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Journal of Lightwave Technology

Experimental Implementation of Four-User Downstream Access Network Continuous-Variable Quantum Key Distribution

Zhenghua Li, Xiangyu Wang, Dengke Qi, Ziyang Chen, Song Yu

Published Jun 11, 2024• 16 citations
Publisher

Abstract

Nowadays, network speeds are increasing rapidly and network security is a growing concern. Quantum key distribution (QKD) can provide unconditionally secure keys at the physical layer for communication networks. The implementation of high-speed QKD quantum networks is important for the security of modern high-speed networks. Here, for the first time, we realized a four-user high-speed downstream access network experiment. The repetition frequency of the system is 100 MHz, considering block size of$10^{8}$, four users achieved secret key rates of 430 kbps, 450 kbps, 150 kbps, and 130 kbps at channel attenuations of 4.4 dB, 4.2 dB, 5.6 dB, and 5.8 dB, respectively. In addition, we find that the block size, the number of users, and the channel attenuation have a significant impact on the system performance in real systems. When the block size is$10^{8}$and the number of user nodes is four, the maximum channel attenuation that can be supported is about 6 dB. Our experimental results demonstrate the feasibility of multi-user downstream CV-QKD access networks, further advancing the practical application of quantum networks in real-world environments.

DOI
10.1109/jlt.2024.3412272

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Nowadays, network speeds are increasing rapidly and network security is a growing concern. Quantum key distribution (QKD) can provide unconditionally secure keys at the physical layer for communication networks. The implementation of high-speed QKD quantum networks is important for the security of modern high-speed networks. Here, for the first time, we realized a four-user high-speed downstream access network experiment. The repetition frequency of the system is 100 MHz, considering block size of10810^{8}108, four users achieved secret key rates of 430 kbps, 450 kbps, 150 kbps, and 130 kbps at channel attenuations of 4.4 dB, 4.2 dB, 5.6 dB, and 5.8 dB, respectively. In addition, we find that the block size, the number of users, and the channel attenuation have a significant impact on the system performance in real systems. When the block size is10810^{8}108and the number of user nodes is four, the maximum channel attenuation that can be supported is about 6 dB. Our experimental results demonstrate the feasibility of multi-user downstream CV-QKD access networks, further advancing the practical application of quantum networks in real-world environments.
Quantum networks provide opportunities and challenges across a range of intellectual and technical frontiers, including quantum computation, communication, and others. Unlike traditional communication networks, quantum networks utilize quantum bits rather than classical bits to store and transmit information. Quantum key distribution (QKD) relying on the principles of quantum mechanics is a key component in quantum networks and enables two parties to produce a shared random secret key, thereby ensuring the security of data transmission. In this work, we propose a cost-effective quantum downstream access network structure in which each user can get their corresponding key information through terminal distribution. Based on this structure, we demonstrate the first four-end-users quantum downstream access network in continuous variable QKD with a local local oscillator. In contrast to point-to-point continuous variable QKD, the network architecture reevaluates the security of each user and accounts for it accordingly, and each user has a lower tolerance for excess noise as the overall network expands with more users. Hence, the feasibility of the experiment is based on the analysis of the theoretical model, noise analysis, and multiple techniques such as the particle filter and adaptive equalization algorithm used to suppress excess noise. The results show that each user can get a low level of excess noise and can achieve secret key rates of 546 kbps, 535 kbps, 522.5 kbps, and 512.5 kbps under a transmission distance of 10 km, respectively, with the finite-size block of 1×10 8 . This not only verifies the good performance but also provides the foundation for the future multi-user quantum downstream access networks.
Quantum key distribution (QKD), which enables the secure distribution of symmetric keys between two legitimate parties, is highly useful in future network security. An access network that connects multiple end users with one network backbone can be combined with QKD to build security for end users in a scalable and cost-effective way. In this paper, we show that a downstream-access network can be implemented using continuous-variable (CV) QKD. The security analysis is conducted independently with other network nodes, which greatly relax the trustfulness requirement on the network nodes. The security is then obtained against other network parties even when the quantum signals are broadcast throughout the network. The passive beam splitter is sufficient to deterministically distribute the quantum signals, and standard CV QKD systems can be directly fitted in the downstream-access network, which makes it more applicable for practical implementations. Numerous simulation results are provided to demonstrate the performance of the CV QKD downstream-access network, where up to 64 end users are shown to be feasible to access the network. We provide the security-analysis framework for realizing CV QKD in the downstream-access network, which will boost the diversity for constructing practical CV QKD networks.
To address the demand for low-cost deployment in quantum key distribution (QKD) networks, this study explores the implementation of unidimensional (UD) modulation continuous-variable quantum key distribution (CV-QKD) protocols within downstream access networks. The UD CV-QKD protocol employs a single modulator for information encoding, offering benefits such as reduced implementation cost and lower random number consumption, which collectively decrease the overall setup expense of QKD systems. Through systematic performance analysis, it is demonstrated that the proposed UD modulation downstream access network scheme exhibits strong scalability and practical applicability. When supporting 32 users, the system maintains secure communication over transmission distances of up to 50 km. As the number of users increases to 64, performance declines slightly; however, the system still achieves a 35 km transmission distance, which remains suitable for many metropolitan access applications. Even in high-density access scenarios involving 128 users, the system sustains a positive key rate within a transmission range of 20 km. Furthermore, this study evaluates the protocol’s practical security under source intensity errors and finite-size effects. These results provide meaningful guidance for deploying low-cost, high-security quantum communication access networks and contribute to advancing QKD technologies toward scalable, real-world implementations.
Quantum key distribution (QKD) which enables the secure distribution of symmetric keys between two legitimate parties is of great importance in future network security. Access network that connects multiple end-users with one network backbone can be combined with QKD to build security for end-users in a scalable and cost-effective way. Though previous QKD access networks are all implemented in the upstream direction, in this paper, we prove that downstream access network can also be constructed by using continuous-variable (CV) QKD. The security of the CV-QKD downstream access network is analyzed in detail, where we show the security analysis is secure against other parties in the network. The security analysis we proved corresponds to the downstream access network where only passive beamsplitter is sufficient to distribute the quantum signals and no other active controls are demanded. Moreover, standard CV-QKD systems can be directly fitted in the downstream access network, which makes it more applicable for practical implementations. Numerous simulation results are provided to demonstrate the performance of the CV-QKD downstream access network, where up to 64 end-users are shown to be feasible to access the network. Our work provides the security analysis framework for realizing QKD in the downstream access network which will boost the diversity for constructing practical QKD networks.
Quantum key distribution (QKD) networks are crucial for the development of quantum networks and the early stages of the quantum internet, enabling unconditional secure key distribution and guaranteeing data security. However, current QKD networks are still limited in their structure, and are hard to achieve super multi-user access due to high system costs associated with the original point-to-point QKD system. In this paper, we introduce two cost-effective continuous-variable QKD (CV-QKD) access network schemes. The first one is a round-trip multi-band CV-QKD access network. Specifically, the frequency division multiplexing (FDM) technology is used to support multi-user access, and the round-trip optical architecture is adopted to realize low cost. As a result of the encoding properties of CV-QKD, different users can utilize the quadrature components in various frequency bands to transmit key information. The proposed network theoretically supports hundreds of users and can be easily expanded using simple equipment. The second one is a CV-QKD access network based on directed detection. With the help of the Kramers-Kronig relation, the receiver at the optical network unit can obtain the quadrature components just using a photodetector. Besides, it can also solve the practical issues in the plug-and-play structure. Compared with a quantum access network based on the point-to-point QKD system, these two schemes can provide cost-effective solutions for the subsequent large-scale verification and implementation of quantum access networks.
In recent years, continuous-variable quantum key distribution (CV-QKD) has become a promising paradigm for enabling secure communication among multiple end users sharing the same telecommunication backbone. CV-QKD with reverse reconciliation naturally enables scalability from conventional point-to-point links to quantum access networks based on passive quantum broadcasting channels. Here, we report an experimental demonstration on an active 1:41:41:4 multi-user CV quantum network (QN) in the finite-size regime. With 1.25⋅1091.25\cdot10^91.25⋅109 coherent states exchanged on each 11km11\text{km}11km quantum channel, the highest performance for secret key generation totaling 1.9⋅10−11.9\cdot10^{-1}1.9⋅10−1 bits/channel use. Furthermore, we investigate adaptable CV-QN protocols that comprehensively allow network operation in various security and key rates requirements of individual users. The results establish the practical security of CV-QN compatible with existing telecommunication for broad deployment, and allowing additional degree of freedom for connected end users in existing infrastructures.