  It was shown [T.S. Cubitt et al., IEEE Trans. Inform. Theory 57, 8114 (2011)]
that there exist quantum channels where a single use cannot transmit classical
information perfectly yet two uses can. This phenomenon is called the
superactivation of the zero-error classical capacity which does not occur in
classical channels. In this paper, it is shown that qubit channels cannot
generate the superactivation.
