  The information carrying capacity of the d-dimensional depolarizing channel
is computed. It is shown that this capacity can be achieved by encoding
messages as products of pure states belonging to an orthonormal basis of the
state space, and using measurements which are products of projections onto this
same orthonormal basis. In other words, neither entangled signal states nor
entangled measurements give any advantage for information capacity. The result
follows from an additivity theorem for the capacity of the product of the
depolarizing channel with an arbitrary channel. We establish the
Amosov-Holevo-Werner p-norm conjecture for this product channel for all p >= 1,
and deduce from this the additivity of the minimal entropy and of the Holevo
quantity.
