\documentclass[12pt]{article}
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\begin{document}
\begin{equation*}
\sigma_+ = {1\over 3}(|0\rangle |1\rangle\langle 0|\langle 1| + |1\rangle |2\rangle\langle 1|\langle 2| + |2\rangle |0\rangle \langle 2|\langle 0|)
\end{equation*}
\end{document}