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 using a passive circulator-based hub built from of 3-port optical circulators. This network topology enables secure key exchange between any pair of the three users-Alice, Bob, and Charlie—without active switching or a trusted intermediary. Each user is connected to the central circulator via standard telecom fibers 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 $10^6$ bits were processed with a composable security parameter $\varepsilon \approx 10^{-9}$. 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 rates. Cascade consistently produced longer final keys due to its higher reconciliation efficiency. These results demonstrate the feasibility of passive circulator-based architectures for secure and scalable multi-user quantum communication.