Quantum key distribution over a three-user fiber network with passive optical circulator

We report the implementation of a quantum key distribution (QKD) system over a three-user fiber-optic network in which each user employs an identical bidirectional transceiver, with a single asymmetric interferometer shared between the encoding and decoding paths to reduce duplicated optical resources at the node level. The three users-Alice, Bob, and Charlie-are interconnected through a passive circulator-based hub built from 3-port optical circulators, which enables secure key exchange between any pair of them without active switching or a trusted intermediary. Each user is connected to the central circulator via standard telecom fiber links of 10-15 km, resulting in effective inter-user distances of 20-25 km. We employed the BB84 protocol with a two-decoy-state method (weak and vacuum) for secure key generation. For each user pair, sifted keys of 10⁶ bits were processed with a composable security parameter ε ~ 10⁻⁸ . Secret key lengths were evaluated using three approaches: (1) theoretical estimation based on measured quantum bit error rate (QBER) and gains, assuming an error correction efficiency of 1.16; (2) experimental post-processing using Winnow; and (3) experimental post-processing using Cascade. All links achieved low QBERs and yielded positive secret key lengths. Cascade consistently produced longer final keys due to its higher reconciliation efficiency. These results demonstrate the feasibility of a resource-sharing bidirectional node architecture for secure multi-user quantum communication using passive circulator-based routing.