\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  Q_v = \begin{bmatrix}\rho & 0 \\ 0 & \rho\end{bmatrix}.
\end{equation*}
\end{document}