\documentclass[convert={density=600,outext=.png}]{standalone}

\usepackage{tikz}

\newcommand{\ket}[1]{|#1\rangle}
\newcommand{\prhalf}{
    \small \textit{ pr=$ 1 \over 2 $ }
}
\newcommand{\prhalfhalf}{
    \small \textit{ pr=$ {1 \over 2} \cdot {1 \over 2} = \color{blue}{1 \over 4} $ }
}

\begin{document}
\begin{tikzpicture}

\draw[->] (6,10) node[above] {$\ket{0}$} 
    --(6,9) 
    node[midway,right] {\textbf{H}}
    node[below] {$ \frac{1}{\sqrt{2}} \ket{0} + \frac{1}{\sqrt{2}} \ket{1} $}
    ;
    
\draw (6,8.3) -- (6,8) node[below]{\textbf{M}};
\draw(5.95,7.5) -- (5.20,6.5) node[midway,left]{ \prhalf } 
    node[below] {$ \ket{0} $};
\draw(6.05,7.5) -- (6.8,6.5) node[midway,right]{ \color{blue}{\prhalf} }
    node[below] {$ \ket{1} $};
    
\draw[->] (5.2,5.9) --(5.2,4.9) 
    node[midway,right] {\textbf{H}}
    node[below] {$ \frac{1}{\sqrt{2}} \ket{0} + \frac{1}{\sqrt{2}} \ket{1} $}
    ;
    
\draw (5.20,4.2) -- (5.20,3.9) node[below]{\textbf{M}};
\draw(5.15,3.4) -- (4.40,2.4) node[midway,left]{ \prhalfhalf } 
    node[below] {$ \ket{0} $};
\draw(5.25,3.4) -- (6,2.4) node[midway,right]{ \prhalfhalf } 
    node[below] {$ \ket{1} $};    

\end{tikzpicture}
\end{document}