Exact quantum capacity of decohering channels in arbitrary dimensions

We derive exact analytical expressions for the quantum capacity of a broad subclass of generalized dephasing channels of the form $\mathrm{\ensuremath{\Lambda}}(\ensuremath{\rho})=(1\ensuremath{-}x)\ensuremath{\rho}+xD(\ensuremath{\rho})$, where $D(\ensuremath{\rho})$ represents a structured decoherence process. These channels are degradable for all noise parameters and in arbitrary dimensions, yielding closed-form single-letter capacity formulas. Our analysis includes fully decohering, block-decohering, and weakly decohering channels, the latter involving coherence preservation within overlapping subspaces. Surprisingly, even under maximal decoherence, the channel may retain nonzero capacity due to residual coherence structure. These results reveal a quantitative role for decoherence-free and partially coherent subspaces in preserving quantum information, offering guidance for encoding strategies in quantum memories and fault-tolerant quantum communication systems.