Qcircuit: How to insert a vertical dashed line in a circuit?

Question

I want to insert vertical dashed lines between parts of the following circuit to make some comments:

\documentclass{amsart}
\usepackage[matrix,frame,arrow]{xy}
\input{Qcircuit}
\vfuzz2pt % Don't report over-full v-boxes if over-edge is small
\begin{document}
\[
    \Qcircuit @C=1em @R=1em {
     \lstick{\ket{x}} & \qw & \ctrl{1} & \qw & \rstick{\ket{x}} \qw \\
     \lstick{\ket{y}} & \gate{H} & \gate{\pi} & \gate{H} & \rstick{\ket{y \oplus x}} \qw
    }
\]
\end{document}

Output:

I have found a similar question (xypic: How to typeset the dashed line in this diagram? ) concerning xy-pic, it is the same idea, it should be the same solution. But it's not working, I don't know why.

Note: Q-circuit is a Qcircuit.tex macro package for drawing quantum circuit diagrams in LaTeX

Answer 1

Here is an example placement. I have used a dotted rather than dashed line for clarity. To get dashes use \ar@{--} instead of \ar@{.}.

\documentclass{article}

\input{Qcircuit}

\begin{document}

\Qcircuit @C=1em @R=1em {
 \lstick{\ket{x}} & \qw \ar@{.}[]+<3.5em,1em>;[d]+<3.5em,-1em>
 & \ctrl{1} & \qw & \rstick{\ket{x}} \qw \\
 \lstick{\ket{y}} & \gate{H} & \gate{\pi} & \gate{H} & \rstick{\ket{y \oplus x}} \qw
}
\end{document}

The command used was:

\ar@{.}[]+<3.5em,1em>;[d]+<3.5em,-1em>

placed at the node where you want the line to start. The [d] points to the node one line down, [dd] would send you two lines down instead, [r] would make the line go to the right. The <3.5em,1em> means the line starts 3.5em to the right and 1em above the starting node. Similarly <3.5em,-1em> puts the end 3.5em to the right and 1em below the end node.