\documentclass[11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{physics}
\usepackage{yquant}


\begin{document}

\section{Spacer}

\yquantdefinebox{dots}[inner sep=0pt]{$\vdots$}
\begin{tikzpicture}
	\begin{yquant}[operator/separation=5mm, register/separation=0mm]
		qubit {$\ket{0}_1$} a1;
		qubit {$\ket{0}_2$} a2;
		nobit adots;
		qubit {$\ket{0}_{q\!}$} af;

		nobit sep;

		qubit {$\ket{0}_1$} b1;
		qubit {$\ket{0}_2$} b2;
		nobit bdots;
		qubit {$\ket{0}_{q\!}$} bf;

		h a1;
		h a2;
		dots adots;
		h af;
		dots bdots;
		box {$U_f(a, N)$} (a1, a2, adots, af, sep, b1, b2, bdots, bf);
		measure b1;
		measure b2;
		dots bdots;
		measure bf;
		barrier a1, a2, adots, af;
		box {$\mathrm{QFT}_q$} (a1, a2, adots, af);
		align a1, a2, adots, af, b1, b2, bdots, bf;
		measure a1;
		measure a2;
		dots adots;
		measure af;
	\end{yquant}
\end{tikzpicture}

\end{document}