Abstract Continuous-variable quantum key distribution (CVQKD) is a highly promising method for building large-scale quantum networks due to its low cost and high capacity. However, current access networks involve hybrid channels and multiple users, where traditional CVQKD protocols may face limitations in performance. At the same time, as an emerging protocol, quantum key distribution with basis-encoding (BE-QKD) is compatible with traditional CVQKD protocols and has the potential to be applied in CVQKD access networks to provide performance advantages. But the performance of the BE-QKD protocol in current CVQKD access networks has not yet been fully exploited. In this work, we conduct an experimental demonstration of a heterogeneous continuous-variable quantum access network, which is based on the BE-QKD protocol with quadrature phase shift keying (QPSK) modulation and the traditional QPSK-CVQKD protocol. In addition, practical CVQKD access networks may involve different types of transmission links. Therefore, this access network is configured with four users, including two free-space channels and two fiber channels, to verify the feasibility of the proposed heterogeneous quantum access network scheme in a hybrid-channel environment. The experimental results demonstrate that the QPSK-BE-QKD protocol offers potential performance advantages across various channel scenarios in quantum access networks, such as the ability to withstand channel loss and secret key rates. And its compatibility with QPSK-CVQKD is confirmed. The proposed scheme is easy to deploy in existing CVQKD access networks and demonstrates significant performance potential, offering a new implementation paradigm for large-scale access scenarios of CVQKD networks with complex and hybrid channels.
Abstract Continuous-variable quantum key distribution (CVQKD) is a highly promising method for building large-scale quantum networks due to its low cost and high capacity. However, current access networks involve hybrid channels and multiple users, where traditional CVQKD protocols may face limitations in performance. At the same time, as an emerging protocol, quantum key distribution with basis-encoding (BE-QKD) is compatible with traditional CVQKD protocols and has the potential to be applied in CVQKD access networks to provide performance advantages. But the performance of the BE-QKD protocol in current CVQKD access networks has not yet been fully exploited. In this work, we conduct an experimental demonstration of a heterogeneous continuous-variable quantum access network, which is based on the BE-QKD protocol with quadrature phase shift keying (QPSK) modulation and the traditional QPSK-CVQKD protocol. In addition, practical CVQKD access networks may involve different types of transmission links. Therefore, this access network is configured with four users, including two free-space channels and two fiber channels, to verify the feasibility of the proposed heterogeneous quantum access network scheme in a hybrid-channel environment. The experimental results demonstrate that the QPSK-BE-QKD protocol offers potential performance advantages across various channel scenarios in quantum access networks, such as the ability to withstand channel loss and secret key rates. And its compatibility with QPSK-CVQKD is confirmed. The proposed scheme is easy to deploy in existing CVQKD access networks and demonstrates significant performance potential, offering a new implementation paradigm for large-scale access scenarios of CVQKD networks with complex and hybrid channels.
Abstract Establishing secure data communication necessitates secure key exchange over a public channel. Quantum key distribution (QKD), which leverages the principles of quantum physics, can achieve this with information-theoretic security. The discrete modulated (DM) continuous variable (CV) QKD protocol, in particular, is a suitable candidate for large-scale deployment of quantum-safe communication due to its simplicity and compatibility with standard high-speed telecommunication technology. Here, we present the first experimental demonstration of a four-state DM CVQKD system, successfully generating composable finite-size keys, secure against collective attacks over a 20 km fiber channel with 2.3 × 109 coherent quantum states, achieving a positive composable key rate of 11.04 × 10−3 bits/symbol. This accomplishment is enabled by using an advanced security proof, meticulously selecting its parameters, and the fast, stable operation of the system. Our results mark a significant step toward the large-scale deployment of practical, high-performance, cost-effective, and highly secure quantum key distribution networks using standard telecommunication components.
Nowadays, quantum communications provide a vast field of research in rapid expansion, with a huge potential impact on the future developments of quantum technologies. In particular, continuous variable systems, employing coherent-state encoding and quadrature measurements, represent a suitable platform, due to their compatibility with both the modulation and detection systems currently employed in standard fiber-optical communications. In this work, we address some relevant aspects of the field, and provide innovative results being also experimentally oriented. In particular, we focus on two relevant paradigms: quantum decision theory and continuous variable quantum key distribution (CVQKD). In the former case, we address the problem of coherent-state discrimination and design new hybrid receivers for binary phase-shift keying discrimination, obtaining a quantum advantage over conventional detection schemes, being also robust against typical experimental imperfections. In the latter scenario, we proceed in two different directions. On the one hand, we design new CVQKD protocols employing discrete modulation of coherent states, being a feasible solution compatible with the state of the art in optical communications technologies. On the other hand, we address the more fundamental problem of performing channel losses mitigation to enhance existing protocols, and investigate the role of optical amplifiers for the task. Finally, we make a first step towards a fully non-Gaussian CVQKD scheme by proposing, for the first time, the adoption of an optimized state-discrimination receiver, commonly adopted for quantum decision theory, within the context of CVQKD, obtaining a genuine quantum enhancement over conventional protocols.
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 1:4 multi-user CV quantum network (QN) in the finite-size regime. With 1.25⋅109 coherent states exchanged on each 11km quantum channel, the highest performance for secret key generation totaling 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.
Continuous variable quantum key distribution that can be implemented using only low-cost and off-the-shelf components reveals great potential in practical large-scale realization. Access networks, as a modern network necessity, connect many end-users to the network backbone. In this work, we first demonstrate upstream transmission quantum access networks using continuous variable quantum key distribution. A two-end-user quantum access network is then experimentally realized. Through phase compensation, data synchronization, and other technical upgrades, we achieve a secret key rate of the total network of 390 kbits/s. In addition, we extend the case of a two-end-user quantum access network to the case of a multiplicity of users, and analyze the network capacity in the case of a multiplicity of users by measuring the additive excess noise from different time slots.
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.
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.