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Asia Communications and Photonics Conference

Integrating of optical frequency with quantum key distribution based on 10-km 7-core fiber

Lai Gui Yu, Lei Liu, Li Zhang, Zhicheng Jin, Jialiang Wang, Fang‐Xiang Wang, Youzhen Gui

Published Nov 5, 2025• 0 citations
Publisher

Abstract

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%.

DOI
10.1109/acp66871.2025.11349775

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•Mar 31, 2025•0 citations
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%.
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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.
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.
We demonstrated ultra fast BB84 quantum key distribution (QKD) transmission at 625 MHz clock rate through a 97 km field-installed fiber using practical clock synchronization based on wavelength-division multiplexing (WDM). We succeeded in over-one-hour stable key generation at a high sifted key rate of 2.4 kbps and a low quantum bit error rate (QBER) of 2.9%. The asymptotic secure key rate was estimated to be 0.78- 0.82 kbps from the transmission data with the decoy method of average photon numbers 0, 0.15, and 0.4 photons/pulse.
In this paper, we explore a hybrid quantum communication protocol that operates concurrently over fiber optic and atmospheric channels. This new protocol addresses challenges in urban settings where laying optical fiber may be impractical or costly. By integrating the subcarrier wave (SCW) quantum key distribution (QKD) with phase coding, our approach enhances the flexibility and reliability of quantum communication systems. We have developed and tested an atmospheric optical module equipped with an auto-tuning system to ensure precise optical axis alignment, crucial for minimizing signal loss in turbulent environments. Experimental results demonstrate stable sifted key rates and low quantum bit error rate (QBER) across various channel lengths, confirming the efficacy of our hybrid protocol in securing communication over diverse transmission environments.
HD-QKD(high-dimensional quantum key distribution) is an alternative to two-dimensional QKD with advantages such as higher data transmission per pulse [54] and noise tolerance [13, 66]. It has been shown that MCFs (multicore fibers) are a viable platform for performing HD-QKD through phase encoding across modes of the same fiber [10]. MCFs have also gathered attention due to their increased data transmission rates and are likely to become part of classical telecom infrastructure in the future [35, 50, 14, 51]. In this thesis we report a proof-of-concept decoy-state BB84 HD-QKDexperiment with an all-fiber setup using installed 4C-MCFs (four core multicore fibers) in a realistic environment. We introduce the theory of QKDand the motivations for decoy states as well as the security bounds for systems with different dimensions. We describe the experiment in detail, including active phase stabilization, state preparation, MCF-based technologies, channels and the detection system. We report a maximum short-range key rate of 7.8 kbit/s and a maximum secure key transmission distance of 105 km, as well as a short-range stability of over 16 hours. A comparison of our experiment with several others is provided, showing that our results are comparable to other works with similar detection technology. Wealso include two papers where we used our experimental platform for experiments simulating non-Markovian noise and implementing single setting quantum state tomography, showing the flexibility of our setup and potential for further work.