Published online by Cambridge University Press: 21 November 2014
For a positive integer $n\geq 2$, let
$M_{n}$ be the set of
$n\times n$ complex matrices and
$H_{n}$ the set of Hermitian matrices in
$M_{n}$. We characterize injective linear maps
${\it\phi}:H_{m_{1}\cdots m_{l}}\rightarrow H_{n}$ satisfying
$$\begin{eqnarray}\text{rank}(A_{1}\otimes \cdots \otimes A_{l})=1\Longrightarrow \text{rank}({\it\phi}(A_{1}\otimes \cdots \otimes A_{l}))=1\end{eqnarray}$$
$A_{k}\in H_{m_{k}}$,
$k=1,\dots ,l$, where
$l,m_{1},\dots ,m_{l}\geq 2$ are positive integers. The necessity of the injectivity assumption is shown. Moreover, the connection of the problem to quantum information science is mentioned.