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Optics Express

Continuous-variable quantum key distribution coexisting with classical signals on few-mode fiber

Hai Zhong, Shanhua Zou, Duan Huang, Ying Guo

Published Apr 19, 2021• 15 citations

Abstract

Continuous-variable quantum key distribution (CVQKD) holds an advantage of well compatibility with classical coherent optical communications. However, there exists a performance trade-off between CVQKD and classical communication on single-mode fiber (SMF) because of the spontaneous Raman scattering. Space-division multiplexing (SDM) technique may provide a feasible way to mitigate this performance trade-off in short-distance communication while CVQKD coexisting with classical signals on few-mode fiber (FMF). Here, we examine the feasibility of CVQKD coexisting with classical signals on FMF and analyze the noise impact in weak coupling regime. We find that the inter-mode crosstalk generated from the mode coupling and re-coupling between modes and the group delay spread originated from the differential group delay (DGD) contribute the main noise sources. DGD may become one of the main limits for FMF-based CVQKD towards high-speed system. In addition, a well channel wavelength management is needed to suppress the inter-mode four-wave-mixing for achieving the positive secret key rates. The numerical simulations identify the key parameters for CVQKD system, enabling a helpful insight for realizing security analysis of the Gaussian modulated coherent state protocol. It shows that CVQKD coexisting with high power classical signals on FMF is feasible to implement with standard telecommunication components and able to operate at higher secret key rates. The results may provide a potential guideline for the practical high-rate CVQKD integrating with the FMF-based configuration.

DOI: 10.1364/oe.420583

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Continuous-variable quantum key distribution (CVQKD) holds an advantage of well compatibility with classical coherent optical communications. However, there exists a performance trade-off between CVQKD and classical communication on single-mode fiber (SMF) because of the spontaneous Raman scattering. Space-division multiplexing (SDM) technique may provide a feasible way to mitigate this performance trade-off in short-distance communication while CVQKD coexisting with classical signals on few-mode fiber (FMF). Here, we examine the feasibility of CVQKD coexisting with classical signals on FMF and analyze the noise impact in weak coupling regime. We find that the inter-mode crosstalk generated from the mode coupling and re-coupling between modes and the group delay spread originated from the differential group delay (DGD) contribute the main noise sources. DGD may become one of the main limits for FMF-based CVQKD towards high-speed system. In addition, a well channel wavelength management is needed to suppress the inter-mode four-wave-mixing for achieving the positive secret key rates. The numerical simulations identify the key parameters for CVQKD system, enabling a helpful insight for realizing security analysis of the Gaussian modulated coherent state protocol. It shows that CVQKD coexisting with high power classical signals on FMF is feasible to implement with standard telecommunication components and able to operate at higher secret key rates. The results may provide a potential guideline for the practical high-rate CVQKD integrating with the FMF-based configuration.

Based on mode crosstalk theory, this paper develops a spontaneous Raman scattering (SpRS) model for the quantum-classical coexistence system using few-mode fiber (FMF) integrated with wavelength-division multiplexing (WDM) and spatial-division multiplexing (SDM). Through numerical calculations, the influence degrees of three factors (mode coupling, the number of modes and wavelengths) on SpRS are analyzed. The investigation identifies the dominant contributors to SpRS and reveals their relative impact magnitudes. Based on these results, a ring-assisted FMF is proposed to mitigate noise impacts on quantum signals. The numerical results show that the optimized FMF enhances quantum signal transmission distance by up to 41.5%.

Quantum key distribution (QKD) is one of the most practical applications in quantum information processing, which can generate information-theoretical secure keys between remote parties. With the help of the wavelength-division multiplexing technique, QKD has been integrated with the classical optical communication networks. The wavelength-division multiplexing can be further improved by the mode-wavelength dual multiplexing technique with few-mode fiber (FMF), which has additional modal isolation and large effective core area of mode, and particularly is practical in fabrication and splicing technology compared with the multi-core fiber. Here, we present for the first time a QKD implementation coexisting with classical optical communication over weakly-coupled FMF using all-fiber mode-selective couplers. The co-propagation of QKD with one 100 Gbps classical data channel at -2.60 dBm launched power is achieved over 86 km FMF with 1.3 kbps real-time secure key generation. Compared with single-mode fiber using wavelength-division multiplexing, given the same fiber-input power, the Raman noise in FMF using the mode-wavelength dual multiplexing is reduced by 86% in average. Our work implements an important approach to the integration between QKD and classical optical communication and previews the compatibility of quantum communications with the next-generation mode division multiplexing networks.

In this paper, the theoretical model of spontaneous Raman scattering (SpRS) in a few-mode fiber (FMF) is discussed. It evaluates the impact of SpRS on FMF-based quantum key distribution (QKD) by integrating wavelength division multiplexing (WDM) and space division multiplexing (SDM) techniques. To effectively suppress the interference of classical signal-induced noise on quantum signals, we propose a nanopore-assisted FMF, which significantly reduces the complexity of mode coupling and notably enhances the signal transmission distance of the system. Through numerical analysis, the transmission performance of quantum signals under different numbers of wavelengths and channel mode configurations is assessed, with the transmission distance increased by nearly 50% at maximum.

Performance analysis of continuous-variable quantum key distribution (CVQKD) has been one of the focuses of quantum communications. In this paper, we propose an approach to enhancing the secret rate of CVQKD with the multi-core fiber (MCF) system that transmits multiple spatial modes simultaneously. The excess noise contributed by the inter-core crosstalk between cores can be effectively suppressed by quantum channel wavelength management, leading to the performance improvement of the MCF-based CVQKD system. In the security analysis, we perform numerical simulations for the Gaussian-modulated coherent state CVQKD protocol, considering simultaneously the extra insert loss of fan-in/fan-out (FIFO), which is the extra optical device that should be used at the input and the output of the fiber. Simulation results show that the performance of the one-way and two-way protocols for each core are slightly degraded because of the insert loss of the FIFO, but the total secret key rate can be increased, whereas the performance of the measurement-device-independent CVQKD protocol will be degraded due to the effect of the insert loss of the FIFO. These results may provide theoretical foundation for the space-division multiplexing CVQKD system.

Quantum Key Distribution (QKD) offers a secure method for generating cryptographic keys crucial for 6G future networks. It relies on quantum mechanics principles to guarantee protection against potential threats from quantum computers. However, QKD signal is fragile and can be easily degraded by external noise sources. Therefore, integrating QKD into existing networks poses a challenge, as it must seamlessly coexist with classical channels, in optical fiber links utilizing wavelength division multiplexing (WDM). Continuous-variable QKD (CV-QKD), which leverages optical coherent technology, is seen as a promising solution for this integration within current telecom infrastructure. Additionally, Software-Defined Networking (SDN) presents opportunities for enhancing efficiency and facilitating dynamic management in coexistence scenarios. This paper reports key noise sources that degrade CV-QKD performance in coexistence and the parameters contributing to these noise sources such as classical channel (CC) power, separation between CC and quantum channel (QC) and position of QC. We also report strategies for effectively managing coexistence scenarios using SDN. We focus on the coexistence of CVQKD in presence of classical channels spaced according a 100 GHz ITU-T grid over a 10 km link length, providing guidelines to optimize the performance.

In this paper, the theoretical model of spontaneous Raman scattering (SpRS) in few-mode fiber (FMF) is discussed. The influence of SpRS on quantum key distribution (QKD) in FMF is evaluated by combining wavelength division multiplexing (WDM) and space division multiplexing (SDM) techniques. On this basis, an improved ring-assisted FMF is designed and characterized; the transmission distance can be increased by up to 54.5% when choosing different multi-channels. The effects of forward and backward SpRS on QKD are also discussed.