Session P3b: Timekeeping for Quantum Networking and Other Science Applications

Towards Quantum Networking: Characterization of White Rabbit Precision Time Protocol Over a Metropolitan Scale Fiber Link
W. McKenzie, Laboratory for Telecommunication Sciences; Y.S. Li-Baboud, National Institute for Standards and Technology; M. Morris, S. Patel, Laboratory for Telecommunication Sciences; A. Rahmouni, P. Kuo, O. Slattery, Y. Shi, I. Bhardvaj, I. Burenkov, National Institute for Standards and Technology; A. Richards, Laboratory for Telecommunication Sciences; M. Ayako, University of Maryland; B. Crabill, Mid-Atlantic Crossroads; M. Merzouki, A. Battou, T. Gerrits, National Institute for Standards and Technology
Location: Beacon B

Abstract
Towards Quantum Networking: Characterization of White Rabbit Precision Time Protocol Over A Metropolitan Scale Fiber Link
Wayne McKenzie, Thomas Gerrits, Anouar Rahmouni, Yicheng Shi, Mark Morris, Gerry Baumgartner, Ishaan Bhardvaj, Ivan Burenkov, Anne-Marie Richards, Millicent Ayako, Shirali Patel, Oliver Slattery, Mheni Merzouki, Abdella Battou, Ya-Shian Li-Baboud
Long distance quantum communication networks pose unique challenges for precision time synchronization. The required level of precision spans nanoseconds to picoseconds, depending on the coherence time of the qubit implementation. Precision synchronization between quantum nodes is required to support quantum state distribution at practical rates and distances. For a metropolitan-scale quantum network, a further requirement is scalability. The White Rabbit Precision Time Protocol (WR-PTP) is an Ethernet-based protocol that can provide nanosecond (over a wide area network) to picosecond level (in a local area network) synchronization precision and is now part of the High Accuracy PTP (HA-PTP) Profile in IEEE 1588-2019. A key advantage of leveraging a standard Ethernet-based protocol for time transfer, is the availability of commercial off-the-shelf components interoperable with existing telecommunications infrastructure to support rapid deployment of time synchronization capabilities in quantum research networks. Additionally, the feasibility of co-existence among classical optical communications for time distribution, along with single-photon level entangled quantum state distribution, could provide the practical benefit of being able to estimate the path delay in-situ and provide active delay compensation for realizing quantum interference between remote sources. Among the primary technical challenges is that optical two-way time and frequency transfer (O-TWTFT) methods rely on the symmetric path delay or a calibrated coefficient to compensate for path delay asymmetry. In deployed optical fibers, the path delay varies due to dispersion effects causing changes in the group delay and the optical pulse characteristics at the receiver. In situations where the environment has a large effect on deployed fibers, such as aerial fiber, variations can degrade the accuracy of the path delay compensation necessary for picosecond level time transfer.
The objective of this work is to investigate active compensation of chromatic and polarization dispersion effects on path delay variation, in order to improve metropolitan-scale time synchronization. This study explores the clock synchronization precision achieved between different WR-PTP topologies and includes the characterization of wavelength and polarization stability for a fiber link pair comprised of both aerial and buried segments. Both loop-back and star topologies, along with bi-directional simplex and duplex fibers using low-loss Dense Wavelength Division Multiplexing (DWDM) were studied. A unidirectional 64 km link and a 128 km loopback link resulted in Maximum Time Interval Errors below 100 ps and 200 ps, respectively, over five days. Diurnal optical path delay variations up to 50 ns have been observed in the link under study. The diurnal optical path delay variation correlates with outdoor temperature, which in turn affects polarization stability, while wavelength stability is strongly correlated with SFP device temperature. Both, the optical path delay and wavelength variation lead to group delay variations as clock signals traverse the fiber. This study provides a characterization of deployed fiber between two remote locations of DC-QNet, a metropolitan-scale quantum communications network testbed, and its impact on clock synchronization intended for a quantum research network. Future work will explore the data characterization in the context of data driven machine learning methods to predict changes in path and clock delay. The anticipated goal is to explore the feasibility of active compensation for optical fiber dispersion effects in a standard time transfer protocol, in particular, WR-PTP, to achieve 10 picosecond synchronization on a metropolitan scale. The characterization metrics and results of WR-PTP over deployed links and the associated measurement methodologies will be used for development of the future high-precision time transfer standards for fiber-based quantum networks.