Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2512.01359

Witnessing the Effective Entanglement in the COW Protocol

F. Rezazadeh, A. Mani, M. Khodabandeh, M. Jaberi, S. A. Madani

quant-phSubmitted Dec 1, 2025• Announced Dec 2, 2025• 0 citations
arXivHTMLPDF

Abstract

We present a rigorous mathematical framework for verifying effective entanglement in a Coherent One-Way (COW) quantum key distribution setup. In particular, we introduce a two-parameter family of entanglement witnesses, identify the parameter ranges where they constitute valid witnesses, and demonstrate their ability to reveal effective entanglement in the COW protocol. Additionally, we analyze previously obtained experimental data from a COW implementation and report clear signatures of effective entanglement.

Related papers

Finite-key analysis for coherent one-way quantum key distribution

Ming-Yang Li, Xiaoyu Cao, Yuan-Mei Xie +2 more

Coherent-one-way (COW) quantum key distribution (QKD) is a significant communication protocol that has been implemented experimentally and deployed in practical products due to its simple equipment requirements. However, existing security analyses of COW-QKD either provide a short transmission distance or lack immunity against coherent attacks in the finite-key regime. In this paper, we present a tight finite-key security analysis within the universally composable framework for a variant of COW-QKD, which has been proven to extend the secure transmission distance in the asymptotic case. We combine the quantum leftover hash lemma and entropic uncertainty relation to derive the key rate formula. When estimating statistical parameters, we use the recently proposed Kato's inequality to ensure security against coherent attacks and achieve a higher key rate. Our paper confirms the security and feasibility of COW-QKD for practical application and lays the foundation for further theoretical study and experimental implementation. Published by the American Physical Society 2024

arXiv:2309.16136•Physical Review Research•Jan 8, 2024•19 citations

Upper Security Bounds for Coherent-One-Way Quantum Key Distribution

J. González-Payo, Róbert Trényi, Weilong Wang +1 more

The performance of quantum key distribution (QKD) is severely limited by multiphoton pulses emitted by laser sources due to the photon-number splitting attack. Coherent-one-way (COW) QKD has been introduced as a promising solution to overcome this limitation, and thus extend the achievable distance of practical QKD. Indeed, thanks to its experimental simplicity, the COW protocol is already used in commercial applications. Here, we derive simple upper security bounds on its secret key rate, which demonstrate that it scales at most quadratically with the system's transmittance, thus solving a long-standing problem. That is, in contrast to what has been claimed, this approach is inappropriate for long-distance QKD transmission. Remarkably, our findings imply that all implementations of the COW protocol performed so far are insecure.

arXiv:2006.16891•Physical Review Letters•Dec 31, 2020•31 citations

Simple security proof of coherent-one-way quantum key distribution

Rui-Qi Gao, Yuan-Mei Xie, Jie Gu +5 more

Coherent-one-way quantum key distribution (COW-QKD), which requires a simple experimental setup and has the ability to withstand photon-number-splitting attacks, has been not only experimentally implemented but also commercially applied. However, recent studies have shown that the current COW-QKD system is insecure and can only distribute secret keys safely within 20 km of the optical fiber length. In this study, we propose a practical implementation of COW-QKD by adding a two-pulse vacuum state as a new decoy sequence. This proposal maintains the original experimental setup as well as the simplicity of its implementation. Utilizing detailed observations on the monitoring line to provide an analytical upper bound on the phase error rate, we provide a high-performance COW-QKD asymptotically secure against coherent attacks. This ensures the availability of COW-QKD within 100 km and establishes theoretical foundations for further applications.

arXiv:2107.09329•Optics Express•Jun 10, 2022•25 citations

Experimental Implementation of Enhanced Security Coherent One-Way Quantum Key Distribution

Amirhosein Dadahhani, Soheil Hajibaba, Hamid Asgari +4 more

Coherent One-Way (COW) is a simple and widely utilized quantum key distribution (QKD) protocol, favored in both experimental and commercial applications. In this paper, we compare the information that an eavesdropper, Eve, can acquire in both the 2-decoy and 1-decoy COW protocols. Our findings demonstrate that the 2-decoy protocol offers enhanced security compared to the original 1-decoy COW, all while preserving the simplicity of implementation. Furthermore, we have experimentally implemented the improved security COW-QKD (2-decoy protocol), achieving a secure key rate exceeding 0.5 kbps over a distance of 110 km (22 dB loss) in optical fiber, with an interference visibility greater than 80%. We also emphasize that monitoring the click rates of the data line detector for each state individually serves as a powerful method for detecting zero-error attacks.

IEEE Access•Jan 1, 2025•2 citations

Quantum Oblivious Transfer through Coherent-One-Way Quantum-Key-Distribution

Javier Faba, Juan José Romero, Laura Ortiz Martín +1 more

In this work, we investigate the feasibility of realizing Quantum Oblivious Transfer (QOT) using the experimental implementation of the Coherent-One-Way (COW) quantum key distribution protocol, requiring only minimal modifications.

Workshop on Quantum Networks and Distributed Quantum Computing (QuNet)•Aug 19, 2025•0 citations

Improved Coherent One-Way Quantum key Distribution for High-Loss Channels

Emilien Lavie, Charles Ci Wen Lim

Coherent one-way quantum key distribution (COW-QKD) is a popular cryptography protocol, due to its fairly simple implementation and passive measurement device. Its security analysis is challenging, though, and extra assumptions or modifications are typically employed. Here the authors propose minor modifications to COW-QKD's prepared states while retaining the original, fully passive measurement device, and generalize the ``universal squashing'' framework to analyze their system's security. The results suggest that performance should be comparable to upper bounds from the literature, making this protocol a viable alternative to one based on phase-modulated coherent states.

arXiv:2206.08490•Physical Review Applied•Dec 19, 2022•18 citations
We present a rigorous mathematical framework for verifying effective entanglement in a Coherent One-Way (COW) quantum key distribution setup. In particular, we introduce a two-parameter family of entanglement witnesses, identify the parameter ranges where they constitute valid witnesses, and demonstrate their ability to reveal effective entanglement in the COW protocol. Additionally, we analyze previously obtained experimental data from a COW implementation and report clear signatures of effective entanglement.
Coherent-one-way (COW) quantum key distribution (QKD) is a significant communication protocol that has been implemented experimentally and deployed in practical products due to its simple equipment requirements. However, existing security analyses of COW-QKD either provide a short transmission distance or lack immunity against coherent attacks in the finite-key regime. In this paper, we present a tight finite-key security analysis within the universally composable framework for a variant of COW-QKD, which has been proven to extend the secure transmission distance in the asymptotic case. We combine the quantum leftover hash lemma and entropic uncertainty relation to derive the key rate formula. When estimating statistical parameters, we use the recently proposed Kato's inequality to ensure security against coherent attacks and achieve a higher key rate. Our paper confirms the security and feasibility of COW-QKD for practical application and lays the foundation for further theoretical study and experimental implementation. Published by the American Physical Society 2024
The performance of quantum key distribution (QKD) is severely limited by multiphoton pulses emitted by laser sources due to the photon-number splitting attack. Coherent-one-way (COW) QKD has been introduced as a promising solution to overcome this limitation, and thus extend the achievable distance of practical QKD. Indeed, thanks to its experimental simplicity, the COW protocol is already used in commercial applications. Here, we derive simple upper security bounds on its secret key rate, which demonstrate that it scales at most quadratically with the system's transmittance, thus solving a long-standing problem. That is, in contrast to what has been claimed, this approach is inappropriate for long-distance QKD transmission. Remarkably, our findings imply that all implementations of the COW protocol performed so far are insecure.
Coherent-one-way quantum key distribution (COW-QKD), which requires a simple experimental setup and has the ability to withstand photon-number-splitting attacks, has been not only experimentally implemented but also commercially applied. However, recent studies have shown that the current COW-QKD system is insecure and can only distribute secret keys safely within 20 km of the optical fiber length. In this study, we propose a practical implementation of COW-QKD by adding a two-pulse vacuum state as a new decoy sequence. This proposal maintains the original experimental setup as well as the simplicity of its implementation. Utilizing detailed observations on the monitoring line to provide an analytical upper bound on the phase error rate, we provide a high-performance COW-QKD asymptotically secure against coherent attacks. This ensures the availability of COW-QKD within 100 km and establishes theoretical foundations for further applications.
Coherent One-Way (COW) is a simple and widely utilized quantum key distribution (QKD) protocol, favored in both experimental and commercial applications. In this paper, we compare the information that an eavesdropper, Eve, can acquire in both the 2-decoy and 1-decoy COW protocols. Our findings demonstrate that the 2-decoy protocol offers enhanced security compared to the original 1-decoy COW, all while preserving the simplicity of implementation. Furthermore, we have experimentally implemented the improved security COW-QKD (2-decoy protocol), achieving a secure key rate exceeding 0.5 kbps over a distance of 110 km (22 dB loss) in optical fiber, with an interference visibility greater than 80%. We also emphasize that monitoring the click rates of the data line detector for each state individually serves as a powerful method for detecting zero-error attacks.
Coherent one-way quantum key distribution (COW-QKD) is a popular cryptography protocol, due to its fairly simple implementation and passive measurement device. Its security analysis is challenging, though, and extra assumptions or modifications are typically employed. Here the authors propose minor modifications to COW-QKD's prepared states while retaining the original, fully passive measurement device, and generalize the ``universal squashing'' framework to analyze their system's security. The results suggest that performance should be comparable to upper bounds from the literature, making this protocol a viable alternative to one based on phase-modulated coherent states.