Characterizing filter suppression requirements for entanglement distribution in quantum-classical optical systems

Although spontaneous Raman scattering is often regarded as the dominant impairment in hybrid quantum– classical fiber links, practical deployment also requires clear limits on linear inter-channel crosstalk between entangled photons and neighboring dense wavelength-division multiplexing (DWDM) channels. In this work, we experimentally determine such limits using a controlled bench-top testbed in which a high-fidelity (F > 0.97) polarization-entangled photon source shares a DWDM demultiplexer with a single classical optical channel separated by 100 GHz. Residual classical leakage into the quantum receiver is controlled through spectral filtering and systematically varied by sweeping the classical power incident on the detector from −120 dBm to −40 dBm. A thin-film bandpass filter with 0.9 nm bandwidth, providing approximately 10 dB of additional suppression, is employed in the quantum path. Entanglement quality is assessed through polarization-interference visibility measured in multiple polarization bases and through the coincidence-to-accidental ratio (CAR). We observe that high visibility and CAR are maintained over a broad range of crosstalk levels, with degradation governed by the relative rate of accidental coincidences rather than by absolute optical power alone. Acceptable performance, defined here as visibility ≥ 90% and CAR ≥ 20, is preserved provided that the classical leakage power at the detector does not exceed the quantum-signal power by more than 12 dB for a continuous-wave carrier or for a 100 GBd dual-polarization QPSK signal. These experimental results establish filtering and isolation requirements for entanglement distribution in hybrid quantum–classical DWDM optical systems.