Formula_9961

\[\sigma_{1}=\left\|\begin{array}{cc}0 & 1 \\1 & 0\end{array}\right\|, \quad \sigma_{2}=\left\|\begin{array}{cc}0 & -i \\i & 0\end{array}\right\|, \quad \sigma_{3}=\left\|\begin{array}{cc}1 & 0 \\0 & -1\end{array}\right\| .\]