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Quantum Computing, Communication, and Simulation

Picosecond clock synchronization across a 7-node metropolitan scale quantum network

Wayne McKenzie, Anne Marie Richards, Ya-Shian Li-Baboud, Ivan A. Burenkov, Anouar Rahmouni, Thomas Gerrits, Adam T. Black, Daniel E. Jones, Alejandro Rodriguez Perez, Thomas G. Akin

Published Mar 19, 2025• 1 citation
Publisher

Abstract

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.

DOI
10.1117/12.3043505

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•Nov 1, 2019•12 citations
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 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.
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.
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.
Accurate time and phase distribution remain central requirements for emerging communication systems, where synchronization affects radio coordination, fronthaul stability, and network-based positioning. Current approaches rely heavily on GNSS, the Precision Time Protocol (PTP), and Synchronous Ethernet (SyncE). Still, their performance is limited by delay asymmetry, accumulated jitter across long paths, and GNSS's vulnerability to blockage or interference. Recent progress in optical and quantum time-transfer techniques, particularly time-correlated entangled photons (TCEP) and optical lattice clocks (OLCs), offers an opportunity to improve both stability and calibration accuracy. This work proposes a practical synchronization framework that integrates OLC-based time references, TCEP-assisted calibration between core nodes, and standards-compliant packet-based delivery using PTP/SyncE in accordance with ITU-T G.8271. We describe how to interface photonic subsystems, FPGA-based timestamping hardware, and telecom boundary clocks without modifying existing radio functions. We also evaluate how improved synchronization propagates to key applications: uplink and downlink TDoA positioning, fronthaul and TSN timing stability, and NTN deployments where GNSS is unreliable. The architecture is designed for incremental deployment and aligns with current operational practices.
This work models metropolitan-scale photonic quantum networks that use time bin encoding for quantum key distribution and quantum state teleportation. We develop and validate theoretical models by comparing them with prior experimental results. We use our newly developed simulator of quantum network communication, called SeQUeNCe, to perform simulations at the individual photon level with picosecond resolution. The simulator integrates accurate models of optical components including light sources, interferometers, detectors, beam splitters, and telecommunication fiber, allowing studies of their complex interactions. Optical quantum networks have been generating significant interest because of their ability to provide secure communication, enable new functionality such as clock synchronization with unprecedented accuracy, and reduce the communication complexity of certain distributed computing problems. In the past few years experimental demonstrations moved from table-top experiments to metropolitan-scale deployments and long-distance repeater network prototypes. As the number of optical components in these experiments increases, simulation tools such as SeQUeNCe will simplify experiment planning and accelerate designs of new network protocols. The modular design of our tool will also allow modeling future technologies such as network nodes with quantum memories and quantum transducers as they become available.