A mixture of (1-a):a of the Bell State $\ket{\Phi^+}$ and a fully mixed state is called the Werner state.\cite{Werner:1989} $$ \begin{aligned} \rho & =\frac{1-a}{2}(\ket{00}+\ket{11})(\bra{00}+\bra{11})+\frac{a}{4} I \\ & =\frac{1}{2}\left(\begin{array}{cccc} 1-a / 2 & 0 & 0 & 1-a \\ 0 & a / 2 & 0 & 0 \\ 0 & 0 & a / 2 & 0 \\ 1-a & 0 & 0 & 1-a / 2 \end{array}\right) . \end{aligned} $$