Analysis and defense against joint-measurement attacks to quantum private query protocols for multi-rounds

Quantum private query (QPQ) protocols based on quantum key distribution (QKD) have gained significant attention due to their practical implementation advantages. However, joint-measurement attacks pose a serious threat to their security, especially in real-world multi-query scenarios. Most existing QKD-based QPQ protocols become highly vulnerable when users make repeated database queries. Attackers can exploit strategies like minimum error discrimination (MED) and unambiguous state discrimination (USD) to completely break database security. This work thoroughly analyzes joint-measurement attacks in multi-round QPQ systems. We demonstrate that these attacks make current protocols practically unusable. To address this critical issue, we propose an effective defense method using classical post-processing. Our solution not only reveals fundamental flaws in existing approaches but also provides a reliable way to build secure QPQ systems. These findings enable the development of robust protocols that can withstand real-world usage patterns, moving QPQ technology from theory to practical application.