Superadditivity of classical communication over quantum channels via random and deterministic permutations
The work proves that classical communication over quantum channels exhibits superadditivity even when using random or deterministic permutations instead of continuous Haar randomness. This nonadditivity persists in finite-dimensional systems, showing a fundamental difference between classical and quantum noise. The paper provides both a theoretical proof linking finite channels to free limits and a concrete numerical bound on the system size required for this effect.