Passive optical network (PON)-based continuous-variable quantum key distribution (CV-QKD) enables an optical line terminal (OLT) to establish multiple information-theoretically secure keys with optical network units (ONUs) over untrusted quantum channels. However, the average secret key rate (SKR) of PON systems is severely constrained by heterogeneous channel conditions among ONUs. Hardware-based solutions for PON systems, such as wavelength-division multiplexing and multi-core fiber, are also burdened by prohibitive costs and implementation complexity. Conventional signal-processing approaches that rely solely on power regulation are suboptimal for CV-QKD because the SKR exhibits a non-monotonic dependence on the channel transmit power. Although unitary precoding can effectively enhance the SKR of point-to-point links by exploiting the inherently non-flat noise frequency spectrum of practical CV-QKD systems, extending this technique to multi-user PONs introduces unique challenges: all users share a single global transmit power budget and a common precoding matrix, thereby requiring joint optimization to maximize the average SKR. To address these challenges, we propose what we believe is a novel time-domain precoding scheme that integrates precoding with power loading and jointly optimizes the unitary precoding matrix and power allocation profile by exploiting the inherently non-flat noise frequency spectrum of practical CV-QKD systems. Numerical simulations demonstrate that, for a standard 20 km link with 64 ONUs, the proposed scheme improves the SKR by factors of 2.52 to 4.57 over conventional unoptimized systems and by factors of 1.02 to 1.90 over PON systems employing a discrete Fourier transform (DFT) precoding matrix with optimized power loading.
Passive optical network (PON)-based continuous-variable quantum key distribution (CV-QKD) enables an optical line terminal (OLT) to establish multiple information-theoretically secure keys with optical network units (ONUs) over untrusted quantum channels. However, the average secret key rate (SKR) of PON systems is severely constrained by heterogeneous channel conditions among ONUs. Hardware-based solutions for PON systems, such as wavelength-division multiplexing and multi-core fiber, are also burdened by prohibitive costs and implementation complexity. Conventional signal-processing approaches that rely solely on power regulation are suboptimal for CV-QKD because the SKR exhibits a non-monotonic dependence on the channel transmit power. Although unitary precoding can effectively enhance the SKR of point-to-point links by exploiting the inherently non-flat noise frequency spectrum of practical CV-QKD systems, extending this technique to multi-user PONs introduces unique challenges: all users share a single global transmit power budget and a common precoding matrix, thereby requiring joint optimization to maximize the average SKR. To address these challenges, we propose what we believe is a novel time-domain precoding scheme that integrates precoding with power loading and jointly optimizes the unitary precoding matrix and power allocation profile by exploiting the inherently non-flat noise frequency spectrum of practical CV-QKD systems. Numerical simulations demonstrate that, for a standard 20 km link with 64 ONUs, the proposed scheme improves the SKR by factors of 2.52 to 4.57 over conventional unoptimized systems and by factors of 1.02 to 1.90 over PON systems employing a discrete Fourier transform (DFT) precoding matrix with optimized power loading.
Continuous-variable quantum key distribution (CV-QKD) suffers from significant secret key rate (SKR) limitations in long-haul transmission. Existing hardware-oriented capacity boosting approaches are hindered by prohibitive costs and intricate system architectures. To address these challenges, this paper proposes a time-domain unitary precoding scheme to enhance the SKR by making full use of the non-flat noise frequency spectrum inherent to practical CV-QKD systems. This paper derives an analytical expression for the optimal precoder when only excess or electronic noise is colored and employs the Riemann Conjugate Gradient (RCG) algorithm for iterative optimization when both types of noise are colored. Numerical simulations demonstrate that, for a 40 km transmission link with both colored excess noise and electronic noise, the optimized scheme attains a 1.07 to 5.6 fold SKR improvement compared to conventional CV-QKD systems.
Quantum key distribution (QKD), providing a way to generate secret keys with information-theoretic security, is arguably one of the most significant achievements in quantum information. The continuous-variable QKD (CV-QKD) offers the potential advantage of achieving a higher secret key rate (SKR) within a metro area, as well as being compatible with the mature telecom industry. However, the SKR and transmission distance of state-of-the-art CV-QKD systems are currently limited. Here, based on the proposed orthogonal-frequency-division-multiplexing (OFDM) CV-QKD protocol, we demonstrate a high-rate multi-carrier (MC) CV-QKD with a 10 GHz symbol rate that achieves Gbps SKR within 10 km and Mbps SKR over 100 km in the finite-size regime under composable security against collective attacks. The results are achieved by suitable optimization of subcarrier number and modulation variance, well-controlled excess noise induced by both the OFDM mechanism and the efficient DSP scheme, and high-performance post-processing capacity realized by the heterogeneous computing scheme. The composable finite-size SKR reaches 1779.45 Mbps@5 km, 1025.49 Mbps@10 km, 370.50 Mbps@25 km, 99.93 Mbps@50 km, 25.70 Mbps@75 km, and 2.25 Mbps@100 km, which improves the SKR by two orders of magnitude and quintuples the maximal transmission distance compared to the most recently reported CV-QKD results [ Nat. Commun. 13 , 4740 ( 2022 ) NCAOBW 2041-1723 10.1038/s41467-022-32161-y ]. Interestingly, it is experimentally verified that the SKR of the proposed MC CV-QKD can approach five times larger than that of the single-carrier CV-QKD with the same symbol rate without additional hardware costs. Our work constitutes a critical step toward future high-speed quantum metropolitan and access networks.
The integration of quantum key distribution (QKD) into passive optical networks (PONs) poses new challenges due to the specific characteristics of the access network, as the high losses from passive splitters and the spontaneous Raman scattering noise generated by classical channels co-propagating in the same optical fiber. In this paper, different QKD technologies and implementations are compared, taking into account both discrete-variable and continuous-variable QKD systems to maximize the achievable secret key rate performance in single-fiber bidirectional PONs.
Quantum cryptography (QC) is currently under investigation to build highly secure optical communication networks. QC requires distribution of quantum keys (also called "secret" keys) on separate wavelength channels than those used to transmit the encrypted data. Hence, we propose a quantum-secured passive optical network (QS-PON) that supports both i) the traditional wavelength channels for secured data transmission, and ii) a quantum key distribution network (QKDN) running on separate dedicated wavelengths. The QKDN generates and stores secret keys that are then assigned to users' demands served on traditional PON channels. To generate secret keys, quantum transmitters at the optical network units (ONUs) exchange qubits with a quantum receiver at the optical line terminal (OLT). Then, the generated secret keys are stored in quantum key pools (QKPs) installed at both OLT and the ONUs and assigned to users' demands. Point-to-multipoint QKD systems have been experimentally demonstrated over various forms of quantum access networks (QANs), showing that an efficient mechanism to generate and assign quantum keys based on traffic requests is a critical component of QANs. In this study, we present a new QS-PON architecture, and we propose a dynamic secret-key provisioning (DSKP) algorithm that effectively generates and assigns secret keys from users' demands. Our proposed DSKP algorithm features two phases, the lowest-first secret-key generation (LF-SKG) phase and the hierarchical-clustering secret-key consumption (HC-SKC) phase. In this study, we also provide an analytical model that describes how secret keys are generated and consumed in QKPs. In our illustrative numerical evaluation, we compare our algorithm for secret-key provisioning with a baseline IPACT-based solution in terms of service-rejection ratio, time-slot utilization, and guard- and relay-time saving. Results show that DSKP reduces service-rejection ratio and guard- and relay-time of about 16% and 39.54%, respectively.
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.
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.