Interconnection of Quantum Networks at Urban scale: Analysis of Temporal Stability of Entangled Photon Sources

As quantum networks scale, the ability to perform entanglement swapping—a process that connects independent quantum links—becomes critical. This process requires photons from separate sources to be indistinguishable in time. This paper investigates whether entangled photon sources can be synchronized with sufficient stability to operate over existing, non-ideal metropolitan telecommunication infrastructure.

The researchers utilized the National Quantum Internet.it testbed at the University of Naples Federico II. They employed two polarization-entangled photon sources (EPSs) operating in the telecommunication C-band. To test synchronization, they used an electrical clock signal from one source to drive the second. They analyzed temporal correlations using time-tagged coincidence measurements across various fiber lengths, including a 7 km metropolitan loop that incorporates real-world infrastructure such as splices, connectors, and background noise from existing network traffic.

The study confirms that while setting identical clock periods is insufficient for synchronization, sharing an electrical clock signal successfully establishes a clear temporal correlation peak. In the metropolitan environment, despite significant signal attenuation (approximately 14 dB) and background noise, the synchronization remained stable over an 8-hour acquisition period. The observed temporal drift was minimal (120 ps), and the correlation peaks remained well-approximated by Gaussian statistics, suggesting that current telecommunication fiber is suitable for supporting synchronized quantum network nodes.

This work provides an essential proof-of-concept for the scalability of the Quantum Internet. By demonstrating that entanglement sources can be synchronized over pre-installed metropolitan fiber, the authors show that building a large-scale quantum infrastructure does not necessarily require the installation of dedicated, expensive dark fibers. This paves the way for integrating quantum communication protocols into existing urban optical networks.