Previously, we reported a framework capable of simulating classical transmission systems and QKD based on weak-coherent prepare-and-measure protocols. The framework’s modular architecture provides a rich library of models for various system components (e.g., lasers, modulators, fibers). QKD components include singlephoton detectors, transmitter- and receiver designs, and performance estimators. For realistic system designs, component models can account for various imperfections (e.g., laser model with linewidth, RIN, side modes). Here, we present further developments of the framework and its applications. We applied our simulation tool to investigate a satellite-based BB84 system in the downlink scenario. For fiber-based applications in a coexistence scenario, we studied the usage of multicore fibers to better separate classical and quantum channels, and various PON configurations. The system performance was estimated by analyzing the QBER dependence on the classical channel power due to Raman scattering. Furthermore, we discuss the simulation results of a Gaussian-modulated CV-QKD system with a realistic model of a true local oscillator. We studied how the utilization of DSP techniques improves the system’s performance. We evaluated the secret key rate for different transmission distances.
Previously, we reported a framework capable of simulating classical transmission systems and QKD based on weak-coherent prepare-and-measure protocols. The framework’s modular architecture provides a rich library of models for various system components (e.g., lasers, modulators, fibers). QKD components include singlephoton detectors, transmitter- and receiver designs, and performance estimators. For realistic system designs, component models can account for various imperfections (e.g., laser model with linewidth, RIN, side modes). Here, we present further developments of the framework and its applications. We applied our simulation tool to investigate a satellite-based BB84 system in the downlink scenario. For fiber-based applications in a coexistence scenario, we studied the usage of multicore fibers to better separate classical and quantum channels, and various PON configurations. The system performance was estimated by analyzing the QBER dependence on the classical channel power due to Raman scattering. Furthermore, we discuss the simulation results of a Gaussian-modulated CV-QKD system with a realistic model of a true local oscillator. We studied how the utilization of DSP techniques improves the system’s performance. We evaluated the secret key rate for different transmission distances.
Due to their compatibility to existing telecom technology, continuous variable (CV) weak coherent state protocols are promising candidates for a broad deployment of quantum key distribution (QKD) technology. We demonstrate how an existing simulation framework for modelling classical optical systems can be utilized for simulations of weak-coherent CV-QKD links. The quantum uncertainties for the measured characteristics of coherent signals are modelled in the electrical domain by shot noise, while a coherent signal in the optical domain is described by its quadrature components. We simulate various degradation effects such as attenuation, laser RIN, Raman noise (from classical channels in the same fibre), and device imperfections and compare the outcome with analytical theory. Having complemented the physical simulation layer by the post-processing layer (reconciliation and privacy amplification), we are able to estimate secure key rates from simulations, greatly boosting the development speed of practical CV-QKD schemes and implementations.
Abstract An experimental Quantum Key Distribution (QKD) implementation requires advanced costly hardware, unavailable in most research environments, making protocol testing and performance evaluation complicated. This has been a major motivation for the development of QKD simulation frameworks, to allow researchers to obtain insight before proceeding into practical implementations. Several simulators have been introduced over the recent years. However, only four are publicly available, only one of which models equipment imperfections. Currently, no open‐source simulator includes all following capabilities: channel attenuation modelling, equipment imperfections and effects on key rates, estimation of elapsed time during classical and quantum‐ channel processes, use of truly random binary sequences for qubits and measurement bases, shared ‐bit fraction customization. In this paper, we present NuQKD, an open‐ source modular, intuitive simulator, featuring all the above capabilities. NuQKD establishes communication between two computer terminals, accepts custom user inputs (iterations, raw key size, attacker interception rate etc.) and evaluates the sifted key, Quantum Bit Error Rate (QBER), elapsed communication time, and more. NuQKD capabilities include optical fiber and free ‐space simulation, modeling of equipment/channel imperfections, bitstrings from True Random Number Generators, decoy‐state protocol support, and automated evaluation of performance metrics. We expect NuQKD to enable convenient and accurate representation of actual experimental conditions.
Fiber-based quantum key distribution (QKD) systems are mature and commercialized, but their integration into existing optical networks is crucial for their widespread use, in particular in passive optical networks (PONs) if end-to-end quantum-secured communications are to be addressed. While discrete-variable QKD coexistence with classical channels is well-studied in point-to-point links, its performance in point-to-multipoint topologies like PONs has received less attention. We thus developed a numerical tool to estimate quantum-available bandwidth and maximum link lengths for QKD systems in single-fiber PON architectures in coexistence with GPON, XG-PON, NG-PON2, and HS-PON standards. The QKD channel performance is obtained by setting thresholds on the quantum bit error rate and the secret key rate, ultimately limited by spontaneous Raman scattering noise and high optical distribution network losses. We perform a comparison between the performance obtained assuming the asymptotic infinite-key generation rate or taking into account actual implementations in the finite-key regime. We evidence that proper design rules can be obtained as a function of both classical and quantum system parameters to support end-to-end quantum security services in existing optical networks.
Space-based quantum communication is at a point where we will soon see satellites be launched to demonstrate complete and usable Quantum Key Distribution (QKD) links between several optical ground-stations in an industrial live environment. Therefore, and given the specificities of Quantum Communication, researchers as well as industrials need tools and models to leverage their existing classical optics heritage to design their future QKD constellations. Based on our experience of QKD at Fraunhofer IOF, we have developed a centralizing system which capitalize on our QKD models and completes it with the simulation of constellations, the atmosphere, the scheduling of operations and pointing specifications. This simulation tool is particularly aimed at system engineers, in both research and industry, to assess the performance of their envisioned quantum communication space. This tool encompasses all major design factors of a constellation and quantum channels for the simulation of quantum communication channels which includes optical systems as well as atmospheric propagation. Results are providing important inputs for the design of QKD space missions and systems as well as the expected level of possible services in a real-life environment. Our QKD models not only incorporate all systems in the quantum channel, and their design parameters from the laser source on the emitting side to the final click on the detectors on the receiving side, but also the post-processing impact and dynamic optimization of the quantum parameters to maximize the final secret key rate. Furthermore, the evolutive and modular design of the simulation tool itself makes it possible to incorporate different models for all parts of the simulation, for instance the atmosphere, the constellation propagation or the QKD protocols. This architecture makes it possible to interface with other experts of any field of this space system and incorporate their models too. Moreover, this allows for the assessment and comparison of different models and protocols on the final performance of the space QKD system. Our current implementation uses an in-house BBM92 model and is planned to incorporate further models developed at IOF on BB84, Hyperentangled QKD and High-Dimensional QKD.
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.
Entanglement-based quantum key distribution (QKD) offers robust key distribution through intrinsically correlated measurements, ensuring any eavesdropping attempt introduces detectable disturbances. Although practical implementations are primarily limited to fixed fiber networks, extending QKD to mobile platforms like drones or small satellites introduces challenges in optical alignment, weight, and power consumption. This work outlines the core components and architecture for implementing BBM92-based QKD in mobile settings. A compact polarization Sagnac interferometer utilizing a PPKTP crystal provides a stable and efficient method for generating entangled photon pairs. Supporting subsystems include a polarization compensation system, a detector module, and an acquisition, pointing, and tracking (APT) system. To assess performance under realistic conditions, we simulate photon loss, detector efficiency, and quantum bit error rates using the SQUANCH framework. Results show an average key generation rate of 5.43 bits/s, a final secure key length of 45.27 bits per block, and a QBER of 4.5%.