Abstract: Deploying dedicated dark fiber for quantum channels across metropolitan areas is cost-prohibitive. This landmark research demonstrates that quantum signals can co-propagate over the same lit optical fiber carrying multi-terabit classical traffic by optimizing spectral placement, optical filtration, and temporal detector gating.
When 100G DP-QPSK or 400G 16-QAM channels operate at 1550 nm with aggregate powers of +15 dBm, forward and backward Raman scattering generates a broad photon background across both C and L bands. We establish closed-form equations modeling Raman noise spectral density as a function of classical launch power, fiber length, and temperature.
We implement a triple-stage optical filter cascade pairing 50 GHz dielectric thin-film DWDM filters with a 100 pm fiber Bragg grating and an auxiliary Fabry-Perot cavity. This configuration suppresses adjacent classical channels by over 115 dB, preserving quantum state signal-to-noise ratios even during traffic bursts.
During a 90-day continuous trial across an operational municipal network in central Europe, our coexistence framework delivered an average secret key rate of 8.4 kbps with quantum bit error rate (QBER) consistently held below 1.45% alongside 800 Gbps of simultaneous classical payload throughput.