\documentclass[12pt]{article}
\usepackage{simplewick}

\begin{document}

\begin{equation}
\contraction{}{a}{{}^{\dagger}_i(t_1)a^{\dagger}_j(t_1)}{a}
%
\contraction[2ex]{a^{\dagger}_i(t_1)}{a}{{}^{\dagger}_j(t_1)a_j(t_1)a_i(t_1)
a^{\dagger}_i(t_2)a^{\dagger}_j(t_2)a_j(t_2)}{a}
%
\contraction[3ex]{a^{\dagger}_i(t_1)a^{\dagger}_j(t_1)a_j(t_1)}{a}{{}_i(t_1)
a^{\dagger}_i(t_2)}{a}
%
\contraction[4ex]{a^{\dagger}_i(t_1)a^{\dagger}_j(t_1)a_j(t_1)a_i(t_1)}
{a}{{}^{\dagger}_i(t_2)a^{\dagger}_j(t_2)}{a}
%
a^{\dagger}_i(t_1)a^{\dagger}_j(t_1)a_j(t_1)a_i(t_1)
a^{\dagger}_i(t_2)a^{\dagger}_j(t_2)a_j(t_2)a_i(t_2)
\end{equation}

\end{document}