# Practical approach to last-mile converged free-space and fiber QKD for secure city-scale networks

## TL;DR

This paper presents a practical and cost-effective approach to deploying quantum key distribution over free-space and fiber networks in urban environments.

## Contribution

The study introduces a low-cost FSO-QKD system with stable performance in city-scale settings, integrated with existing fiber infrastructure.

## Key findings

- QBER values below 3.5% were achieved for a 0.5 km free-space link during nighttime.
- The system maintained acceptable QBER values below 5.5% even with coexisting C-band light signals.
- The setup enables secure key sharing over kilometer-scale wireless links in urban last-mile scenarios.

## Abstract

Current Free-Space Optical Quantum Key Distribution (FSO-QKD) systems are mainly focusing on the development of critical components needed towards the realization of satellite-based QKD, relying on bulky optical telescopes equipped with expensive optical systems. The cost and complexity of these systems hinder the establishment of a practical FSO-QKD infrastructure deployed as the wireless last-mile segment of Quantum Communication Infrastructure for applications in urban environments. In this work, we experimentally demonstrate successful transmission of polarization encoded qubits over 0.5 km free space metropolitan link with enhanced noise suppression for both nighttime and daylight operation. Βy deploying two low-cost, Single Mode Fiber-coupled, optical terminals along with a prototype quantum sender station operating in C-band, Quantum Bit Error Rate (QBER) values below 3.5% were achieved, with expected Secure Key Rates up to 1 kbps during nighttime. The FSO link was integrated with existing in-building fiber infrastructure, demonstrating stable operation and acceptable QBER values below 5.5%, even when coexisting with intense C-band light signals (> 10 mW) propagating alongside the photonic qubits. The field trial and its comprehensive verification paves the way for practical FSO-QKD in city-scale environments, enabling end users to share quantum keys over kilometer-scale wireless links between building rooftops in last-mile connectivity scenarios.

The online version contains supplementary material available at 10.1038/s41598-025-34184-z.