Angeles Vazquez-Castro

Quantum communication is emerging as a foundational element of future secure information systems, with applications ranging from key distribution to direct message transmission. Widely used standards such as the Digital Video Broadcasting – Satellite – Second Generation Extension (DVB-S2X) can be considered for satellite-based quantum communication scenarios, where resource constraints and channel impairments must be carefully addressed. While much of the early work in quantum security focused on asymptotic analyses or relied on models rooted in classical wiretap theory, there is a growing need for frameworks that provide operational security guarantees in finite-length and non-asymptotic regimes. In this work, we address that gap by introducing a composable security metric based on the trace distance, derived from α-order Rényi information. Our model, illustrated in Fig. 1 (left), serves as a general abstraction of quantum communication systems subject to eavesdropping, which includes protocols of the family known as Quantum Direct Secure Communication (QDSC), which aim to transmit confidential messages directly over quantum channels. The proposed framework allows for precise evaluation of secrecy leakage under realistic conditions and offers an alternative to traditional key-based paradigms, thereby contributing to the broader effort of enabling keyless secure and efficient quantum communication. Our key result is a composable bound on the trace distance, which solely depends on an αparameterized mutual information term. Unlike conventional methods based on ε-smooth min-entropy, our approach avoids smoothing altogether while still ensuring composability. This leads to analytically tractable bounds and a clearer understanding of the trade-off between coding rate and secrecy. As a practical application, we apply our bounds to a one-way information flow where BPSK-modulated coherent quantum states carry secret information over lossy bosonic channels, consistent with DVB-S2X satellite links. Our results, illustrated in Fig. 1 (right) provide two-fold insights. First, we demonstrate the usefulness of our bound for practical design of reliable and secret space links. Second, we quantify the reliability-secrecy trade-off by numerically showing that the finitelength physical-layer secrecy can be guaranteed only if coding rates are appropriately adjusted.