Quantum networks are resources for scaling quantum computers and distributed sensing technologies while offering post-quantum security benefits. Teleporting non-classical resources like entanglement, via so called entanglement swapping, is essential for networks in particular overcoming rate-loss limits via quantum repeaters. Deploying these systems on real infrastructure will likely require multiplexing photonic qubits into fibers carrying 'classical' light encoding standard Internet communications and control plane signals for multi-node quantum protocols. Here, we report the first demonstration of entanglement swapping and conventional communications operating over the same fibers. Entanglement is swapped across a five-node quantum relay topology connected by four long-distance fibers, each populated with classical data signals. Time-bin entangled photons in the C-band are multiplexed alongside C-band classical signals using dense-wavelength division multiplexing, introducing noise photons generated by high-power classical light. We experimentally and theoretically characterize the trade-off between quantum fidelity and Raman noise photons. Entanglement swapping is demonstrated over a maximum fiber length of 40 km (four 10-km fibers) while simultaneously transmitting 10-Gbps classical data through all fibers. These results represent a significant advancement in the demonstrated complexity of coexisting quantum and classical networks and provide a roadmap for achieving the widespread deployment of advanced quantum technologies.
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.
With the rise of 5G X-haul networks and emerging infrastructure segments, efficient multiplexing is essential for enabling secure quantum communication alongside classical data. In this work, we present a polarization-encoded Quantum Key Distribution (QKD) system deployed within the C-band over a live 5G network in Athens, demonstrating coexistence of QKD and 5G services on shared infrastructure. Our setup enables simultaneous transmission of 10 Gb/s classical data and QKD raw click rates of 4.3 kbps (22.6 km backhaul) and 51.9 kbps (2.2 km fronthaul). We propose a novel multiplexing approach using high-isolation, commercial-off- the-shelf (COTS) fiber filters and multiplexers, requiring no changes to existing infrastructure. The system achieves Quantum Bit Error Rates (QBER) below 1.5% and expected Secret Key Rates (SKR) of 650 bps and 11.3 kbps for backhaul and fronthaul, respectively, while maintaining uninterrupted 5G performance. This work offers a practical and scalable path for integrating QKD into future quantum-secured mobile networks.