Provably Secure and Practical Quantum Key Distribution over 307 km of Optical Fibre (1407.7427v1)

Abstract: Proposed in 1984, quantum key distribution (QKD) allows two users to exchange provably secure keys via a potentially insecure quantum channel. Since then, QKD has attracted much attention and significant progress has been made in both theory and practice. On the application front, however, the operating distance of practical fibre-based QKD systems is limited to about 150 km, which is mainly due to the high background noise produced by commonly used semiconductor single-photon detectors (SPDs) and the stringent demand on the minimum classical- post-processing (CPP) block size. Here, we present a compact and autonomous QKD system that is capable of distributing provably-secure cryptographic key over 307 km of ultra-low-loss optical fibre (51.9 dB loss). The system is based on a recently developed standard semiconductor (inGaAs) SPDs with record low background noise and a novel efficient finite-key security analysis for QKD. This demonstrates the feasibility of practical long-distance QKD based on standard fibre optic telecom components.

• The paper introduces a QKD system using a coherent one-way protocol and semiconductor SPDs to achieve secure key exchange over 307 km of optical fiber.
• It implements a finite-key security analysis that accounts for collective attacks, addressing prior overestimations in long-distance QKD performance.
• Experimental results show a 3.18 bps secret key rate at 307 km and robust, extended operation, paving the way for practical quantum communication networks.

Provably Secure and Practical Quantum Key Distribution over 307 km of Optical Fibre

The paper discusses a significant enhancement in the field of Quantum Key Distribution (QKD), presenting a system that demonstrates provably secure key exchange over 307 km of optical fiber. This breakthrough addresses the long-standing challenge of extending the operating distance in fiber-based QKD systems beyond the previous practical limits, approximately 150 km.

Key Contributions

1. System Design: The described QKD system employs coherent one-way (COW) QKD protocol. It advances the practical utility of semiconductor single-photon detectors (SPDs), particularly InGaAs SPDs, significantly reducing dark count rates (DCR) without requiring cryogenic temperatures. This is a shift away from superconducting nanowire single-photon detectors (SNSPDs), typically necessary for long-distance QKD.
2. Finite-Key Security Analysis: An innovative security analysis framework is applied, accounting for finite-key effects. This development is particularly pertinent as previous demonstrations often ignored these corrections, leading to overestimations of achievable distances. The new security model assumes collective attacks and provides a well-defined security parameter for any block size used in data post-processing.
3. Experimental Achievements: The system achieves a 307 km distribution range with minimal loss (51.9 dB) while using conventional telecom-grade fiber. It maintains a high quantum bit error rate (QBER) and visibility throughout the tests, surpassing previous records without sacrificing compactness or practicality.

Significant Experimental Results

• Secret Key Rates (SKR): At a distance of 307 km, the SKR achieved is 3.18 bps. At shorter distances, such as 104 km, a significantly higher SKR of 12.7 kbps is obtained.
• Operational Stability: The system demonstrated the ability to operate stably over extended periods (e.g., 70 hours at 200 km) with consistent SKR and QBER, showcasing robustness for real-world applications.

Implications and Theoretical Advancements

This research indicates a promising future for QKD systems employing standard optical fiber infrastructure, significantly enhancing the feasibility of large-scale quantum communication networks. From a theoretical standpoint, the improved finite-key security analysis deepens the understanding of QKD security in non-ideal conditions, paving the way for further scalability in quantum networks.

Future Directions

Further exploration of ultra-low-loss fiber could enhance QKD performance, potentially extending operational ranges even beyond 307 km. Moreover, refining semiconductor SPD technologies could alleviate temperature-related restrictions, allowing room-temperature operations which would broaden commercial appeal and integration into existing telecommunication frameworks.

Conclusion

The research represents a substantial advancement in QKD, enhancing both the operational range and practical security of quantum communication over long distances. By leveraging semiconductor SPDs and robust finite-key analysis, the authors provide a viable pathway for practical quantum networks, supporting the ongoing transition from experimental setups to commercial systems.