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International Conference on Transparent Optical Networks

Coexistence of Classical and Quantum Signals in Hybrid Fiber and Free-Space Optics Links for QKD Integration

Nelson Jesus Muga, Sara T. Mantey, Diogo Cruz, Ana M. Rocha, Nuno A. Silva, Marco A. Fernandes, Gil M. Fernandes, Fernando P. Guiomar, Paulo P. Monteiro, Armando N. Pinto

Published Jul 14, 2024• 2 citations
Publisher

Abstract

Quantum key distribution (QKD) exploits the unique properties of quantum physics. It has evolved tremendously in recent years, attracting considerable attention as a privileged solution capable of enabling future-proof secure communication systems. This contrasts with classical cryptographic systems, whose security is based on computational security. However, before quantum cryptographic systems, e.g. QKD, could be widely implemented, in a coexistence scenario with classical signals, and made widely available to applications and users, they face several crucial challenges related to their real-world implementation. This paper presents our recent investigations to evaluate the coexistence between classical and quantum signals in hybrid fiber (multi-core fiber) and free-space optical links for an optimized integration of QKD systems in the installed fiber communication networks. We present results in terms of spontaneous Raman scattering, inter-core cross-talk and turbulence-induced optical power fluctuations.

DOI
10.1109/icton62926.2024.10647645

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Quantum key distribution (QKD) exploits the unique properties of quantum physics. It has evolved tremendously in recent years, attracting considerable attention as a privileged solution capable of enabling future-proof secure communication systems. This contrasts with classical cryptographic systems, whose security is based on computational security. However, before quantum cryptographic systems, e.g. QKD, could be widely implemented, in a coexistence scenario with classical signals, and made widely available to applications and users, they face several crucial challenges related to their real-world implementation. This paper presents our recent investigations to evaluate the coexistence between classical and quantum signals in hybrid fiber (multi-core fiber) and free-space optical links for an optimized integration of QKD systems in the installed fiber communication networks. We present results in terms of spontaneous Raman scattering, inter-core cross-talk and turbulence-induced optical power fluctuations.
Quantum key distribution (QKD) is a secure communication method for sharing symmetric cryptographic keys based on the principles of quantum physics. Its integration into the fiber-optic network infrastructure is important for ensuring privacy in optical communications. Multi-core fibers (MCFs), the likely building blocks of future high-capacity optical networks, offer new opportunities for such integration. Here, we experimentally demonstrate, for the first time, the coexistence of discrete-variable QKD and high-throughput classical communication in the C-band over a field-deployed MCF with industry standard cladding diameter of 125 μm. Specifically, we demonstrate successful secure-key establishment in one core of a 25.2-km uncoupled-core MCF, while simultaneously loading the remaining three cores with full C-band counter-propagating classical traffic at an aggregate net rate of 110.8 Tb/s. By proposing and experimentally validating an improved analytical model for inter-core spontaneous Raman scattering noise, we find that this configuration is optimal for our deployed MCF link as it is immune to four-wave mixing, that becomes relevant when the quantum and classical signals are propagating in the same direction. Our findings make an important step forward in demonstrating the integration of QKD and classical transmission in uncoupled-core multi-core fibers for next-generation optical communication networks.
In this paper, we investigate the impact of classical optical communications in quantum key distribution (QKD) over hollow-core fiber (HCF), multi-core fiber (MCF) and single-core fiber (SCF) and propose wavelength allocation schemes to enhance QKD performance. Firstly, we theoretically analyze noise interference in QKD over HCF, MCF and SCF, such as spontaneous Raman scattering (SpRS) and four-wave mixing (FWM). To mitigate these noise types and optimize QKD performance, we propose a joint noise suppression wavelength allocation (JSWA) scheme. FWM noise suppression wavelength allocation and Raman noise suppression wavelength allocation are also proposed for comparison. The JSWA scheme indicates a significant enhancement in extending the simultaneous transmission distance of classical signals and QKD, reaching approximately 100 km in HCF and 165 km in MCF under a classical power per channel of 10 dBm. Therefore, MCF offers a longer secure transmission distance compared with HCF when classical signals and QKD coexist in the C-band. However, when classical signals are in the C-band and QKD operates in the O-band, the performance of QKD in HCF surpasses that in MCF. This research establishes technical foundations for the design and deployment of QKD optical networks.
Quantum key distribution (QKD) is regarded as an alternative to traditional cryptography methods for securing data communication by quantum mechanics rather than computational complexity. Towards the massive deployment of QKD, embedding it with the telecommunication system is crucially important. Homogenous optical multi-core fibers (MCFs) compatible with spatial division multiplexing (SDM) are essential components for the next-generation optical communication infrastructure, which provides a big potential for co-existence of optical telecommunication systems and QKD. However, the QKD channel is extremely vulnerable due to the fact that the quantum states can be annihilated by noise during signal propagation. Thus, investigation of telecom compatibility for QKD co-existing with high-speed classical communication in SDM transmission media is needed. In this paper, we present analytical models of the noise sources in QKD links over heterogeneous MCFs. Spontaneous Raman scattering and inter-core crosstalk are experimentally characterized over spans of MCFs with different refractive index profiles, emulating shared telecom traffic conditions. Lower bounds for the secret key rates and quantum bit error rate (QBER) due to different core/wavelength allocation are obtained to validate intra- and inter-core co-existence of QKD and classical telecommunication.
Abstract In the contemporary era of information technology, cybersecurity has emerged as a pivotal subject. Quantum key distribution (QKD) networks have been implemented in various regions worldwide, demonstrating their superior secure communication capabilities in practice. The integration of QKD with traditional fiber optic networks will facilitate the deployment of upcoming quantum technologies. The recently proposed mode-pairing quantum key distribution (MP-QKD) protocol can drastically improve the transmission distance of QKD, thereby obviating the necessity for complex optical field setups. This paper focuses on the co-transmission of MP-QKD with classical signals in three types of optical fibers (single-mode fiber, few-mode fiber, and multi-core fiber). The Raman noise power in the co-transmission system is calculated, and the performance of MP-QKD is analyzed. The analytical results demonstrate the superiority of MP-QKD compared to other MDI-types in co-fiber transmission scenarios.
Quantum key distribution (QKD) is approaching industrial maturity, but its integration with high-power classical signals in shared optical fiber is hindered by noise from nonlinear effects like spontaneous Raman scattering and four-wave mixing. Hollow-core fiber (HCF) offers a promising solution by inherently suppressing these nonlinearities. However, a comprehensive noise analysis for pure HCF or standard single-mode-fiber–HCF hybrid links, validated experimentally under realistic high-power conditions, remains unavailable. This work bridges that gap through a thorough theoretical and experimental investigation of an HCF-featured coexistence system. We demonstrate a silicon-photonics-based QKD system operating with an optical transport network (OTN). The system achieves multiplexed launch powers of 20 dBm for counter-propagation and 14 dBm for co-propagation, meeting the highest requirements of modern OTNs. Over a 10 km HCF link, it simultaneously supports 12×583 Gbps classical channels across the C-band and achieves secure key rates of 5.7 kbps (counter-propagation) and 2.5 kbps (co-propagation) over a 1550 nm quantum channel. Our results provide critical benchmarks and a practical framework for deploying HCF-based quantum-classical networks, paving the way for future inherently secure telecommunications.
Quantum key distribution (QKD) coexisting with ultra-high-capacity classical optical signals over the same fiber is widely regarded as a cornerstone of future quantum-safe networks. This approach eliminates the need for dedicated fibers for QKD, significantly reducing infrastructure costs and facilitating large-scale deployment. As a promising foundation for next-generation high-capacity optical networks, multi-core fiber (MCF) opens new possibilities for such coexistence, offering an elegant and spectrum-efficient solution for the integration of quantum and classical channels. Here, we report the first field trial of QKD securely coexisting with bidirectional 106.7 Tb/s full-C-band dense wavelength-division-multiplexed coherent classical transmission over 25.2 km of installed standard-cladding diameter (125 μm) uncoupled-core four-core fiber in L'Aquila, Italy. Despite a classical launch power of 20 dBm per core, a secure key rate of 1.7 kb/s was achieved, using a commercial QKD system. We used the secure keys to achieve real-time encryption of up to 132 Gb/s live traffic within a 400 Gb/s channel, with key refresh intervals as short as 15 seconds. We used hardware encryption, introducing only 52 μs end-to-end latency and jitter below 4 μs compared to the unencrypted case. These results represent a major milestone toward practical, secure, and scalable integration of quantum and classical communication over deployed metropolitan multi-core fiber infrastructure.