On-chip coexistence of classical optical communication and quantum key distribution on etchless thin-film lithium niobate

Integrated photonics-based coexistence systems enable classical data channels and quantum channels to share the same optical waveguide by integrating quantum key distribution (QKD) into existing telecommunications infrastructure, reducing the costs and complexities of deploying quantum-secure links. Achieving this requires photonic components that efficiently reject intense classical light while preserving the integrity of ultra-weak single-photon quantum signals. Here, we demonstrate the first on-chip quantum-classical coexistence platform based on thin-film lithium niobate (TFLN). The chip integrates a polarization splitter-rotator (PSR) with two four-channel wavelength-division multiplexers, creating eight distinct channels encoded by wavelength and polarization in a highly compact millimeter-scale footprint. The device features ultra-low on-chip insertion loss ( IL ), high polarization isolation, and robust suppression of nonlinear noise, minimizing crosstalk from classical to quantum channels. Single-photon experiments verify its compatibility with QKD operations under realistic scenarios of simultaneous classical and quantum transmission. Its fabrication-robust design eases lithography constraints, enabling scalable and cost-effective manufacturing. Overall, this chip offers a viable path to integrate QKD into dense optical networks and advances the development of scalable quantum-secure communications.