In a groundbreaking advance for the future of secure telecommunications, an international team of researchers has successfully demonstrated the seamless integration of quantum key distribution (QKD) with ultrahigh-speed classical optical communications over field-deployed multi-core fibers (MCFs). This pioneering achievement heralds a new era in optical networking, where the seemingly incompatible worlds of quantum and classical data traffic coexist within the same fiber infrastructure, uniting unbreakable security with massive data throughput in a real-world environment. The study, spearheaded by Dr. Qi Wu from Hong Kong Polytechnic University in collaboration with Prof. Cristian Antonelli and Prof. Antonio Mecozzi from the University of L’Aquila in Italy, represents a pivotal breakthrough by operating the entire C-band spectrum alongside quantum channels over a deployed fiber in L’Aquila, Italy.
Quantum key distribution exploits the fundamental principles of quantum mechanics to guarantee information-theoretic security, ensuring that encryption keys exchanged between parties remain invulnerable to eavesdropping. Despite its remarkable promise, QKD has traditionally faced significant obstacles in practical deployment, especially when integrated with existing classical optical networks. One of the main challenges arises from the extreme sensitivity of quantum signals, which can be easily disturbed or overwhelmed by noise generated by the petabytes of classical data simultaneously transmitted over the same fiber. Laboratory experiments previously illustrated coexistence possibilities over separated wavelengths or distinct channels, but these controlled environments could not fully replicate the complexities encountered in live telecommunication infrastructures.
The unique approach taken by Dr. Wu’s team involves the use of a four-core multi-core fiber, a modern fiber optic cable featuring multiple spatial channels or cores within a single cladding, enabling parallel transmission paths to boost spectral efficiency dramatically. In this setup, one dedicated core carries the delicate quantum signals, while the three remaining cores transport full-capacity classical data streams modulated across the entire C-band, totaling an unprecedented 110.8 terabits per second. This architectural decision leverages spatial division multiplexing along with wavelength division multiplexing to maximize channel density and exploit the fiber’s available capacity without compromising the fragile quantum transmissions.
A key technical challenge tackled by the researchers centers on inter-core spontaneous Raman scattering (SpRS), a nonlinear optical effect where photons in the high-power classical cores scatter and generate broadband noise photons that can invade the quantum core’s spectral domain. This noise, which manifests even when quantum signals and classical data operate at different wavelengths, is one of the main bottlenecks impairing quantum signal integrity and efficiency. To mitigate this, the team developed a comprehensive theoretical model that guides wavelength allocation and directional transmission strategies to minimize the detrimental Raman noise impacting the quantum channel.
The model advocated adopting counter-propagation for classical channels relative to the quantum core, meaning the classical data flows in the opposite direction to quantum signals. This configuration significantly reduces the overlap of spontaneous Raman scattering in the quantum core by exploiting the directional dependency of Raman processes. Furthermore, system parameters such as launch powers, modulation formats, and core assignments were meticulously optimized based on real-time measurements and simulations, ensuring the fine balance necessary for stable key generation and maximum data throughput.
In field trials conducted over 25.2 kilometers of the deployed multi-core fiber in L’Aquila, the integration achieved stable and continuous quantum key generation, marking the first demonstration of its kind over an operational metropolitan-scale fiber network with full C-band classical traffic simultaneously present. Maintaining record-class classical data rates alongside quantum channel stability in a noisy, uncontrolled field environment validates the robustness and scalability of the proposed coexistence methodology. The results provide compelling evidence that future quantum-classical hybrid networks can be realized without the need for costly parallel fiber deployments or major infrastructural overhauls.
This research carries profound implications for the realization of the quantum internet—a visionary network capable of enabling secure quantum information exchange at scale—and secure metropolitan area networks where dense classical traffic and quantum encryption must coexist seamlessly. Data centers, financial institutions, government communication facilities, and critical infrastructure sectors stand to benefit enormously from such integrated solutions, gaining unprecedented security assurances without sacrificing network performance or throughput capacities.
Moreover, this work aligns with ongoing advancements in spatial division multiplexing and optical fiber technology, indicating a future where quantum communication layers are embedded naturally alongside classical data transmission within existing fiber optic networks. Such integration reduces capital expenditure, operational complexity, and spatial constraints by maximizing the utility of every fiber deployed while paving the way for a more secure and interconnected global communication landscape.
The experimental validation of this coexistence framework also establishes new practical guidelines for network architects and operators to manage wavelength channel assignment, signal launch power, and propagation directions within multi-core fiber systems. These insights will be instrumental in guiding future standards and protocols that integrate quantum cryptography with the increasingly dense and complex architecture of modern optical networks.
The team emphasized that harnessing the full potential of the C-band spectrum for classical data transmission simultaneously with quantum signals is unprecedented in a real field setting. Their theoretical model, backed by rigorous experimental confirmation, represents a scalable blueprint for future deployments that require the coexistence of quantum and classical channels at terabit-per-second rates with high fidelity and operational stability.
In summary, the successful integration of QKD and high-throughput classical optical communications over field-deployed multi-core fibers epitomizes a critical milestone towards secure, high-capacity telecommunication infrastructures. The seamless marriage of quantum technology with conventional optical networks promises to accelerate the deployment of quantum-secure communications on a global scale, overcoming previous technical barriers and establishing a resilient foundation for next-generation networks.
This study was published in the renowned journal Light: Science & Applications, underscoring its significance within the photonics and quantum communication communities. The results presented highlight not only the scientific ingenuity but also the practical relevance and immediacy of bringing quantum key distribution into widespread operational use, thus marking an inflection point in secure optical networking.
Subject of Research: Integration of quantum key distribution and classical high-throughput communications in multi-core optical fibers.
Article Title: Integration of quantum key distribution and high-throughput classical communications in field-deployed multi-core fibers
Web References:
10.1038/s41377-025-01982-z
Image Credits: Wu, Q., Ribezzo et al.
