Field-Trial Quantum Key Distribution with Qubit-Based Frame Synchronization

TL;DR

This paper demonstrates a practical, long-term quantum key distribution system using qubit-based frame synchronization over urban fiber networks, achieving high stability, low error rates, and secure key rates without extra hardware.

Contribution

It introduces a novel qubit-based frame synchronization method for QKD, eliminating the need for dedicated synchronization hardware in real-world urban environments.

Findings

Achieved a secure key rate of 26.6 kbit/s at 18 dB loss.

Maintained a low QBER of 1.12% over 12 hours of continuous operation.

Validated the system's stability and high-loss tolerance in a real urban setting.

Abstract

Quantum key distribution (QKD) is a cryptographic technique that uses quantum mechanical principles to enable secure key exchange. Practical deployment of QKD requires robust, cost-effective systems that can operate in challenging field environments. A major challenge is achieving reliable clock synchronization without adding hardware complexity. Conventional approaches often use separate classical light signals, which increase costs and introduce noise that degrades quantum channel performance. To address this limitation, we demonstrate a QKD system incorporating a recently proposed qubit-based distributed frame synchronization method, deployed over a metropolitan fiber network in Nanning, China. Using the polarization-encoded one-decoy-state BB84 protocol and the recently proposed qubit-based distributed frame synchronization method, our system achieves synchronization directly from the quantum signal, eliminating the need for dedicated synchronization hardware. Furthermore, to counteract dynamic polarization disturbances in urban fibers, the system integrates qubit-based polarization feedback control, enabling real-time polarization compensation through an automated polarization controller using data recovered from the qubit-based synchronization signals. During 12 hours of continuous operation, the system maintained a low average quantum bit error rate (QBER) of 1.12/%, achieving a secure key rate of 26.6 kbit/s under 18 dB channel loss. Even under a high channel loss of 40 dB, a finite-key secure rate of 115 bit/s was achieved. This study represents the first successful long-term validation of a frame-synchronization based QKD scheme in a real urban environment, demonstrating exceptional stability and high-loss tolerance, and offering an alternative for building practical, scalable, and cost-efficient quantum-secure communication networks.