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Journal of Optical Communications and Networking

Continuous variable QKD in flexible optical networks for future quantum secure connectivity

Michela Svaluto Moreolo, Masab Iqbal, Arturo Villegas, Ramon Casellas, Laia Nadal, Raül Muñoz

Published May 9, 2025• 7 citations
Publisher

Abstract

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.

DOI
10.1364/jocn.553771

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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.
Several technological challenges are currently posed by future 6G networks to support ultra-high capacity and dynamic connectivity, while ensuring environmental and societal sustainability, scalability, and reliability. Among them, security is a crucial aspect, particularly considering open and disaggregated networks. Continuous variable quantum key distribution (CV -QKD) is proposed as an appealing solution to ensure long-term security, while facilitating the coexistence with conventional optical communications for a sustainable integration of QKD technology in future 6G networks. In this work, the adoption of CV -QKD in future optical networks is discussed, focusing on the network resources and infrastructure sharing, exploring advanced features, flexibility and programmability, further enabled by software defined networking (SDN).
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.
Continuous variable quantum key distribution (CV-QKD) is arising as an attractive technology for quantum secure communications in future optical networks, thanks to its higher compatibility with conventional telecommunication systems. In fact, CV-QKD adopts opto-electronic devices of coherent optical systems, enabling the use of commercial off-the-shelf components (COTS) and photonic integrated circuits (PICs). This favors a faster and sustainable adoption of this technology in optical networks. Also, smoother integration within the network infrastructure can be envisioned, in coexistence with classical communications, further facilitated and accelerated by software defined networking (SDN). In this invited contribution, the adoption of CV-QKD in future optical networks is discussed, presenting our recent results.
Optical transport networks are evolving toward unprecedented capacity, sustainability, and security requirements. We present an SDN-enabled architecture for integrating continuous variable quantum key distribution (CV-QKD) into programmable optical networks, adopting sliceable bandwidth/bitrate variable transceivers (S-BVTs) and/or pluggable modules. The proposed solution combines a TeraFlowSDN-based SD-QKD network controller with a reconfigurable CV-QKD system through interfaces compliant with ETSI GS QKD 015. Experimental validation demonstrates real-time monitoring and dynamic wavelength (re)configuration, facilitating coexistence. Alternative dense wavelength-division multiplexing (DWDM) co-propagation scenarios are presented, and flexible channel allocation strategies are analyzed and discussed. Simulation results further evaluate evolutionary scenarios involving coexistence with 400G pluggable modules at 0 dBm launch power. The analysis identifies the maximum number of supported classical channels (up to 20 over a 20 km fiber link) and suitable spectral allocation in accordance with the experimental results. Furthermore, we compare the coexistence performance of CV-QKD adopting discrete components and an early version of a photonic integrated circuit (PIC) receiver, supporting half the number of 400G channels. Performance enhancement can be envisioned with further PIC design improvement and optimization. The obtained results confirm the viability of SDN-controlled CV-QKD as an enabler for quantum secure operation in future optical transport networks and a promising evolutionary path toward the adoption of this technology.
To face the challenges of open and disaggregated 6G networks and the prospect/threat of quantum computing, we propose to adopt continuous-variable QKD (CV-QKD), as a promising technology compatible and suitable to be integrated with conventional optical systems, in combination with software defined networking (SDN). In this work, we present our most recent advances on SDN-enabled flexible CV-QKD for future secure communications. We describe the proposed network architecture, identifying the relevant parameters and interfaces. We consider and analyze a Gaussian-modulated coherent state CV-QKD system, reporting recent experimental results at the varying of the QKD system wavelength, as relevant adaptable parameter, that can be configured by the SDN controller, for distributing highly secure keys in metropolitan networks. The capabilities and potentialities of our proposed approach for future secure communications in open and disaggregated 6G networks are also discussed.
The need to increase throughput and reduce the latency in the workload distribution and communications between different Data Centers (DCs) demands the use of dedicated or rented fibers. In view of enabling quantum secure communication in centralized and distributed DC, the integration of quantum key distribution (QKD) systems appears as a crucial milestone to safeguard data against quantum computing threats and the "Store-Now, Decrypt-Later" risk. Specifically, tailored technological solutions must be considered to enable practical deployment. In this context, continuous-variable QKD (CV-QKD) presents a promising approach due to its compatibility with optical systems and potential for cost savings and scalability. The adoption of photonic integrated circuits further enhances feasibility by enabling component miniaturization, reducing power consumption, and facilitating large-scale deployment. An important aspect to consider is optimizing fiber usage to support infrastructure sharing and resource efficiency. Therefore, the coexistence of the proposed QKD solutions with inter-DC communications must be carefully assessed. This invited contribution explores the integration of CV-QKD in DC networks to address flexible and efficient quantum secure connectivity. Envisioned co-propagation scenarios and associated challenges will be discussed, offering insights into the evolutionary path towards quantum secure DC infrastructures.