Continuous Variable QKD Coexists with Classical Data in the C-Band

Quantum Key Distribution has long been seen as a technology that demands special treatment: dedicated fiber, cleared spectrum, or costly infrastructure upgrades. A joint research effort between Luxquanta, Telefónica, and the Centre Tecnològic de Telecomunicacions de Catalunya (CTTC) proves otherwise.

Published at ECOC 2024, the European Conference on Optical Communication, our results confirm that commercial CV-QKD can coexist with fully loaded classical traffic on standard metro fiber links today, using off-the-shelf components.

Luxquanta’s Nova lq® system transmitted a continuous quantum key distribution signal alongside 39 active DWDM channels (100G and 400G) over a 20 km amplified metro link without removing or shifting a single classical data channel, and without requiring dedicated fiber.

Quantum-safe security was added to a live, fully occupied network.

Most QKD deployments to date have required operators to set aside dedicated fiber or clear spectrum to accommodate the quantum channel. That constraint has been one of the primary barriers to adoption:it means extra cost, operational complexity, and a difficult business case for carriers already running high-capacity networks.+

This demonstration eliminates that barrier. The quantum signal coexists within the full C-band alongside production 100G and 400G traffic, using only two standard notch filters for isolation.

Experimental Setup

The experimental setup was deliberately close to real-world conditions:

• Network link: 20 km amplified metro link based on FOADM (Fixed Optical Add-Drop Multiplexer) architecture, a standard configuration in metro deployments
• QKD platform: Luxquanta’s Nova lq®, implementing the Gaussian Modulated Coherent State (GMCS) protocol with a true local oscillator
• Classical traffic: 39 DWDM channels at 100G and 400G, using commercial CFP2 and QSFP-DD transceivers from multiple vendors
• Filtering: Just two standard notch filters, significantly simpler than what is typically assumed for QKD deployment
• Amplification: Standard EDFA amplifiers and 100 GHz multiplexers throughout

All 39 classical channels remained in the C-band. The quantum channel was allocated to the anti-Stokes region of the C-band.

Nova lq® CV-QKD Features

No dedicated fiber. CV-QKD operates over the same fiber that already carries your data traffic.

Full spectrum preserved. Classical capacity is untouched. Quantum security is additive, not a trade-off.

Standard components throughout. EDFA amplifiers, commercial transceivers, and 100 GHz mux/demux equipment.

Simple filtering. Two-notch filters are sufficient for channel isolation, keeping deployment costs and complexity low.

Validated in a real network. The experiment’s topology, equipment, and traffic profiles reflect actual metro network deployments.

Part of a broader commitment to quantum-safe infrastructure

This research was conducted under the EU Horizon-funded QUARTER project, a collaborative initiative bringing together industry and research partners to accelerate the real-world deployment of quantum-safe networking technologies. The work also contributes to the EU Horizon ALLEGRO project and Spain’s 6G-OPENSEC-KEYS initiative, both of which reinforce Europe’s commitment to building a sovereign, quantum-resilient digital infrastructure.

Luxquanta continues to work with network operators, system integrators, and enterprise security teams to make quantum-safe connectivity a practical, deployable reality.

Reference: A. Melgar et al., “Coexistence of Commercial CV-QKD and DWDM 100G/400G Transmission in Amplified FOADM-based Metro Links,” ECOC 2024; 50th European Conference on Optical Communication, Frankfurt, Germany, 2024, pp. 1416–1419. Read the paper →