= Pauli correlation tensor
{c}
{title2=$T_{ij}=\operatorname{Tr}(\rho\,\sigma_i\otimes\sigma_j)$}
For a two-qubit <density operator>, the real three-by-three <matrix> $T$ encodes the spin-product <expectation values>: unit measurement axes $\mathbf a,\mathbf b$ have $E(\mathbf a,\mathbf b)=\mathbf a^TT\mathbf b$. The tensor does not include local Bloch-vector data and alone need not specify the complete <density operator>. The <CHSH optimum from the correlation tensor> depends on the two largest <eigenvalues> of $T^TT$.
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