Clock Synchronization Characterization of the Washington DC Metropolitan Area Quantum Network (DC-QNet)

Distributed protocols relying on quantum interference, event synchronization, network telemetry, timestamping, and precise time-of-flight measurements require high-precision clock and time synchronization. This study describes the design, development, and characterization of two optical time transfer methods between a network of reference clocks at each laboratory node in the Washington Metropolitan Quantum Network Research Consortium (DC-QNet). With optical time transfer using active electronic stabilization, sub-picosecond time deviation (TDEV) was achieved at integration times between 1 s and 105 s over 35 km of deployed fiber. Using the White Rabbit-Precision Time Protocol (WR-PTP), we achieved sub-10 picosecond TDEV at integration times ranging from sub-second to over one day. There is potential to improve WR-PTP time transfer using in-situ compensation methods. Measurement methods were developed to understand the impact of environmental and time-of-flight fluctuations on clock synchronization. Path delay gradients, chromatic dispersion, polarization drift, and optical power variations all contributed to clock synchronization instabilities. For protocols requiring the coexistence of quantum and classical communications, we deployed WR-PTP in a bi-directional configuration using 1270 nm and 1290 nm wavelengths over 64 km. The results from this study will inform future advancements in the development of metropolitan-scale, telecommunications-compatible clock synchronization networks for enabling near-term experimental research in quantum networking protocols.