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.
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.
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 promising solution for providing high secure key rates in moderate loss channels. A great advantage with respect to discrete-variable (DV) systems is the use of a technology similar to the one used in classical coherent communication, in particular for the detection system, which can operate at room temperature and benefits from an easier integration process. In addition to this, the use of advanced digital signal processing (DSP) techniques developed for classical communication allows for bandwidth-efficient temporal shaping, which optimizes the performance of the CV-QKD system. These techniques applied to the detected signal are also fundamental for using a locally generated local oscillator, correcting frequency and phase differences using frequency-multiplexed pilots generated by the transmitter. In this presentation, we will describe how these DSP techniques can be applied to a CV-QKD system and show some recent experimental results obtained by our research group, including results for a receiver based on a Photonic Integrated Circuit (PIC).
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.
Continuous-variable quantum key distribution (CV-QKD) enables two remote parties to establish information-theoretically secure keys and offers high practical feasibility due to its compatibility with mature coherent optical communication technologies. However, as CV-QKD systems progress toward digital implementations, device nonidealities drive the optical field from a single-mode to a continuous-mode region, thereby underscoring the mismatch between theoretical models and practical systems. Here, we introduce temporal modes to construct an entanglement-based scheme that more accurately captures device nonidealities and develop a corresponding secret key rate calculation method applicable to continuous-mode scenarios. We demonstrate that optimizing the pulse-shaping format can significantly improve performance under detector-bandwidth-limited conditions. Experimental results also confirm that the proposed model effectively describes the impact of sampling-time deviations. We further analyze a linear weighted-reconstruction digital signal processing method,which improves the secret key rate by approximately 50% in a 30-km fiber experiment without requiring additional hardware, demonstrating a substantial performance enhancement at metropolitan distances. The proposed theoretical framework accommodates a broader range of experimental conditions and can guide the optimization of digital CV-QKD systems.
Quantum information processing uses the laws of quantum physics to perform tasks that are infeasible or impossible classically. Within this broader programme, quantum communication has emerged as one of the most practically mature sub-fields of quantum information science, with several proposed protocols having already been demonstrated over deployed optical fibre and free-space links. Of the various platforms available for encoding quantum information for communication, the continuous-variable (CV) framework encodes information in the quadratures of the electromagnetic field and is therefore directly compatible with the existing machinery of classical coherent optical communications. The most prominent CV protocol is continuous-variable quantum key distribution (CV-QKD), which establishes an information-theoretically secure cryptographic key between two spatially separated parties. Because CV-QKD reuses much of the hardware of classical coherent telecommunications, including off-the-shelf lasers, modulators, and balanced detectors, it is a leading candidate for the deployment of quantum-secured communication over standard optical fibre networks. The development of CV-QKD has typically proceeded with the underlying physics dictating the engineering. Security proofs, often relying on the Gaussian optimality of collective attacks, set the structure of the protocol, and the practical system is then built to satisfy the assumptions on which those proofs depend. Contributions of this thesis adopt the opposite view, taking the practical engineering of high-rate CV-QKD systems as the starting point and developing the theoretical machinery required to support it. The first contribution is the formalisation of one-round sliced reconciliation with guard-band post-selection as a key distillation scheme designed for real-time, high-throughput operation. Originally introduced experimentally as a means to stabilise bit error rates and enable the use of fixed-rate error correction codes at high symbol rates, guard-band post-selection is shown here to identify the guard-band span as a free parameter that can be tuned to hold the quantum bit error rate approximately constant across a range of channel conditions. However, conditioning on accepted data produces non-Gaussian post-selected statistics that lie outside the reach of standard Gaussian-optimality security arguments, which motivates the remaining two contributions. The second contribution is an asymptotic security analysis of guard-band post-selected CV-QKD against arbitrary collective attacks, no longer restricted to Gaussian adversaries. By using a quantum channel estimate as side information and by lifting reduced-state distances to purification distances via entropic continuity bounds, security is certified against the class of channels lying close to a Gaussian thermal-loss reference, with a key-rate penalty that scales with the energy-constrained diamond distance to that reference. The third contribution is the experimental basis required to apply this framework in practice. A lightweight three-stage protocol for quantum channel estimation is proposed and demonstrated, which repurposes the Gaussian probe constellation already present in the CV-QKD signal and requires no additional hardware or modifications to the underlying protocol. Over single-mode optical fibre at distances from 10 km to 50 km, the protocol produces physically valid process tensor estimates whose energy-constrained diamond distances to a Gaussian reference channel remain on the order of 10^(-3) throughout. Together, these three contributions provide a treatment of post-selected Gaussian-modulated CV-QKD in which the engineering, the security theory, and the channel characterisation are designed to operate together within a deployable system.
Quantum key distribution (QKD) leverages quantum information theory to enable two remote parties to establish a random key with unconditional secrecy. Performing QKD with coherent light states integrates with modern optical communication techniques but presents new challenges. This survey reviews advancements in applying digital signal processing (DSP) techniques, transformative in coherent optical communications, to continuous-variable QKD (CV-QKD). We discuss how DSP methods enhance CV-QKD performance, security, and feasibility. The areas examined include phase estimation, channel equalization, error correction, and noise reduction, highlighting associated challenges and future research directions.