\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}\cal{ C}_{ij}:=
\mbox{span}\{ |\alpha\rangle_i \otimes |\beta\rangle_j\,/\, \alpha,\beta =0,1\}
\subset \cal{H}_i\otimes\cal{H}_j\cong {\bf{C}}^9.
\end{equation*}
\end{document}