\begin{array}{ccccc}
\multirow{4}{*}{$\begin{array}{c} \uparrow \\ U_{1} \\ \downarrow \end{array}$} & \multirow{4}{*}{$\begin{array}{c} \uparrow \\ U_{2} \\ \downarrow \end{array}$} & \multirow{4}{*}{$\begin{array}{c} \uparrow \\ \cdots \\ \downarrow \end{array}$} & \multirow{4}{*}{$\begin{array}{c} \uparrow \\ U_{m} \\ \downarrow \end{array}$} & \begin{array}{c} \Qcircuit@C=1em @R=.7em{
& & \qout{b~\mbox{part}~1~\mbox{on}~\{1~\ldotp \ldotp~k\}}
} \end{array} \\
& & & & \begin{array}{c} \Qcircuit@C=1em @R=.7em{
& & \qout{b~\mbox{part}~2~\mbox{on}~\{1~\ldotp \ldotp~k\}}
} \end{array} \\
& & & & \vdots \\
& & & & \begin{array}{c} \Qcircuit@C=1em @R=.7em{
& & \qout{b~\mbox{part}~k~\mbox{on}~\{1~\ldotp \ldotp~k\}}
} \end{array} \\
\end{array}