\documentclass{standalone}
\usepackage{tikz}

\begin{document}
\hfil\begin{tikzpicture}

\draw(-1,0)--++(4,0)--++(0,4)--++(-4,0)--cycle;

\draw[line width=4pt](0,0)..controls(0,2)and(1,2)..(1,0);
\draw[line width=4pt](0,4)..controls(0,2)and(1,2)..(1,4);
\draw[line width=4pt](2,0)--++(0,4);

\end{tikzpicture}

\end{document}
