Symmetry-Enforced Quadratic Approximate-Degradability Bounds for Noisy Landau-Streater Channels
The study investigates why certain symmetric quantum channels exhibit enhanced approximate degradability, even when exact degradability fails. It shows that this enhancement stems from symmetry-induced orthogonality between signal and noise components, leading to a quadratic suppression of non-degradability in all system dimensions. This mechanism is universal across different channel models.