Reconfigurable MDI-QKD and BB84 over 20 km optical channels via EOM-tailored weak coherent states

Measurement-device-independent quantum key distribution (MDI-QKD) is designed to eliminate detector side-channel vulnerabilities. However, its practical deployment remains experimentally demanding because it requires two-photon interference (TPI) between mutually phase-randomized optical states. In this study, we demonstrate a reconfigurable platform that supports both polarization encoded MDI-QKD and BB84 measurements utilizing the same optical hardware over 20 km optical fiber channels. Two mutually phase-randomized weak coherent states (WCSs) are generated from a shared continuous-wave (CW) laser via electro-optic phase modulation and subsequent etalon-based first-order sideband filtering. Channel indistinguishability is verified through Hong-Ou-Mandel (HOM) interference, combining time-resolved coincidence measurements and polarization mismatch scans, confirming a high degree of indistinguishability that robustly approaches the classical upper limit of 0.5 for WCSs. The transmitted states go through partial Bell-state measurement (BSM) to implement MDI-QKD. Here, the sytem can be directly reconfigured for BB84 simply by rotatinga single half-wave plate (HWP) by 22.5 degree in one arm of the module. This seamless reconfiguration drastically reduces hardware redundancy and enhances operational flexibility in dynamic network environments. These results indicate that EOM-based frequency engineering using a shared CW laser offers a highly practical route toward scalable and reconfigurable quantum communication systems.