  The quantum capacity of degradable quantum channels has been proven to be
additive. On the other hand, there is no general rule for the behavior of
quantum capacity for non-degradable quantum channels. We introduce the set of
partially degradable (PD) quantum channels to answer the question of additivity
of quantum capacity for a well-separable subset of non-degradable channels. A
quantum channel is partially degradable if the channel output can be used to
simulate the degraded environment state. PD channels could exist both in the
degradable, non-degradable and conjugate degradable family. We define the term
partial simulation, which is a clear benefit that arises from the structure of
the complementary channel of a PD channel. We prove that the quantum capacity
of an arbitrary dimensional PD channel is additive. We also demonstrate that
better quantum data rates can be achieved over a PD channel in comparison to
standard (non-PD) channels. Our results indicate that the partial degradability
property can be exploited and yet still hold many benefits for quantum
communications.
