<gh_stars>1-10
\begin{equation}
\begin{aligned}
\vec{\sigma} = \left[\begin{matrix}
    \sigma_{1} & 0 & 0 \\ 0 & \sigma_{1} & 0 \\ 0 & 0 & \sigma_{1}
\end{matrix}\right]
\qquad
\vec{\sigma} = \left[\begin{matrix}
    \sigma_{1} & 0 & 0 \\ 0 & \sigma_{2} & 0 \\ 0 & 0 & \sigma_{3}
\end{matrix}\right]
\qquad
\vec{\sigma} = \left[\begin{matrix}
    \sigma_{1} & \sigma_{4} & \sigma_{5} \\ \sigma_{4} & \sigma_{2} & \sigma_{6} \\ \sigma_{5} & \sigma_{6} & \sigma_{3}
\end{matrix}\right]
\end{aligned}
\end{equation}
