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:2601.04543 · The European Physical Journal D

Increasing the secret key rates and point-to-multipoint extension for experimental coherent-one-way quantum key distribution protocol

Venkat Abhignan, Mohit Mittal, Aditi Das, Megha Shrivastava

quant-phphysics.opticsSubmitted Jan 8, 2026• Announced Jan 9, 2026• 0 citations
v1 · Jan 8, 2026v2 · Jul 20, 2026
arXivHTMLPDF

Abstract

Using quantum key distribution (QKD) protocols, a secret key is created between two distant users (transmitter and receiver) at a particular key rate. Quantum technology can facilitate secure communication for cryptographic applications, combining QKD with one-time-pad (OTP) encryption. In order to ensure the continuous operation of QKD in real-world networks, efforts have been concentrated on optimizing the use of components and effective QKD protocols to improve secret key rates and increase the transmission between multiple users. Generally, in experimental implementations, the secret key rates are limited by single-photon detectors, which are used at the receivers of QKD and create a bottleneck due to their limited detection rates (detectors with low detection efficiency and high detector dead-time). We experimentally show that secret key rates can be increased by combining the time-bin information of two such detectors on the data line of the receiver for the coherent-one-way (COW) QKD protocol with a minimal increase in quantum bit error rate (QBER, the proportion of erroneous bits). Further, we implement a point-to-multipoint COW QKD protocol, introducing an additional receiver module. The three users (one transmitter and two receivers) share the secret key in post-processing, relying on OTP encryption. Typically, the dual-receiver extension can improve the combined secret key rates of the system; however, one has to optimise the experimental parameters to achieve this within security margins. These methods are general and can be applied to any implementation of the COW protocol.

Journal reference
Eur. Phys. J. D (2026) 80:75
DOI
10.1140/epjd/s10053-026-01180-5

Related papers

Fiber-based higher dimensional quantum key distribution implementation using time-bin qudits

Gokul Agasthilingam, Harinee Natarajan, Varun Raghunathan

Higher Dimensional Quantum Key Distribution (HD-QKD) improves information capacity and security compared to binary QKD systems. Previous HD-QKD implementations have required d+1 (d refers to encoding dimension) single-photon detectors, limiting large-scale implementation. In contrast, another HD-QKD implementation using coherent one-way (COW) protocol hardware with two detectors achieved a secret key rate (SKR) of 155 kbps for an 8-D system and 2.7% QBER tolerance for a 16-D system with qubit scrambling across a time window. In this paper, we theoretically simulate and experimentally demonstrate time-bin encoded HD-QKD using COW protocol hardware with two single-photon detectors operating at standard telecom wavelengths, achieving a remarkable maximum encoding of 6 bits per photon (i.e., 56-d system). Our HD-QKD system incorporates a practical decoy-state technique to counter the photon number splitting attack, allowing greater flexibility in choosing higher mean photon number thereby achieving higher SKR and distances. This implementation demonstrates a remarkable ninefold increase in SKR for a 16-d system (210 kbps over 25 km of standard single-mode fiber) compared to qubit-based systems, with a QBER tolerance of 13.5% for d = 16. Notably, we achieved positive key rates for emulated loss up to 131 km of fiber length for d = 8. By optimizing detector deadtime and mean photon number using our theoretical model, we achieve enhanced key rates and security. Our findings highlight the superiority of our approach and lay the foundation for high throughput HD-QKD implementation, offering a secure and efficient method for transmitting sensitive information.

Quantum Computing, Communication, and Simulation•Mar 12, 2024•2 citations

Multi-protocol quantum key distribution decoding chip

Chunxue Zhang, Jianguang Li, Yue Wang +3 more

Abstract Quantum key distribution (QKD) is a method for secure communication that utilizes quantum mechanics principles to distribute cryptographic keys between parties. Integrated photonics offer benefits such as compactness, scalability, energy efficiency and the potential for extensive integration. We have achieved BB84 phase encoding and decoding, time-bin phase QKD, and the coherent one-way (COW) protocol on a planar lightwave circuit (PLC) platform. At the optimal temperature, our chip successfully prepared quantum states, performed decoding and calculated the secure key rate of the time-bin phase-decoding QKD to be 80.46 kbps over a 20 km transmission with a quantum bit error rate (QBER) of 4.23%. The secure key rate of the COW protocol was 18.18 kbps, with a phase error rate of 3.627% and a time error rate of 0.377%. The uniqueness of this technology lies in its combination of high integration and protocol flexibility, providing an innovative solution for the development of future quantum communication networks.

Chinese Physics B•Feb 17, 2025•1 citation

High-Rate Point-to-Multipoint Quantum Key Distribution using Coherent States

Yiming Bian, Yichen Zhang, Chao Zhou +3 more

Quantum key distribution (QKD) which enables information-theoretically security is now heading towards quantum secure networks. It requires high-performance and cost-effective protocols while increasing the number of users. Unfortunately, qubit-implemented protocols only allow one receiver to respond to the prepared signal at a time, thus cannot support multiple users natively and well satisfy the network demands. Here, we show a 'protocol solution' using continuous-variable quantum information. A coherent-state point-to-multipoint protocol is proposed to simultaneously support multiple independent QKD links between a single transmitter and massive receivers. Every prepared coherent state is measured by all receivers to generate raw keys, then processed with a secure and high-efficient key distillation method to remove the correlations between different QKD links. It can achieve remarkably high key rates even with a hundred of access points and shows the potential improvement of two orders of magnitude. This scheme is a promising step towards a high-rate multi-user solution in a scalable quantum secure network.

arXiv:2302.02391•Feb 5, 2023•4 citations

Implementation of coherent one way protocol for quantum key distribution up to an effective distance of 145 km

Priya Malpani, Satish Kumar, Anirban Pathak

Abstract It’s well known that many of the traditional cryptographic schemes (like RSA and DH) will be vulnerable when scalable quantum computers will be available. However, quantum cryptographic protocols can provide unconditional security. One such protocol for unconditionally secure quantum key distribution (QKD) is coherent one way (COW) protocol. In the present work, we report experimental realization of an optical fiber based COW protocol for QKD in the telecom wavelength (1550 nm) where the attenuation in the optical fiber is minimum. A laser of 1550 nm wavelength, attenuator and intensity modulator is used for the generation of pulses having average photon number 0.5 and repetition rate of 500 MHz. The experiment is performed over 40 km, 80 km and 120 km of optical fiber and several experimental parameters like disclose rate, compression ratio, dead time and excess bias voltage of the detector are varied for all the cases (i.e., for 40 km, 80 km and 120 km distances) to observe their impact on the final key rate. Specifically, It is observed that there is a linear increase in the key rate as we decrease compression ratio or disclose rate. The key rate obtains its maximum value for least permitted values of disclose rate, compression ratio and dead time. It seems to remain stable for various values of excess bias voltage. While changing various parameters, we have maintained the quantum bit error rate (QBER) below 6% (5%) for distance ≥ 120 km (≤ 120 km). The key rate obtained is also found to remain stable over time. Experimental results obtained here are also compared with the earlier realizations of the COW QKD protocol. Further, to emulate key rate at intermediate distances and at a distance larger than 120 km, an attenuator of 5 dB loss is used which can be treated as equivalent to ∼ 25 km of the optical fiber used in the present implementation. This has made the present implementation equivalent to the realization of COW QKD upto ∼ 145 km.

arXiv:2309.07555•Optical and Quantum Electronics•Jul 30, 2024•10 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

Foiling zero-error attacks against coherent-one-way quantum key distribution

Marcos Curty

To protect practical quantum key distribution (QKD) against photon-number-splitting attacks, one could measure the coherence of the received signals. One prominent example that follows this approach is coherent-one-way (COW) QKD, which is commercially available. Surprisingly, however, it has been shown very recently that its secret key rate scales quadratically with the channel transmittance and thus this scheme is unsuitable for long-distance transmission. This result was derived by using a zero-error attack, which prevents the distribution of a secure key without introducing any error. Here we study various countermeasures to foil zero-error attacks against COW QKD. They require either monitoring some additional available detection statistics or increasing the number of quantum states emitted. We obtain asymptotic upper security bounds on the secret key rate that scale close to linearly with the channel transmittance, thus suggesting the effectiveness of the countermeasures to boost the performance of this protocol.

arXiv:2108.12393•Physical Review A•Dec 10, 2021•7 citations
Using quantum key distribution (QKD) protocols, a secret key is created between two distant users (transmitter and receiver) at a particular key rate. Quantum technology can facilitate secure communication for cryptographic applications, combining QKD with one-time-pad (OTP) encryption. In order to ensure the continuous operation of QKD in real-world networks, efforts have been concentrated on optimizing the use of components and effective QKD protocols to improve secret key rates and increase the transmission between multiple users. Generally, in experimental implementations, the secret key rates are limited by single-photon detectors, which are used at the receivers of QKD and create a bottleneck due to their limited detection rates (detectors with low detection efficiency and high detector dead-time). We experimentally show that secret key rates can be increased by combining the time-bin information of two such detectors on the data line of the receiver for the coherent-one-way (COW) QKD protocol with a minimal increase in quantum bit error rate (QBER, the proportion of erroneous bits). Further, we implement a point-to-multipoint COW QKD protocol, introducing an additional receiver module. The three users (one transmitter and two receivers) share the secret key in post-processing, relying on OTP encryption. Typically, the dual-receiver extension can improve the combined secret key rates of the system; however, one has to optimise the experimental parameters to achieve this within security margins. These methods are general and can be applied to any implementation of the COW protocol.
Higher Dimensional Quantum Key Distribution (HD-QKD) improves information capacity and security compared to binary QKD systems. Previous HD-QKD implementations have required d+1 (d refers to encoding dimension) single-photon detectors, limiting large-scale implementation. In contrast, another HD-QKD implementation using coherent one-way (COW) protocol hardware with two detectors achieved a secret key rate (SKR) of 155 kbps for an 8-D system and 2.7% QBER tolerance for a 16-D system with qubit scrambling across a time window. In this paper, we theoretically simulate and experimentally demonstrate time-bin encoded HD-QKD using COW protocol hardware with two single-photon detectors operating at standard telecom wavelengths, achieving a remarkable maximum encoding of 6 bits per photon (i.e., 56-d system). Our HD-QKD system incorporates a practical decoy-state technique to counter the photon number splitting attack, allowing greater flexibility in choosing higher mean photon number thereby achieving higher SKR and distances. This implementation demonstrates a remarkable ninefold increase in SKR for a 16-d system (210 kbps over 25 km of standard single-mode fiber) compared to qubit-based systems, with a QBER tolerance of 13.5% for d = 16. Notably, we achieved positive key rates for emulated loss up to 131 km of fiber length for d = 8. By optimizing detector deadtime and mean photon number using our theoretical model, we achieve enhanced key rates and security. Our findings highlight the superiority of our approach and lay the foundation for high throughput HD-QKD implementation, offering a secure and efficient method for transmitting sensitive information.
Abstract Quantum key distribution (QKD) is a method for secure communication that utilizes quantum mechanics principles to distribute cryptographic keys between parties. Integrated photonics offer benefits such as compactness, scalability, energy efficiency and the potential for extensive integration. We have achieved BB84 phase encoding and decoding, time-bin phase QKD, and the coherent one-way (COW) protocol on a planar lightwave circuit (PLC) platform. At the optimal temperature, our chip successfully prepared quantum states, performed decoding and calculated the secure key rate of the time-bin phase-decoding QKD to be 80.46 kbps over a 20 km transmission with a quantum bit error rate (QBER) of 4.23%. The secure key rate of the COW protocol was 18.18 kbps, with a phase error rate of 3.627% and a time error rate of 0.377%. The uniqueness of this technology lies in its combination of high integration and protocol flexibility, providing an innovative solution for the development of future quantum communication networks.
Quantum key distribution (QKD) which enables information-theoretically security is now heading towards quantum secure networks. It requires high-performance and cost-effective protocols while increasing the number of users. Unfortunately, qubit-implemented protocols only allow one receiver to respond to the prepared signal at a time, thus cannot support multiple users natively and well satisfy the network demands. Here, we show a 'protocol solution' using continuous-variable quantum information. A coherent-state point-to-multipoint protocol is proposed to simultaneously support multiple independent QKD links between a single transmitter and massive receivers. Every prepared coherent state is measured by all receivers to generate raw keys, then processed with a secure and high-efficient key distillation method to remove the correlations between different QKD links. It can achieve remarkably high key rates even with a hundred of access points and shows the potential improvement of two orders of magnitude. This scheme is a promising step towards a high-rate multi-user solution in a scalable quantum secure network.
Abstract It’s well known that many of the traditional cryptographic schemes (like RSA and DH) will be vulnerable when scalable quantum computers will be available. However, quantum cryptographic protocols can provide unconditional security. One such protocol for unconditionally secure quantum key distribution (QKD) is coherent one way (COW) protocol. In the present work, we report experimental realization of an optical fiber based COW protocol for QKD in the telecom wavelength (1550 nm) where the attenuation in the optical fiber is minimum. A laser of 1550 nm wavelength, attenuator and intensity modulator is used for the generation of pulses having average photon number 0.5 and repetition rate of 500 MHz. The experiment is performed over 40 km, 80 km and 120 km of optical fiber and several experimental parameters like disclose rate, compression ratio, dead time and excess bias voltage of the detector are varied for all the cases (i.e., for 40 km, 80 km and 120 km distances) to observe their impact on the final key rate. Specifically, It is observed that there is a linear increase in the key rate as we decrease compression ratio or disclose rate. The key rate obtains its maximum value for least permitted values of disclose rate, compression ratio and dead time. It seems to remain stable for various values of excess bias voltage. While changing various parameters, we have maintained the quantum bit error rate (QBER) below 6% (5%) for distance ≥ 120 km (≤ 120 km). The key rate obtained is also found to remain stable over time. Experimental results obtained here are also compared with the earlier realizations of the COW QKD protocol. Further, to emulate key rate at intermediate distances and at a distance larger than 120 km, an attenuator of 5 dB loss is used which can be treated as equivalent to ∼ 25 km of the optical fiber used in the present implementation. This has made the present implementation equivalent to the realization of COW QKD upto ∼ 145 km.
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.
To protect practical quantum key distribution (QKD) against photon-number-splitting attacks, one could measure the coherence of the received signals. One prominent example that follows this approach is coherent-one-way (COW) QKD, which is commercially available. Surprisingly, however, it has been shown very recently that its secret key rate scales quadratically with the channel transmittance and thus this scheme is unsuitable for long-distance transmission. This result was derived by using a zero-error attack, which prevents the distribution of a secure key without introducing any error. Here we study various countermeasures to foil zero-error attacks against COW QKD. They require either monitoring some additional available detection statistics or increasing the number of quantum states emitted. We obtain asymptotic upper security bounds on the secret key rate that scale close to linearly with the channel transmittance, thus suggesting the effectiveness of the countermeasures to boost the performance of this protocol.