Enhancing the security of coherent one-way quantum key distribution using CHSH correlations
The coherent one-way (COW) protocol is a quantum key distribution scheme that has attracted significant attention, leading to the development and commercialization of practical implementations. Despite this progress, the security of the COW protocol has remained a fundamental challenge since its introduction. Numerous studies have investigated its security, and several security proofs have been proposed over the years. More recently, a number of works have questioned the security of this protocol. In particular, one of the latest studies introduced an attack that severely limits the security of COW-QKD and reported a maximum secure distance of less than 20km. In this work, we introduce minimal alteration to the COW protocol that can enhance its security. Specifically, instead of monitoring the coherence between successive pulses, we propose to monitor quantum correlations through the violation of Bell inequalities. This approach enables the detection of a broader class of potential attacks. Our simulation results indicate that, by employing this method, the maximum secure distance of the protocol can be extended to approximately 259km.