\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\sigma_- = {1\over 3}(|1\rangle |0\rangle \langle 1|\langle 0| + |2\rangle |1\rangle \langle 2|\langle 1| + |0\rangle |2\rangle \langle 0|\langle 2|)
\end{equation*}
\end{document}