  The zero-error classical capacity of a quantum channel is the asymptotic rate
at which it can be used to send classical bits perfectly, so that they can be
decoded with zero probability of error. We show that there exist pairs of
quantum channels, neither of which individually have any zero-error capacity
whatsoever (even if arbitrarily many uses of the channels are available), but
such that access to even a single copy of both channels allows classical
information to be sent perfectly reliably. In other words, we prove that the
zero-error classical capacity can be superactivated. This result is the first
example of superactivation of a classical capacity of a quantum channel.
