Application of α-order information metrics for secure communication in quantum physical layer design

Quantum physical layer security protocols offer significant advantages, particularly in space communications, but their achievable secrecy rates are often limited to the asymptotic regime. Realistic communication systems, however, demand non-asymptotic solutions. Recently, the α-order information-theoretic metrics based on Rényi entropy has been proposed. We study their practical applicability to engineering secure quantum communication systems. By deriving a composable security bound using sandwiched Rényi entropy, we apply our framework to a practical scenario involving Binary Phase Shift Keying modulation over a lossy bosonic channel, consistent with Digital Video Broadcasting—Satellite—Second Generation Extension standards. We highlight a critical tradeoff between reliability and security, by showing that current coding rates may fall short of supporting positive secure coding rates under physical layer security considerations. This emphasizes the need for higher coding rates or additional frame lengths to balance reliability-security tradeoffs effectively. Through this application, we demonstrate the utility of α-order measures for advancing secure communication in quantum physical layer designs.