Quantum entanglement in the C band coexisting with data transmission in the L band

The coexistence of quantum information and classical signals within the same fiber link is essential for realizing future quantum networks that utilize the well-established optical fiber infrastructure. In this work, we demonstrate the coexistence of $C$-band energy-time entanglement and $L$-band classical data. By employing wavelength-division multiplexing techniques and polarization management, we achieve quantum entanglement distribution over a 40-km-long fiber carrying $4\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{W}$ of bidirectional received power. Under the highest noise conditions, we obtain a visibility of $84.59\ifmmode\pm\else\textpm\fi{}0.68%$ from Franson interference and calculate a secret key rate of 1.89 kbps using the Bennett-Brassard-Mermin 1992 (BBM92) protocol, showing improved performance compared to an all-$C$-band setup. These results pave the way for large-scale quantum-classical coexistence within existing optical fiber infrastructure.