Passive Synchronization of Nonlocal Franson Interferometry for Fiber-Based Quantum Networks Using Co-propagating Classical Clock Signals

TL;DR

This paper presents a passive synchronization method for fiber-based quantum networks using co-propagating classical clock signals, achieving high-visibility nonlocal Franson interferometry over 50 km without external timing infrastructure.

Contribution

It introduces a novel co-propagation scheme with cross-band allocation for quantum and classical signals, enabling scalable, high-precision synchronization in quantum networks.

Findings

Achieved 88.35% interference visibility over 50 km fiber

Suppressed Raman scattering noise via cross-band allocation

Demonstrated picosecond-level passive synchronization

Abstract

We demonstrate a robust, high-visibility nonlocal Franson interferometry for fiber-based quantum networks by co-propagating a classical Radio-over-Fiber clock signal with energy-time entangled photon pairs in the same fiber. Utilizing cross-band allocation (O-band for classical, L-band for quantum signals), the spontaneous Raman scattering noise photons are effectively suppressed. At the same time, their environmental delay fluctuations remain highly correlated for common-mode noise cancellation, achieving a passive synchronization with picoseconds precision. Over 50 km of single-mode fiber, this co-propagation enables nonlocal quantum interference with a visibility of (88.35\pm3.62)%, without relying on external dedicated timing infrastructure. This work provides a practical, scalable synchronization solution for metropolitan-scale entanglement-based quantum networks.