in communication technologies, enabling communication security unattainable through classical signals and the interconnection of quantum computers, sensors, and devices at various scales. Leveraging classical telecommunication infrastructure, such as optical fibers and associated devices, is pivotal for practical quantum network implementation. The Washington Metropolitan Quantum Network Research Consortium (DC-QNet) is developing a quantum network testbed in the Washington, DC area based on this infrastructure. However, adapting classical systems for quantum networks introduces significant challenges, including crosstalk between classical and quantum channels, stringent synchronization precision demands, polarization drift in optical fibers, and the limitations of classical optical devices. We present experimental demonstrations of these challenges and novel solutions, including a new crosstalk characterization technique. Results are derived from real-world DC-QNet infrastructure, showcasing the feasibility and progress of quantum network implementation.
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.