Integrated polarization-entangled photon source for wavelength-multiplexed quantum networks

Entangled photons are fundamental resources for quantum communication, computing, and networking. Among them, polarization-entangled photon pairs play an important role due to their straightforward state manipulation and direct use in quantum key distribution, teleportation, and network protocols. However, realizing compact, efficient, and scalable polarization-entangled sources that meet the requirements of practical deployment remains a major challenge. Here, we present a simple yet high-performance on-chip polarization-entangled photon-pair source on thin-film lithium niobate. Our device employs dual quasi-phase matching that sequentially supports type-0 and type-I spontaneous parametric down-conversion in a single nanophotonic waveguide, eliminating the need for interferometers, polarization rotators, or other complex circuits. The source directly produces high-fidelity Bell states with broad bandwidth, high brightness, and low noise. Using this integrated approach, we realize wavelength-multiplexed entanglement distribution in a four-user quantum network deployed over loopback metropolitan fiber links up to 50 km. Researchers use dual quasi-phase matching in a thin-film lithium niobate chip to directly generate polarization-entangled photons, enabling scalable quantum communication over a 50 km metropolitan fiber network.