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Optica Quantum

High-speed phase-encoded quantum key distribution over a 1.4-km free-space channel

Li Zhang, Fang‐Xiang Wang, Haiyang Fu, Xincai Tang, Guo-Wei Zhang, Hongyan Song, yongjun Qian, Dawei Li, Da-Chuang Li, Dong Pan, Guang‐Can Guo, Zheng‐Fu Han, Gui‐Lu Long

Published Feb 9, 2026• 1 citation
Publisher

Abstract

Quantum Key Distribution (QKD) represents a groundbreaking cryptographic technology that enables theoretically unconditionally secure key generation between communication parties. With recent advancements in network integration, QKD systems have been successfully deployed across diverse complex scenarios. For last-mile access network applications, free-space optical channels emerge as an optimal transmission medium. In this work, we implement a free-space QKD system employing a high-repetition-rate phase-encoding BB84 protocol. The experimental setup incorporates a simplified, highly integrable synchronization apparatus and self-compensating phase drift correction mechanism, which enhances both system stability and secure key generation rate. Experimental results demonstrate robust performance over a 1.4-km free-space channel with approximately 13 dB attenuation, achieving a secure key rate (SKR) of 160.19 kbps while maintaining a quantum bit error rate (QBER) of 2.3%. These findings establish the feasibility and efficiency of phase-encoded free-space QKD for last-mile access network applications.

DOI
10.1364/opticaq.577031

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Quantum Key Distribution (QKD) represents a groundbreaking cryptographic technology that enables theoretically unconditionally secure key generation between communication parties. With recent advancements in network integration, QKD systems have been successfully deployed across diverse complex scenarios. For last-mile access network applications, free-space optical channels emerge as an optimal transmission medium. In this work, we implement a free-space QKD system employing a high-repetition-rate phase-encoding BB84 protocol. The experimental setup incorporates a simplified, highly integrable synchronization apparatus and self-compensating phase drift correction mechanism, which enhances both system stability and secure key generation rate. Experimental results demonstrate robust performance over a 1.4-km free-space channel with approximately 13 dB attenuation, achieving a secure key rate (SKR) of 160.19 kbps while maintaining a quantum bit error rate (QBER) of 2.3%. These findings establish the feasibility and efficiency of phase-encoded free-space QKD for last-mile access network applications.
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As quantum computing threatens to compromise classical cryptographic systems, the need for future-proof, information-theoretically secure methods of communication has become critical. Quantum Key Distribution (QKD) leverages the fundamental principles of quantum mechanics to enable the secure generation and exchange of encryption keys, immune to computational attacks. This paper investigates the integration of QKD into telecom infrastructures to enable ultra-secure communication. We examine the performance of key QKD protocols, such as BB84 and E91, over fiber-optic and free-space channels, and simulate their deployment within realistic metropolitan network topologies. Results show that secure key rates of up to 10 kbps can be maintained over 50–80 km fiber links with Quantum Bit Error Rates (QBER) below 5%. The study also explores architecture models based on trusted relay nodes and WDM coexistence, highlighting practical deployment considerations. Our findings suggest that QKD is a viable enhancement for telecom security and a necessary foundation for long-term quantum-resilient communication systems.
Quantum key distribution (QKD) leverages the principles of quantum mechanics to exchange a secret key between two parties. Despite its promising features, QKD also faces several practical challenges such as transmission loss, noise in quantum channels and finite key size effects. Addressing these issues is crucial for the large-scale deployment of QKD in fiber and satellite networks. In this paper, we present a 1550 nm QKD system realizing the efficient-BB84 protocol and based on the iPOGNAC scheme. The system achieved repetition rates up to 1.5~GHz and showed an intrinsic QBER of ∼0.4%\sim 0.4\%∼0.4%. The system was first tested on a laboratory fiber link and then on an intermodal link in the field, consisting of both deployed fiber and a 620 m free-space channel. The experiment was performed in daylight conditions, exploiting the Qubit4Sync synchronization protocol. With this trial, we achieved a new benchmark for free-space BB84 QKD systems by generating a sustained secret key rate (SKR) above 1~Mb/s for 1 hour. Finally, exploiting a recently discovered finite-size bound, we achieved a secure key rate of about 10 Mb/s at low losses (5 dB), and around 6.5~kb/s in the high-loss (38.5 dB), low block length (N=104N=10^4N=104) regime. The latter results demonstrate the system's suitability for highly lossy and time-constrained scenarios such as QKD from low Earth orbit satellites.
Quantum communication technologies capable of operating reliably across heterogeneous optical channels are essential for scalable metropolitan quantum networks. Here we demonstrate high-dimensional time-bin-encoded quantum key distribution over a hybrid metropolitan link comprising 1.7 km free-space transmission and 685 m of optical fiber. Operating at a clock rate of 500 MHz in the C-band, we implement both 2- and 4-dimensional protocols, and obtain estimated secure finite-key rates of (95 +- 28) kbit/s for 4D at (25.0 +- 2.0) dB loss and (59 +- 27) kbit/s for 2D at (23.5 +- 2.3) dB loss. Crucially, we achieve continuous operation over 48 h in a fully self-referenced architecture: initial synchronization, interferometric phase stabilization, and long-term drift compensation are performed exclusively using the detected quantum signals, without auxiliary optical reference channels. Our results thus establish a practical and versatile platform for hybrid free-space-to-fiber quantum communication and show that the encoding dimensionality can be adapted to the optimal operating regime of realistic metropolitan channels, providing a pathway toward efficient, autonomous and deployable quantum network nodes.