[Paper Explained] Continuous Variable QKD Coexists with Classical Data in the C-Band

Research Papers

Published on 16 January, 2025

CV-QKD transmits quantum signals using coherent states, which are detected by the receiver through coherent detectors, similar to those commonly used in classical telecommunications. In this study, we evaluate its ability to coexist with classical data signals over the same optical fiber.

“Coexistence of Commercial CV-QKD and DWDM 100G/400G Transmission in Amplified FOADM-based Metro Links” 

In this brief entry, we will try to explain one of the main results of QUARTER-funded research: A joint project between QUARTER members LuxQuanta & Telefónica in demostrating co-existance of CV-QKD signal within classical data channels in real-world production networks. 

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.

Introduction

Quantum-safe security solutions are rapidly evolving to enable the seamless adoption of next-generation technologies within today’s telecommunications networks. While Quantum Key Distribution (QKD) systems are already commercially available and mature, they still face significant challenges before widespread adoption by companies of all sizes becomes a reality.

PArt of QUARTEr’s efforts is directed to address one of the most critical challenges: integrating the quantum link, responsible for generating and distributing quantum keys, into existing optical infrastructure. We achieve this without displacing classical data channels, ensuring minimal impact and maximum compatibility with current network operations. 

In a joint research effort between LuxQuanta, Telefónica, and the Centre Tecnològic de Telecomunicacions de Catalunya (CTTC), recently published in a 2024 peer-reviewed paper and presented at 50th European Conference on Optical Communication (ECOC 2024) , we demonstrate that this integration is not only possible, it is already viable using commercial technology and standard network infrastructure.

Coexistance in full-capacity C-band

LuxQuanta’s commercial CV-QKD system, branded as NOVA LQ®, successfully coexists with fully occupied C-band DWDM traffic, without displacing classical data channels or requiring dedicated optical fiber.

This result was validated through extensive experimentation in a metro network scenario, confirming that quantum and classical optical signals can share the same spectrum and physical infrastructure under real-world conditions.

The experimental setup included:

  • A 20 km amplified metro link based on FOADM architecture
  • LuxQuanta’s NOVA LQ® CV-QKD platform, implementing the Gaussian Modulated Coherent State (GMCS) protocol with a true local oscillator
  • 39 DWDM channels (100G and 400G), using off-the-shelf CFP2 and QSFP-DD transceivers from multiple vendors
  • Standard telecom components, including EDFA amplifiers and 100 GHz multiplexers

Only two notch filters were used to ensure effective isolation of the quantum channel, significantly reducing filtering complexity compared to typical QKD deployment expectations.

Quantum Flagship area where QUARTER members LuxQuanta & Quside stands

Live QKD Demo: LuxQuanta Systems & Quside’s QRNG

Importantly, all classical data channels remained within the C-band. No spectrum was shifted, and no classical traffic was removed to accommodate the quantum signal.

Spectrum Efficiency
Full C-band utilization is preserved. Quantum security can be added without sacrificing classical capacity, making it fully compatible with current and future high-throughput network designs.

Infrastructure Compatibility
The CV-QKD system was deployed on a standard metro link using commercial transceivers and switching equipment, proving that quantum-secure upgrades can be achieved without any architectural changes.

Cost-Effective Integration
The use of just two standard filters simplifies deployment and reduces both cost and complexity. No dedicated fiber is required.

Performance Optimization
Experimental results showed improved secure key rates when the quantum channel was allocated to the anti-Stokes region of the C-band. This finding provides a practical guideline for minimizing noise—especially Raman scattering—in coexistence scenarios.

Supporting a Quantum-Resilient Infrastructure
This study was conducted as part of broader European initiatives focused on secure, future-proof communications, including the EU Horizon ALLEGRO project, the Digital QUARTER project, and Spain’s 6G-OPENSEC-KEYS initiative.

These programs aim to accelerate the adoption of quantum-safe technologies and reinforce Europe’s leadership in secure digital infrastructure.

Written by Sergi Vizcaino

Strategy & Outreach Manager at LuxQuanta