Vector figure in LaTeX/Tikz

vector.tex

\begin{tikzpicture} [nodes in empty cells,
      nodes={minimum width=0.5cm, minimum height=0.5cm},
      row sep=-\pgflinewidth, column sep=-\pgflinewidth]
      border/.style={draw}
    
      \matrix(vector)[matrix of nodes,
          row 1/.style={nodes={draw=none, minimum width=0.3cm}},
          nodes={draw}]
      {
          \tiny{0} & \tiny{1} & \tiny{2} & \tiny{3}\\
          $a_{0}$ & $a_{1}$ & $a_{2}$ & $a_{3}$\\
      };
\end{tikzpicture}