  The diamond norm is a norm defined over the space of quantum transformations.
This norm has a natural operational interpretation: it measures how well one
can distinguish between two transformations by applying them to a state of
arbitrarily large dimension. This interpretation makes this norm useful in the
study of quantum interactive proof systems.
  In this note we exhibit an efficient algorithm for computing this norm using
convex programming. Independently of us, Watrous recently showed a different
algorithm to compute this norm. An immediate corollary of this algorithm is a
slight simplification of the argument of Kitaev and Watrous [STOC 2000] that
QIP is contained in EXP.
