Quantum Communication Breaks 1000km Barrier with New Secure Transmission Method

Scientists have long sought to extend the practical range of quantum key distribution, a secure communication method. Georgi Bebrov from the Technical University of Varna, alongside collaborators, demonstrate a novel sending-or-not-sending phase-matching quantum key distribution protocol (SNS-PM-QKD) that significantly improves resilience to phase mismatch. This advancement directly addresses a key limitation in long-distance quantum communication systems and, through rigorous security analysis and performance comparisons with existing protocols, including standard phase-matching QKD and twin-field QKD systems reaching 1002km, establishes SNS-PM-QKD as a promising pathway towards substantially increased transmission distances for secure data transmission.

This work introduces sending-or-not-sending phase-matching QKD, or SNS-PM-QKD, a protocol designed to improve tolerance to phase mismatch and extend achievable transmission distances in secure communication.

A detailed security analysis has been performed within the asymptotic regime under collective attacks, confirming the protocol’s robustness against potential eavesdropping strategies. Performance comparisons against standard phase-matching QKD, theoretical twin-field QKD protocols, and an experimental twin-field QKD system operating over distances up to 1002km demonstrate a clear advantage for SNS-PM-QKD in terms of transmission range.
The central limitation of QKD deployment remains the restricted operational transmission distance imposed by optical loss and phase drift within communication channels. This research addresses this challenge by focusing on key-rate dependence on channel transmissivity and tolerance to errors. Achieving square-root transmissivity scaling of the key rate is a key feature of twin-field QKD protocols, which rely on single-photon interference at a beam splitter.

The newly proposed SNS-PM-QKD builds upon phase-matching QKD, simplifying the key-generation mechanism without compromising security. A high tolerance to errors is attained through the absence of interference in key-generation events and the implementation of specific postselection criteria. Existing SNS-TF-QKD schemes have already achieved the longest distance range among fiber-based QKD systems, reaching approximately 910km in theoretical models and 1002km in experimental setups.

Motivated by the need for further improvements in long-distance quantum communication, researchers constructed a protocol that reduces error rates by integrating outcome postselection and a sending-or-not-sending paradigm into a phase-matching QKD framework. The protocol functions by having Alice and Bob each prepare two coherent states, a signal state and a secondary state, with identical phases and then randomly deciding whether to send them to an intermediate node.

A preliminary interference stage at the intermediate node, utilising a coupler, induces a phase shift, leading to destructive interference under specific conditions. Key generation is based on the sending-or-not-sending choices, with postselection of measurement outcomes further mitigating the error rate and improving the key-rate-to-error-rate ratio. This ultimately enables greater operational distances for secure quantum communication.

Experimental protocol and comparative performance analysis are detailed in Appendix A

A sending-or-not-sending phase-matching quantum key distribution protocol (SNS-PM-QKD) underpinned this research, designed to enhance resilience against phase mismatch and extend transmission distances. The study implemented a protocol where coherent states are either sent or not sent, leveraging phase-matching techniques to improve key distribution security.

Security analysis proceeded within the asymptotic regime, specifically addressing collective attacks to rigorously evaluate the protocol’s robustness. Researchers compared the performance of SNS-PM-QKD against standard phase-matching QKD, theoretical twin-field QKD protocols of the SNS type (SNS-TF-QKD), and an experimental SNS-TF-QKD system.

The experimental SNS-TF-QKD operated across transmission distances extending up to 1002km, providing a benchmark for comparison. Results demonstrated that SNS-PM-QKD consistently achieved greater transmission distances than these established protocols, indicating a significant advancement in long-distance quantum communication capabilities.

Phase values φb and τa were modulated at couplers Cs and Cr by introducing an additional π phase shift, influencing the reflection of coherent states eiφb√μ Q′ and eiτa√μ T. Correlation between variables k and y, determined through experimental observations, directly informed the security assessment and was found to be proportional to observed correlations. The research noted that observations yielding [κ′ =?,κ′′ = +] produced identical outcomes, simplifying the analysis and confirming the protocol’s consistency.

Extended transmission distances via a novel sending-or-not-sending phase-matching quantum key distribution protocol are now demonstrated

Scientists developed a sending-or-not-sending phase-matching quantum key distribution protocol (SNS-PM-QKD) demonstrating extended transmission distances compared to existing methods. This new protocol improves tolerance to phase mismatch, a critical factor limiting the range of quantum communication systems.

The research focused on achieving greater distances through protocol design while maintaining security against potential attacks. The study presents a security analysis of SNS-PM-QKD within the asymptotic regime under collective attacks, establishing its theoretical robustness. Performance comparisons were conducted against standard phase-matching QKD, theoretical SNS-type twin-field QKD (SNS-TF-QKD) protocols, and an experimental SNS-TF-QKD system operating over distances up to 1002km.

Results indicate that SNS-PM-QKD surpasses these existing protocols in achievable transmission distance, suggesting its potential for long-distance quantum communication networks. Key generation in this protocol relies on the sending-or-not-sending choice, similar to previous SNS-QKD approaches. Outcome postselection, sifting measurement outcomes to exclude those not corresponding to destructive interference, effectively mitigates error rates.

This mitigation improves the key-rate-to-error-rate ratio, directly contributing to the extended operational distances achieved. The work adopts a security framework established in prior research to ensure the integrity of the key generation process. Evaluations demonstrate the protocol’s advantages in terms of operational distances when contrasted with both theoretical and experimental implementations of sending-or-not-sending protocols.

The proposed scheme incorporates a preliminary interference stage at an intermediate node, inducing a phase shift that enhances the destructive interference effect. This design feature, combined with outcome postselection, allows for a reduction in error rates and a corresponding increase in the maximum achievable transmission distance for secure quantum communication.

Enhanced transmission distances via phase-matching sending-or-not-sending quantum key distribution are now demonstrated

Researchers have developed a new quantum key distribution (QKD) protocol, sending-or-not-sending phase-matching QKD (SNS-PM-QKD), which demonstrably extends achievable transmission distances for secure communication. This protocol builds upon the sending-or-not-sending principle and incorporates improvements to tolerate phase mismatch, a significant limitation in long-distance QKD systems.

Security analysis within the asymptotic regime and collective attacks confirms the protocol’s viability for generating cryptographic keys. Performance comparisons against standard phase-matching QKD, theoretical twin-field QKD protocols, and existing experimental implementations reveal that SNS-PM-QKD surpasses these methods in terms of operational transmission distances.

Specifically, the protocol achieves greater distances than previously reported, suggesting a substantial advancement in the practicality of long-distance secure communication. The authors acknowledge that the analysis assumes ideal conditions and that real-world implementations will be subject to imperfections in devices and channels.

Future research could focus on mitigating these practical limitations and exploring the protocol’s performance in more complex network topologies. These findings represent a step towards more robust and extended-range quantum communication networks.