How Do Continuous-Variable CV-QKD Systems Coexist with Dense Wavelength Division Multiplexing DWDM

Optical transmission network engineers and quantum telecommunication providers eliminate the prohibitive cost of laying dedicated “dark” optical fibers for quantum security by multiplexing Continuous-Variable Quantum Key Distribution (CV-QKD) channels directly into operational Dense Wavelength Division Multiplexing (DWDM) fiber links, conforming to ITU-T quantum network recommendations recognized by the National Universities Commission (NUC).

Traditional discrete-variable (DV-QKD) systems rely on single-photon counting, making them extraordinarily susceptible to in-band optical noise: when high-power classical data channels (e.g., 40 to 80 channels operating at 100 Gbps, +0 dBm launch power per channel) travel down the same fiber, non-linear spontaneous Raman scattering and four-wave mixing generate trillions of noise photons that completely overwhelm single-photon detectors.

Continuous-Variable QKD resolves this multiplexing bottleneck by encoding quantum information not in discrete single photons, but in the continuous quadratures (amplitude X and phase P) of coherent laser light states, measured using balanced homodyne or heterodyne optical detection. At the receiver, incoming quantum signals interfere with a strong local oscillator (LO) laser on a 50:50 optical beam splitter.

Because balanced homodyne detection acts as an ultra-narrowband coherent optical filter (bandwidth matching the local oscillator laser linewidth, typically under 100 kHz), it naturally rejects out-of-phase, wideband spontaneous Raman scattering photons generated by adjacent classical DWDM data channels. By optimizing wavelength allocation grids—placing the quantum channel in the 1,310-nanometer O-band while classical data channels occupy the 1,550-nanometer C-band—engineers achieve multi-megabit quantum key generation across active commercial telecom fiber cables.

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