Towards a Point-to-point CV-QKD System: Implementation Challenges and Perspectives
Davi Juvêncio Gomes de Sousa, Nelson Alves Ferreira Neto, Christiano M. S. Nascimento, Lucas Queiroz Galvão, Mauro Q. Nooblath Neto, Micael Andrade Dias, Cássio de Castro Silva, B. Pinheiro da Silva, Alexandre B. Tacla, Valéria Loureiro da Silva
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 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.
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.
Continuous-Variable Quantum Key Distribution (CV-QKD) is a form of quantum cryptography that encodes information in the amplitude and phase of light, enabling provably secure key sharing over optical channels. Building on previous research, subcarrier multiplexing configurations have been proposed to allow a quantum channel and a classical data channel to share a single optical carrier and transmitter. In these schemes, advanced Digital Signal Processing (DSP) enables the high Signal-to-Noise Ratio (SNR) classical signal to serve as a phase reference for the quantum channel, facilitating phase recovery without dedicated pilot tones. This thesis integrates these concepts into a complete system architecture that combines CV-QKD with One-Time Pad encrypted communication over a single fiber and transmitter, bringing practical implementation closer to a scenario where classical and quantum communication modes can be selected purely via software. A detailed link-budget analysis quantifies trade-offs between classical and quantum data rates and ensures sufficient bandwidth for essential post-processing tasks such as reverse reconciliation and parameter estimation.Beyond system integration and experimental improvements, this work rigorously addresses practical security. Three contributions enhance parameter estimation under realistic conditions. First, the Multi-Level Parameter Estimation scheme allows continuous receiver-noise calibration via an intensity modulator without interrupting key generation, significantly increasing throughput. Second, a DSP-induced mode mismatch is characterized: the receiver's digital filters and hardware response define the measured quantum mode, which may deviate from the sender's calibration and bias critical parameter estimation. Third, the Time-Gated Variance formalism quantifies calibrations in the presence of realistic non-white noise, accounting for the impact of calibration duration on variance estimation. Together, these methods provide a more accurate, quantitative foundation for assessing transmittance and excess noise, improving both security and performance.Although demonstrated on a single-fiber, subcarrier-multiplexed system, these techniques are broadly applicable to other CV-QKD architectures. By combining such system designs with rigorous, quantitative analysis of noise calibrations, this thesis takes a step toward scalable, certifiable quantum-secured communication, bridging the gap between theoretical security and real-world deployment.
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.
Future optical networks, envisioning the support of 6G services and related demanding requirements, should provide ultra-high-capacity and reliable connectivity, ensuring sustainability and security. Quantum key distribution (QKD) is a technology to address the limitations of classical cryptography, enabling quantum secure communications. Continuous variable QKD (CV-QKD) offers potential cost savings and enhanced compatibility with classical systems. This facilitates the integration within the network infrastructure, particularly in synergy with software-defined networking (SDN), further promoting interoperability and resource sharing/saving. For the adoption and practical implementation of CV-QKD, it is relevant to consider composable security and a finite number of symbols employed in the protocol (block size). In this work, we present the comparison of two models, to provide a figure of the required block size. We also assess the results using a simulation software. Furthermore, we explore a coexistence scenario considering a flexible grid, to exploit the CV-QKD capability of arbitrarily tuning the operating wavelength. This is a relevant feature enabling flexible allocation of the quantum channel to mitigate impairment and saving spectral resources. Considering realistic parameters aligned to commercially available CV-QKD systems and finite block size, we show an improvement with respect to our previous results. A minimum quantum-classical channel spacing of 112.5 GHz is required in a coexistence scenario with 8×200G transceivers and 9 dBm of total power over a 15 km link. In case of 10 dBm of total power, 125 GHz quantum-classical channel spacing is required to generate keys for practical implementation over the analyzed links. Finally, we discuss SDN aspects relevant for dynamic quantum channel allocation and flexible network management, enabling coexistence and efficient resource sharing, facilitating QKD technology integration in deployed networks. The presented results and envisioned potentialities of CV-QKD for practical implementation in optical networks pave the way for its adoption in the network infrastructure toward future quantum secure communications.
Continuous Variable Quantum Key Distribution (CV-QKD) is currently experiencing rapid and significant growth. This can be attributed to its capacity to increase resilience to noise and its ease of integration into current optical communication networks. In this article, we review and survey existing work on CV-QKD post-processing techniques, privacy amplification, and key reconciliation. We also classify protocols based on transmission, preparation, and measurement, which allows for a more systematic and organized understanding of protocols, as well as identifying potential vulnerabilities or weaknesses in each component and allowing for targeted improvements to enhance the overall security and efficiency of protocols. Finally, we outline potential research directions and future challenges that require attention to advance the optimization of CV-QKD technology.
Continuous-variable quantum key distribution (CV-QKD) promises high rates and seamless integration with classical beams within a single optical fiber. Over the years, implementations have been performed by transmitting a local oscillator reference along with the quantum channel, opening security loopholes for eavesdroppers and limiting potential applications. Here, we report on a Gaussian CV-QKD implementation using fully independent transmitter and receiver lasers (local-oscillator sources) over a 12 km fiber spool. The system was experimentally evaluated using logical frames containing approximately 107 coherent states, each composed of ten independently processed subframes of approximately 106 states, and security was assessed in both asymptotic and finite-size regimes under a trusted-device model. The full-fledged classical post-processing is capable of recovering the channel parameters and extracting secret key rates of 5.11 Mbit/s in the asymptotic regime and 4.67 Mbit/s in the finite-size regime, showing good agreement with theoretical predictions. This work establishes the foundation for metropolitan fiber deployment of CV-QKD under strict security constraints.