  We introduce a new quantum decoder based on a variant of the pretty good
measurement, but defined via an alternative matrix quotient. We use this
decoder to show new lower bounds on the error exponent both in the one-shot and
asymptotic regimes for the classical-quantum and the entanglement-assisted
channel coding problem. Our bounds are expressed in terms of measured (for the
one-shot bounds) and sandwiched (for the asymptotic bounds) channel R\'enyi
mutual information of order between 1/2 and 1. Our results are not comparable
with some previously established bounds for general instances, yet they are
tight (for rates close to capacity) when the underlying channel is classical.
