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

Rui Guan, Jingchun Yu, Zhaoyun Li, Hongbo Xie, Yuxing Wei, Sen Li, Jing Wen, Xiaodong Liang, Yanwei Li, Kejin Wei

TL;DR

This work tackles the practical barrier of clock synchronization in field-deployed QKD by implementing qubit-based distributed frame synchronization that derives timing from the quantum signal itself, removing the need for separate synchronization hardware. Coupled with qubit-based polarization feedback, the system auto-compensates dynamic polarization disturbances in urban fibers, enabling stable operation over a metropolitan link. Field tests show a low average QBER around 1.12% and a secure key rate of 26.6 kbit/s at 18 dB loss, with finite-key secure rates of 115 bit/s at 40 dB, demonstrating strong high-loss tolerance. The results support the viability of scalable, cost-efficient quantum-secure networks using frame-synchronization-based QKD in real-world environments.

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.

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

TL;DR

This work tackles the practical barrier of clock synchronization in field-deployed QKD by implementing qubit-based distributed frame synchronization that derives timing from the quantum signal itself, removing the need for separate synchronization hardware. Coupled with qubit-based polarization feedback, the system auto-compensates dynamic polarization disturbances in urban fibers, enabling stable operation over a metropolitan link. Field tests show a low average QBER around 1.12% and a secure key rate of 26.6 kbit/s at 18 dB loss, with finite-key secure rates of 115 bit/s at 40 dB, demonstrating strong high-loss tolerance. The results support the viability of scalable, cost-efficient quantum-secure networks using frame-synchronization-based QKD in real-world environments.

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.
Paper Structure (4 sections, 2 equations, 4 figures, 1 table)

This paper contains 4 sections, 2 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Setup
  3. Experimental Results
  4. Conclusion

Figures (4)

  • Figure 1: Aerial view of the fiber link used in the QKD field trial performed in Nanning, China. [©2024 Baidu]. The link connects the transmitter (Alice), located at the Guangxi Zhuang Autonomous Region Information Center, with the receiver (Bob), situated at the School of Physical Science and Technology, Guangxi University.
  • Figure 2: Schematic representation of the setup employed in the experiment. In the upper-left and upper-right boxes we represent the setup for the transmitter(Alice) and receiver(Bob), respectively. BS, beam splitter; PM, phase modulator; CPM, customized polarization module; VOA, variable optical attenuator; DWDM, dense wavelength division multiplexing; EPC, electronic polarization controller; PAM, polarization analysis module; SPD, single photon detector; PM fiber, polarization-maintaining fiber; SM fiber, single mode fiber.
  • Figure 3: QBER on $Z (X)$ basis of the system (left axis) and the feedback voltage of EPC (right axis, for EPC channels 1, 2, and 3 in blue, purple, and orange) over a 12-hour continuous operation in a field-deployed fiber link with 18 dB channel loss. The red (blue) point denotes the quantum bit error in $Z (X)$ basis, which are recorded every 2.5 minutes.
  • Figure 4: The secure key rate under different transmission losses. The blue line shows the simulation results based on our experimental parameters, the red stars represent the experimental results obtained over real fiber links. The red circle and triangle markers denote the results measured over the same field fiber with additional 12 dB and 22 dB losses emulated by VOA, corresponding to total losses of 30 dB and 40 dB, respectively.