Integrated silicon photonic transceivers for bidirectional continuous-variable quantum key distribution in metropolitan ring networks

Metropolitan quantum key distribution (QKD) networks face scalability bottlenecks from limited fiber resources and high deployment costs. Although photonic integration enables miniaturization, current implementations are largely restricted to unidirectional configurations, limiting topological flexibility. Here, we report a monolithic silicon photonic continuous-variable (CV) QKD transceiver tailored for simultaneous bidirectional operation. Integrating high-linearity modulators and high-sensitivity coherent detectors on a silicon-on-insulator (SOI) platform, we employ a frequency division duplexing (FDD) strategy combined with pilot-aided digital signal processing to effectively suppress crosstalk and backscattering. The system achieves a total asymptotic secret key rate (SKR) of 4.65 Mbps in a single-fiber, same-band bidirectional configuration over 50.4 km. Demonstrating practical viability, we further establish a finite-size SKR of 1.41 Mbps over a 65.5 km link. By resolving the crosstalk challenges inherent to bidirectional integration, this work provides a hardware-ready foundation for the uni-transceiver quantum key distribution ring network (UT-QKDRN). These results mark a pivotal step in transitioning quantum secure networks from laboratory demonstrations to scalable metropolitan infrastructure.