Entropy of the Quantum-Classical Interface: A Potential Metric for Security.

Hybrid quantum-classical systems are emerging as key platforms in quantum computing, sensing, and communication technologies, but the quantum-classical interface (QCI)-the boundary enabling these systems-introduces unique and largely unexplored security vulnerabilities. This position paper proposes using entropy-based metrics to monitor and enhance security, specifically at the QCI. We present a theoretical security outline that leverages well-established information-theoretic entropy measures, such as Shannon entropy, von Neumann entropy, and quantum relative entropy, to detect anomalous behaviors and potential breaches at the QCI. By linking entropy fluctuations to scenarios of practical relevance-including quantum key distribution, quantum sensing, and hybrid control systems-we promote the potential value and applicability of entropy-based security monitoring. While explicitly acknowledging practical limitations and theoretical assumptions, we argue that entropy-based metrics provide a complementary approach to existing security methods, inviting further empirical studies and theoretical refinements that can strengthen future quantum technologies.