[Tex/LaTex] Positioning a node relative to the center of another node in tikz

Question

How can I position the node 's2' above 's1' so that leftmost 0 (of node 's2') is exactly above and at center of 's1'

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}

\begin{frame}
  \begin{tikzpicture}[overlay,remember picture]
    \node[draw,fill=cyan!40,minimum size=1.2cm,rectangle,yshift=.5cm] at (current page.center) 
    (s1) {q};

    \node[draw=none,above=of s1,anchor=south west] (s2) {0 0 0 1 1 1};
    \draw[->] (s1) -- (s2);
  \end{tikzpicture}
\end{frame}
\end{document}

Answer 1

One way is to anchor the node to the south west (as you have done), then shift it to the left by the amount necessary to align the first zero:

\documentclass{beamer}
\usepackage{tikz}
\usepackage{calc}
\usetikzlibrary{positioning}
\begin{document}

\newlength{\zerooffset}
\begin{frame}
  \begin{tikzpicture}[overlay,remember picture]
    \node[draw,fill=cyan!40,minimum size=1.2cm,rectangle,yshift=.5cm] at (current page.center) 
    (s1) {q};
    \setlength{\zerooffset}{\widthof{0}}%<=== get the width of the 0
    \node[draw=none,above= of s1,anchor=south west,xshift={\dimexpr-0.5\zerooffset-\pgfkeysvalueof{/pgf/inner xsep}}] (s2) {0 0 0 1 1 1};
    \draw[->] (s1) -- (s2);
  \end{tikzpicture}
\end{frame}
\end{document}

And if you want the arrow to be vertical:

\documentclass{beamer}

\usepackage{tikz}
\usepackage{calc}
\usetikzlibrary{positioning}
\usetikzlibrary{calc}
\begin{document}

\newlength{\zerooffset}
\begin{frame}
  \begin{tikzpicture}[overlay,remember picture]
    \node[draw,fill=cyan!40,minimum size=1.2cm,rectangle,yshift=.5cm] at (current page.center) 
    (s1) {q};
    \setlength{\zerooffset}{\widthof{0}}%<=== get the width of the 0
    \node[draw=none,above= of s1,anchor=south west,xshift={\dimexpr-0.5\zerooffset-\pgfkeysvalueof{/pgf/inner xsep}}] (s2) {0 0 0 1 1 1};
    \draw[->] let \p1=(s1.north),\p2=(s2.south) in (\x1,\y1)--(\x1,\y2);%<=== vertical arrow
  \end{tikzpicture}
\end{frame}
\end{document}