\begin{tikzpicture}

% draw axis
\draw[thick, ->] (-3cm, 0cm) -- (3cm, 0cm) node[anchor=south] {$\uptau$};
\draw[thick, ->] (-0.75cm, -1cm) -- (-0.75cm, 3cm) node[anchor=east] {$\upsigma$};

% draw defects
\filldraw[fill=white, draw=black] (-2cm, 0cm) circle (2pt) node[anchor=north] {$\hat{\uptau}_{(t+1)}$};
\filldraw[fill=white, draw=black] (0.25cm, 0cm) circle (2pt) node[anchor=north] {$\hat{\uptau}_{(t)}$};
\filldraw[fill=white, draw=black] (2.25cm, 0cm) circle (2pt) node[anchor=north] {$\hat{\uptau}_{(t-1)}$};

% draw the endlines
\draw[dotted] (-2cm, 2pt) -- (-2cm, 2cm);
\draw[dotted] (0.25cm, 2pt) -- (0.25cm, 2cm);
\draw[dotted] (2.25cm, 2pt) -- (2.25cm, 2cm);

% draw the second D-brane
\draw[dashed] (-3cm, 2cm) -- (3cm, 2cm);
\node[anchor=south west] at (-0.75cm, 2cm) {$\uppi$};

% draw the string
\draw[decorate, decoration={snake, segment length=0.75cm}] (1cm, 0cm) -- (1cm, 2cm);
\draw[->] (1.1cm, 1cm) -- (1.6cm, 1cm);

\end{tikzpicture}

% vim: ft=tex
