= Choi's theorem on completely positive maps
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= Choi criterion for complete positivity
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A finite-dimensional <linear map> is <completely positive> if and only if its <Choi matrix> is <positive semidefinite>. One direction applies the map tensored with the identity to a maximally entangled positive operator. For the converse, factor the positive Choi matrix into rank-one terms and reshape their vectors into matrices; the <spectral Kraus decomposition> reconstructs the map as a <Kraus representation>, which is completely positive.
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