Box with incoming and outgoing arrows
If you only change the last four lines so that the arrows start relative to a.west
and a.east
, you can achieve want you want.
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning}
\begin{document}
\pagestyle{empty}
%
\tikzstyle{int}=[draw, fill=blue!20, minimum size=7em]
\tikzstyle{init} = [pin edge={to-,thin,black}]
\begin{tikzpicture}[node distance=2.5cm,auto,>=latex']
\node [int,align=center] (a) {$U$\\evolution operator};
\draw[<-] ([yshift=5pt]a.west) -- node[above]{$a1$} ++(-4em,0em);
\draw[<-] ([yshift=-5pt]a.west) -- node[below]{$a2$} ++(-4em,0em);
\draw[->] ([yshift=5pt]a.east) -- node[above]{$b1$} ++(4em,0em);
\draw[->] ([yshift=-5pt]a.east) -- node[below]{$b2$} ++(4em,0em);
\end{tikzpicture}
\end{document}
This solved the problem, after working around I got the solution. Thanks to you all people
\documentclass[border=2mm]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\node [draw,minimum height=3cm] (a) {$U$ \vspace{1cm} evolution operator};
\path (a.south east) -- (a.north east)
coordinate [pos=0.2] (p1)
coordinate [pos=0.5] (p2)
coordinate [pos=0.8] (p3);
\path (a.south west) -- (a.north west)
coordinate [pos=0.2] (p4)
coordinate [pos=0.5] (p5)
coordinate [pos=0.8] (p6);
\draw [->] (p1) -- +(1cm,0) node[right]{$b_1$};
\draw [->] (p2) -- +(1cm,0) node[right]{$b_2$};
\draw [->] (p3) -- +(1cm,0) node[right]{$b_3$};
\draw [<-] (p4) -- +(-1cm,0) node[left]{$a_1$};
\draw [<-] (p5) -- +(-1cm,0) node[left]{$a_2$};
\draw [<-] (p6) -- +(-1cm,0) node[left]{$a_3$};
\end{tikzpicture}
\end{document}