Photon level crosstalk between parallel fibers installed in urban area

TL;DR

This study measures photon-level crosstalk in urban-installed dark fibers over 45 km, revealing that stray light from public internet fibers can cause crosstalk, affecting quantum key distribution security.

Contribution

It provides the first detailed measurement of photon crosstalk in urban dark fibers at single-photon levels, highlighting potential security risks for QKD systems.

Findings

Stray light from public fibers is the dominant crosstalk source.

Crosstalk occurs mainly at cable bending points.

Crosstalk can increase bit error rates in QKD.

Abstract

We estimate the channel characteristics of field-installed dark fibers at single photon levels for quantum key distribution (QKD). Measured fibers are telecom single-mode dark fibers over 45 km connecting a center and suburbs of Tokyo. Their total losses are about 14dB, and 50% of the whole lengths are aerial lines. We find that stray light from other public internet fibers is dominant and crosstalk occurs at bending points in laying cables. These results mean the crosstalk from public networks can increase the bit error rate in the QKD system, and imply an underlying information leakage through an adjacent covered-fiber.