= Størmer-Woronowicz decomposability theorem
{c}
Every <positive linear map> between $M_2(\mathbb C)$ and $M_2(\mathbb C)$, or between $M_2(\mathbb C)$ and $M_3(\mathbb C)$ in either direction, is a <decomposable positive map>. The low-dimensional result is the ingredient that makes the <positive partial transpose criterion> sufficient in two-qubit and qubit-qutrit systems. https://doi.org/10.1016/0034-4877(76)90038-0[Woronowicz's low-dimensional positive-map paper] establishes the $M_2$ to $M_3$ case; taking adjoints gives the reverse direction.
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