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Journal of Lightwave Technology

Coexistence of Optical Frequency Transmission With Quantum Key Distribution Over Multi-Core Media

Lai Yu, Li Zhang, Lei Liu, Zhicheng Jin, Jialiang Wang, Hongyan Song, Wenjing Ding, Fang‐Xiang Wang, Wei Chen, Zheng‐Fu Han, Youzhen Gui

Published Jan 12, 2026• 0 citations
Publisher

Abstract

The quantum-classical coexistence can be obtained using wavelength division multiplexing (WDM), however, the crosstalk between quantum and classical signals due to Raman noise must be handled with extra attention. Multi-core fiber (MCF) provides a good solution for the coexistence based on space division multiplexing (SDM). In this work, we propose and experimentally demonstrate hybrid transmission of ultrastable optical frequency reference and quantum key distribution (QKD) over homogeneous MCF. The scheme achieves optical frequency stabilization at the remote site by beating the loopback frequency signal with a local reference to extract link phase noise, which is then compensated via voltage-controlled oscillator (VCO) feedback. Prior to the experiment, we first analyze the critical challenges in MCFs including inter-core crosstalk and inter-core spontaneous Raman scattering. To minimize the crosstalk between the signals, the quantum signal and the optical frequency propagate through separate fiber cores. Successful hybrid transmission of optical frequency references and QKD is achieved across 20 km seven-core fiber. The frequency instability of$1.2 \times 10^{-17}$at 1 s and$4.2 \times 10^{-19}$at 10000 s, and secure key rate of 317.28 kbps can be realized in this integrated transmission system. The scheme benefits from two merits, ultrastable optical frequency offers high-precision clocks to QKD, and QKD provides one-time-pad keys to encrypt the time data. These results demonstrate the feasibility and tolerance of our QKD system with high-precision frequency reference and the viability of multi-core fibers for next-generation quantum networks.

DOI
10.1109/jlt.2026.3652175

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The quantum-classical coexistence can be obtained using wavelength division multiplexing (WDM), however, the crosstalk between quantum and classical signals due to Raman noise must be handled with extra attention. Multi-core fiber (MCF) provides a good solution for the coexistence based on space division multiplexing (SDM). In this work, we propose and experimentally demonstrate hybrid transmission of ultrastable optical frequency reference and quantum key distribution (QKD) over homogeneous MCF. The scheme achieves optical frequency stabilization at the remote site by beating the loopback frequency signal with a local reference to extract link phase noise, which is then compensated via voltage-controlled oscillator (VCO) feedback. Prior to the experiment, we first analyze the critical challenges in MCFs including inter-core crosstalk and inter-core spontaneous Raman scattering. To minimize the crosstalk between the signals, the quantum signal and the optical frequency propagate through separate fiber cores. Successful hybrid transmission of optical frequency references and QKD is achieved across 20 km seven-core fiber. The frequency instability of1.2×10−171.2 \times 10^{-17}1.2×10−17at 1 s and4.2×10−194.2 \times 10^{-19}4.2×10−19at 10000 s, and secure key rate of 317.28 kbps can be realized in this integrated transmission system. The scheme benefits from two merits, ultrastable optical frequency offers high-precision clocks to QKD, and QKD provides one-time-pad keys to encrypt the time data. These results demonstrate the feasibility and tolerance of our QKD system with high-precision frequency reference and the viability of multi-core fibers for next-generation quantum networks.
We report the first experimental demonstration hybrid transmission of ultra-stable optical frequency references and quantum key distribution (QKD) signals over a homogeneous multi-core fiber (MCF). The frequency transfer system achieves frequency stabilization at the remote site via active phase noise compensation. A QKD system implementing the BB84 phase-encoding protocol with decoy states is integrated within the platform. Successful hybrid transmission of optical frequency references and quantum signals is achieved over 10 km 7-core fiber. The integrated transmission system achieves a frequency stability of 8×10−18at 1s and 3.45×10−19at 10000 s, with a quantum bit error rate (QBER) of 2.54%.
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.
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) 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.
Multicore-fibres (MCF) have emerged as an attractive solution for capacity scaling in optical communication. Securing such links with Quantum Key Distribution (QKD) is critical with the advent of Quantum computers. For practical deployment, QKD is required to operate alongside high-speed classical communication over field-deployed fiber infrastructure. In this work, the performance of QKD is experimentally evaluated in coexistence with high-speed classical data transmission over field-deployed underground and aerial multicore fiber links. The QKD system is implemented with Coherent One-Way Slim protocol, with dedicated Quantum and classical channels for Quantum communication and QKD post-processing. The deployed 4-core MCF infrastructure at IIT Madras provides an effective transmission length of 48 km through loopback operation. QKD performance over the MCF is evaluated in terms of quantum bit error rate (QBER) under varying attenuation, controlled noise injection, and inter-core crosstalk over both singlecore fiber and MCF links. The experimental results show that inter-core crosstalk has a stronger impact in MCF, leading to increased excess noise at the quantum receiver. For a 32-GBaud PM 16-QAM classical modulation format, stable QKD operation is maintained when the spectral separation is approximately six to eight times the occupied bandwidth.