  We investigate how special relativity influences the transmission of
classical information through quantum channels by evaluating the Holevo bound
when the sender and the receiver are in (relativistic) relative motion. By
using the spin degrees of freedom of spin-1/2 fermions to encode the classical
information we show that, for some configurations, the accessible information
in the receiver can be increased when the spin detector moves fast enough. This
is possible by allowing the momentum wave packet of one of the particles to be
wide enough while the momentum wave packets of other particles are kept
relatively narrow. In this way, one can take advantage of the fact that boosts
entangle the spin and momentum degrees of freedom of spin-1/2 fermions to
increase the accessible information in the former. We close the paper with a
discussion of how this relativistic quantum channel cannot in general be
described by completely positive quantum maps.
