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Optics Express

Realizing multi-user continuous-variable quantum key distribution via a shared entangled source and passive beam splitters

Pu Wang, Zengliang Bai, Liwei Chang, Yan Feng Tian

Published Aug 22, 2025• 3 citations
Publisher

Abstract

Establishing a high-rate, cost-effective, and secure quantum communication network is critical for developing the quantum internet. A promising approach is to place a complex and expensive entangled source at a shared central node, which allows for efficient utilization of quantum resources in constructing a star-shaped downstream quantum access network. In this study, we propose a continuous-variable quantum passive optical network (CV-QPON) that employs a shared entangled source. We evaluate the network's security under two scenarios: one with a fully characterized entangled source and the other with a partially characterized entangled source. Both scenarios involve practical mixed entangled states. By combining the biased basis scheme, the renovated protocol outperforms the previous coherent state protocols in the secret key rate, transmission distance, the number of supported users, and tolerable excess noise. The results indicate that for a fully characterized entangled source, even at a transmission distance of 50 km, a moderate entangled source with 7 dB squeezing can still support up to 256 users accessing the network. Our scheme for a partially characterized entangled source relaxes the security assumptions regarding the central node, which is crucial for the downstream CV-QPON system. In this case, a 7 dB squeezing entangled source achieves a positive key rate for 64 users over a 30 km transmission distance, and it can even support up to 128 users at a 10 km transmission distance. With a 10 dB squeezing entangled source, 256 users can still be supported at a 30 km transmission distance. Furthermore, we analyze the protocol's security in the finite-size scenario and identify an optimal biased basis ratio that maximizes the key rate of the protocol. Given the rapid advancements in integrated entangled sources, our scheme offers a promising pathway for developing low-cost, high-rate, high-security, and large-scale quantum communication networks.

DOI
10.1364/oe.567204

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Establishing a high-rate, cost-effective, and secure quantum communication network is critical for developing the quantum internet. A promising approach is to place a complex and expensive entangled source at a shared central node, which allows for efficient utilization of quantum resources in constructing a star-shaped downstream quantum access network. In this study, we propose a continuous-variable quantum passive optical network (CV-QPON) that employs a shared entangled source. We evaluate the network's security under two scenarios: one with a fully characterized entangled source and the other with a partially characterized entangled source. Both scenarios involve practical mixed entangled states. By combining the biased basis scheme, the renovated protocol outperforms the previous coherent state protocols in the secret key rate, transmission distance, the number of supported users, and tolerable excess noise. The results indicate that for a fully characterized entangled source, even at a transmission distance of 50 km, a moderate entangled source with 7 dB squeezing can still support up to 256 users accessing the network. Our scheme for a partially characterized entangled source relaxes the security assumptions regarding the central node, which is crucial for the downstream CV-QPON system. In this case, a 7 dB squeezing entangled source achieves a positive key rate for 64 users over a 30 km transmission distance, and it can even support up to 128 users at a 10 km transmission distance. With a 10 dB squeezing entangled source, 256 users can still be supported at a 30 km transmission distance. Furthermore, we analyze the protocol's security in the finite-size scenario and identify an optimal biased basis ratio that maximizes the key rate of the protocol. Given the rapid advancements in integrated entangled sources, our scheme offers a promising pathway for developing low-cost, high-rate, high-security, and large-scale quantum communication networks.
To establish a scalable and secure quantum network, a critical milestone is advancing from basic point-to-point quantum key distribution (QKD) systems to the development of inherently multi-user protocols designed to maximize network capacity. Here, we propose a quantum passive optical network (QPON) protocol based on continuous-variable (CV) systems, particularly the quadrature of the coherent state, which enables deterministic, simultaneous, and high-rate secret key generation among all network users. We implement two protocols with different trust levels assigned to the network users and experimentally demonstrate key generation in a quantum access network with 8 users, each with an 11 km span of access link. Depending on the trust assumptions about the users, we reach 1.5 and 2.1 Mbits/s of total network key generation (or 0.4 and 1.0 Mbits/s with finite-size channels estimation). Demonstrating the potential to expand the network's capacity to accommodate tens of users at a high rate, our CV-QPON protocols open up new possibilities in establishing low-cost, high-rate, and scalable secure quantum access networks serving as a stepping stone towards a quantum internet.
Building scalable and secure quantum networks with many users has a high application potential but also holds many practical challenges. A significant stride in this pursuit involves extending quantum key distribution, an information-theoretically secure method for establishing cryptographic keys between two distant users, from a point-to-point protocol implemented on direct optical connections to a quantum access network. Yet, realizations of quantum access networks have, so far, relied on probabilistic or time-sharing strategies. Here, we show theoretically and experimentally that a solution without these constraints can come from the exclusive features of continuous-variable systems. Based on coherent states, we propose continuous-variable quantum passive-optical-network (CV-QPON) protocols, enabling deterministic and simultaneous secret key generation among all network users. We achieve this by leveraging the inherent wave-like property of coherent states split at a beam splitter and electric-field quadrature measurements. We show two protocols with different trust levels assigned to the network users and experimentally demonstrate key generation in a quantum access network with 8 users, each with an 11 km span of access link. Depending on the trust assumptions about users, we reach 1.5 Mbits/s and 2.1 Mbits/s of total network key generation. Demonstrating the potential to expand the network's capacity to accommodate tens of users at a high rate, our CV-QPON protocols offer a pathway toward establishing low-cost, high-rate, and scalable quantum access networks using standard telecom technologies and directly exploiting the existing access network infrastructure.
Locking the sophisticated and expensive entanglement sources at the shared relay node is a promising choice for building a star-type quantum network with efficient use of quantum resources, where the involved parties only need to equip low-cost and simple homodyne detectors. Here, to our best knowledge, we demonstrate the first experimental continuous variable quantum key distribution with an entanglement source between the two users. We consider a practical partially characterized entangled source and establish the security analysis model of the protocol under realistic conditions. By applying a biased base technology, the higher key rate than that of the original protocol is achieved. The experimental results demonstrate that the distance between two users can reach up to 60 km over telecom single-mode fiber, implying the feasibility for high-rate and secure communication with a shared entangled source at metropolitan distances.
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.
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.
Introduction: Continuous-variable quantum teleportation and entanglement distribution are fundamental building blocks for future quantum internet infrastructures. Quantum passive optical networks (QPONs) provide a promising architecture for scalable multi-user quantum communication by exploiting the mature passive optical network infrastructure. Materials and methods: We develop a theoretical framework for continuous-variable entanglement distribution and quantum teleportation in QPONs using two-mode squeezed vacuum states propagating through splitter–combiner–loss channels. Both downlink and uplink architectures are investigated under assisted and non-assisted protocols. The performance is analyzed using covariance matrix formalism, logarithmic negativity, and teleportation fidelity, and closed-form analytical expressions are derived together with numerical evaluations. Results: The analysis demonstrates that downlink entanglement distribution consistently outperforms the uplink architecture in terms of both logarithmic negativity and teleportation fidelity except in noise-less channels. Assisted protocols further enhance the achievable performance compared with non-assisted schemes. We derive the optimum squeezing parameter for non-assisted teleportation, showing that excessive squeezing may become detrimental because of network loss and splitting. Numerical results quantify the effects of network size, transmission distance, channel excess noise, detector imperfections, and measurement-added noise on entanglement distribution and teleportation performance. Also, we have showed that the time-division multiple-access is the preferred scheduling scheme between the users. Conclusions: The proposed framework establishes fundamental performance limits for continuous-variable QPONs and identifies practical design guidelines for scalable quantum access networks. The results demonstrate the advantages of downlink entanglement distribution and assisted teleportation, providing theoretical support for the development of future multi-user quantum communication and quantum internet infrastructures.