Quantum key distribution (QKD) provides a secure and effective way of exchanging cryptographic symmetric keys without the possibility of an eavesdropper being undetected. The transmission and detection of the quantum states at the physical layer is the first step to defining a common source of entropy, where both parties will extract a raw key. Over the last few years, continuous variable quantum key distribution (CV-QKD) protocols have become an attractive technology for QKD, providing a low-cost solution that can be easily integrated with the current optical network devices and infrastructure. Unlike other QKD technologies, a CV protocol requires complex and highly computationally demanding digital signal processing (DSP) operations. In that sense, parallel computation tools such as field programmable gate arrays (FPGAs) represent a key element for the practical implementation of this quantum technology and to scale its performance in terms of distance and secret key rate indicators. This work presents an FPGA-based implementation of the physical layer of a CV-QKD transmitter. The proposed architecture and employed protocols are described, and hardware implementation aspects are discussed, including resource occupation analysis, configuration parameters, and timing issues.
Quantum key distribution (QKD) provides a secure and effective way of exchanging cryptographic symmetric keys without the possibility of an eavesdropper being undetected. The transmission and detection of the quantum states at the physical layer is the first step to defining a common source of entropy, where both parties will extract a raw key. Over the last few years, continuous variable quantum key distribution (CV-QKD) protocols have become an attractive technology for QKD, providing a low-cost solution that can be easily integrated with the current optical network devices and infrastructure. Unlike other QKD technologies, a CV protocol requires complex and highly computationally demanding digital signal processing (DSP) operations. In that sense, parallel computation tools such as field programmable gate arrays (FPGAs) represent a key element for the practical implementation of this quantum technology and to scale its performance in terms of distance and secret key rate indicators. This work presents an FPGA-based implementation of the physical layer of a CV-QKD transmitter. The proposed architecture and employed protocols are described, and hardware implementation aspects are discussed, including resource occupation analysis, configuration parameters, and timing issues.
Abstract Quantum key distribution (QKD) represents a cornerstone of secure communication in the quantum era. While discrete-variable QKD (DV-QKD) protocols were historically the first to demonstrate secure key exchange, continuous-variable QKD (CV-QKD) has emerged as a more practical alternative due to its seamless compatibility with current telecommunications infrastructure. CV-QKD relies on coherent and squeezed states of light, offering significant advantages for integration into modern optical networks. This review comprehensively explores the theoretical underpinnings, technological advancements, and practical challenges of CV-QKD. Special attention is given to the role of photonic integrated circuits (PICs) in enabling scalable and efficient implementation of CV-QKD systems. Furthermore, recent advances in machine learning have been leveraged to optimize CV-QKD performance, with data-driven techniques enhancing noise estimation, parameter optimization, and system security. Additionally, tensor networks provide efficient computational tools for analyzing complex quantum correlations, improving the efficiency and robustness of quantum key distribution protocols. These developments, combined with ongoing improvements in quantum photonic integration, pave the way for the practical deployment of large-scale, high-speed quantum-secure networks.
This article presents an analysis of the practical challenges and implementation perspectives of point-to-point continuous-variable quantum key distribution (CV-QKD) systems over optical fiber. The study addresses the physical layer, including the design of transmitters, quantum channels, and receivers, with emphasis on impairments such as attenuation, chromatic dispersion, polarization fluctuations, and coexistence with classical channels. We further examine the role of digital signal processing (DSP) as the bridge between quantum state transmission and classical post-processing, highlighting its impact on excess noise mitigation, covariance matrix estimation, and reconciliation efficiency. The post-processing pipeline is detailed with a focus on parameter estimation in the finite-size regime, information reconciliation using LDPC-based codes optimized for low-SNR conditions, and privacy amplification employing large-block universal hashing. From a hardware perspective, we discuss modular digital architectures that integrate dedicated accelerators with programmable processors, supported by a reference software framework (CV-QKD-ModSim) for algorithm validation and hardware co-design. Finally, we outline perspectives for the deployment of CV-QKD in Brazil, starting from metropolitan testbeds and extending toward hybrid fiber/FSO and space-based infrastructures. The work establishes the foundations for the first point-to-point CV-QKD system in Brazil, while providing a roadmap for scalable and interoperable quantum communication networks.
Quantum key distribution with continuous variables (CV-QKD) enables two parties to share a secret key by using coherent states of light and homodyne detection. This can be carried out by employing low-cost and mature technology developed for telecommunications. It has been shown that at short distances, CV-QKD allows for higher bit rates than conventional discrete variable QKD protocols. Furthermore, CV-QKD could potentially be employed to build a link for secure communication between two parties inside a building or campus and potentially in satellite communications. In this work, we will direct efforts into developing a fiber-based, low-cost, robust, and high-bitrate CV-QKD link. The theoretical concepts, specifics of the QKD system subsequently put into practice in the implementation, have been detailed. Finally, interference experiments have been performed parallel to the proof-of-principle QKD system.
Abstract This article presents an analysis of the practical challenges and implementation perspectives of point-to-point continuous-variable quantum key distribution (CV-QKD) systems over optical fiber. The study addresses the physical layer, including the design of transmitters, quantum channels, and receivers, with emphasis on impairments such as attenuation, chromatic dispersion, polarization fluctuations, and coexistence with classical channels. We further examine the role of digital signal processing (DSP) as the bridge between quantum state transmission and classical post-processing, highlighting its impact on excess noise mitigation, covariance matrix estimation, and reconciliation efficiency. The post-processing pipeline is detailed with a focus on parameter estimation in the finite-size regime, information reconciliation using LDPC-based codes optimized for low-SNR conditions, and privacy amplification employing large-block universal hashing. From a hardware perspective, we discuss modular digital architectures that integrate dedicated accelerators with programmable processors, supported by a reference software framework ( CV-QKD-ModSim ) for algorithm validation and hardware co-design. Finally, we outline perspectives for the deployment of CV-QKD in Brazil, starting from metropolitan testbeds and extending toward hybrid fiber/FSO and space-based infrastructures. The work establishes the foundations for the first point-to-point CV-QKD system in Brazil, while providing a roadmap for scalable and interoperable quantum communication networks.
Abstract Quantum key distribution (QKD) provides secure communication using quantum mechanics, with continuous-variable QKD (CV-QKD) being an attractive solution due to its compatibility with existing telecommunication technology. Its main drawback is susceptibility to signal loss in fibres and free-space links, including satellites, which limits performance. Here we show a software-based protocol enhancing CV-QKD by applying adaptive filters at the transmitter and receiver, allowing the system to dynamically respond to changing channel conditions. Our security analysis avoids relying on Gaussian extremality, giving accurate bounds on an eavesdropper’s information. The protocol can also extract keys in regions that would normally be considered insecure. We demonstrate a threefold increase in secret-key rates compared with the best existing CV-QKD protocol, and in satellite simulations, up to a 400-fold improvement. Because it requires no hardware modifications, our method can be readily integrated into existing systems, paving the way for more practical and robust quantum communication networks.
Quantum key distribution (QKD) is a quantum communication technology that offers absolute security. Among its various implementations, continuous-variable (CV) QKD has garnered significant attention due to its high secret key generation rates and compatibility with conventional optical telecommunication infrastructure, which operates under ambient conditions. However, chip-based integration is crucial for realizing practical and scalable CV-QKD networks, and photonic integrated circuit technology is a potential solution, leveraging the on-chip integration of essential optical components onto silicon photonic circuits. Therefore, this work presents a compact, stable, and cost-effective quantum photonic CV-QKD system on a chip. Our architecture combines time- and polarization-multiplexing with high-extinction-ratio pulse generation to mitigate excess noise. At the same time, a real-time feedback control mechanism compensates for polarization drift, ensuring robust system performance. Fiber-based evaluations reveal secret key generation rates of 2.55 Mbps over 25 km and 0.737 Mbps over 50 km with 12 h stable operation, establishing, to our knowledge, a new benchmark for integrated photonic CV-QKD systems with both the transmitter and receiver on-chip. These results mark a significant milestone in on-chip quantum communication, paving the way for widespread adoption of photonic integrated circuits in future metropolitan quantum-secure networks.