Quantum key distribution (QKD) offers unbreakable security based on quantum physics, but integrating it with classical high-speed data transmission remains a long-standing challenge due to the extreme sensitivity of quantum signals to noise. In laboratory settings, QKD and classical systems have been shown to coexist over different wavelengths or channels, but real-world deployments often suffer from unpredictable interferences, especially in multi-core fiber systems.
In a new study published in Light: Science & Applications, a team of researchers led 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, Italy, successfully demonstrate the first real-world integration of QKD with 110.8 Tbit/s classical coherent optical communication over field-deployed multi-core fibers (MCFs) in L'Aquila.
The researchers used a four-core MCF where one core was dedicated to quantum signals and the other three carried full-capacity C-band classical data. To reduce inter-core spontaneous Raman scattering (SpRS) noise—a key source of quantum degradation—they proposed and experimentally verified a theoretical model to optimize wavelength allocation and directionality. By adopting counter-propagation for classical cores and fine-tuning system parameters, they achieved stable quantum key generation over 25.2 kilometers while maintaining record-level classical data throughput.
This work represents a major milestone in building secure, high-throughput optical communication networks for the future. The ability to combine quantum and classical signals over the same physical fiber infrastructure not only saves deployment cost but also accelerates the realization of the quantum internet. The research opens up exciting possibilities for secure metropolitan networks, data centers, and long-haul communication systems where security and capacity must coexist.
"This is the first time such a demonstration has been carried out in a real field environment with full C-band utilization," the team said. "Our model provides a practical guideline for scalable integration of QKD and classical communication in future multi-core fiber networks."
