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

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.

Figures (8)

• Figure 1: (a) Schematic of a typical nonlocal Franson interferometry with co-propagating classical clock signals. (b) Corresponding coincidence envelopes in the time domain, showing the central interference peak and the two non-interference side peaks.
• Figure 2: Maximum achievable Franson interference visibility as a function of time synchronization error with $B = 10^8$ cps and $\sigma_0 = 30$ ps. The inset shows the corresponding optimal coincidence window $\tau_{\text{opt}}$ for each $\sigma_{\text{sync}}$. The shaded region in the inset indicates a $\pm 20\%$ tolerance range for coincidence window selection in practical implementations.
• Figure 3: Measured SpRS photon counts as a function of fiber length at 1555 nm (blue dots) and 1575 nm (green stars). The dashed lines represent fits based on Eqn. (\ref{['eq:SpRS']}).
• Figure 4: Interference visibility as a function of fiber length for a fixed source brightness $B = 10^8$ cps, single-photon detector DCR = 100 Hz and time synchronization errors of $\sigma_{\text{sync}} = 100$ ps.
• Figure 5: Setup for non-local Franson interference with co-Propagating RoF Synchronization over 50 km optical fiber.