Generalized State Discrimination for Tunable Quantum Key Distribution
We introduce a tunable framework for generalized quantum state discrimination (GSD) and apply it to quantum key distribution (QKD) through a protocol we call phiQKD. Building upon the two-state B92 protocol, phiQKD replaces the traditional unambiguous state discrimination (USD) measurement with a one-parameter family of hybrid POVMs characterized by a tilting angle $ϕ$. This allows for continuous control over the trade-off among correct, incorrect, and inconclusive outcomes. While offering improvement in key rate over B92, the primary practical advantage of phiQKD lies in its adaptability to noise and channel imperfections via measurement tunability. By evaluating the protocol under asymptotic, finite-key, and composable security models, we show that, treating quantum measurement as a tunable design parameter, rather than a fixed operation, enables flexible protocol optimization and improved performance under realistic constraints.