  We investigate linear maps between matrix algebras that remain positive under
tensor powers, i.e., under tensoring with $n$ copies of themselves. Completely
positive and completely co-positive maps are trivial examples of this kind. We
show that for every $n\in\mathbb{N}$ there exist non-trivial maps with this
property and that for two-dimensional Hilbert spaces there is no non-trivial
map for which this holds for all $n$. For higher dimensions we reduce the
existence question of such non-trivial "tensor-stable positive maps" to a
one-parameter family of maps and show that an affirmative answer would imply
the existence of NPPT bound entanglement.
  As an application we show that any tensor-stable positive map that is not
completely positive yields an upper bound on the quantum channel capacity,
which for the transposition map gives the well-known cb-norm bound. We
furthermore show that the latter is an upper bound even for the LOCC-assisted
quantum capacity, and that moreover it is a strong converse rate for this task.
