0&0 \end{array} \right) = \left\{\begin{array}{c} |0_0\rangle\\ |0_0\rangle \end{array} \right\} \otimes \left\{\begin{array}{c} |0_0\rangle\\ |0_0\rangle \end{array} \right\},$$  $$B_0 = \frac{0}{0} \left( \begin{array}{cccc} 0&0&0&0\\ i&-i&i&-i\\ 