  We investigate minimum output (R\'enyi) entropy of qubit channels and unital
quantum channels. We obtain an exact formula for the minimum output entropy of
qubit channels, and bounds for unital quantum channels. Interestingly, our
bounds depend only on the operator norm of the matrix representation of the
channels on the space of trace-less Hermitian operators. Moreover, since these
bounds respect tensor products, we get bounds for the capacity of unital
quantum channels, which is saturated by the Werner-Holevo channel. Furthermore,
we construct an orthonormal basis, besides the Gell-Mann basis, for the space
of trace-less Hermitian operators by using discrete Weyl operators. We apply
our bounds to discrete Weyl covariant channels with this basis, and find new
examples in which the minimum output R\'enyi $2$-entropy is additive.
