Reconfigurable measurement scheme compatible with MDI-QKD and BB84 protocols using single-laser sideband generation

A quantum network can be operated efficiently by switching between measurement-device-independent quantum key distribution (MDI-QKD) and BB84 according to the trust level of an intermediate node, since MDI-QKD is immune to all detector side-channel attacks while generally yielding a lower secret-key rate. Here, we present a proof-of-principle demonstration of a reconfigurable measurement scheme compatible with both protocols using a single laser. Two mutually independent weak coherent states (WCSs) are generated from a shared continuous-wave laser via electro-optic phase modulation and subsequent first-order sideband filtering. Channel indistinguishability is verified using two complementary Hong–Ou–Mandel (HOM) measurements: time-resolved coincidence measurements and polarization mismatch scans, both yielding HOM visibilities close to the theoretical limit of 0.5 for WCSs. The measurement module performs a partial Bell-state measurement for MDI-QKD, while rotating a single half-wave plate to reconfigures the same module into a dual-basis polarization analyzer for BB84. The quantum bit error rates measured in both configurations agree well with theoretical expectations, verifying the operational feasibility of the reconfigurable receiver at the physical layer. By eliminating the need for active frequency locking while retaining independent sideband tunability, this approach provides a practical foundation for flexible and cost-effective quantum networks.