smc

We use the logarithm property of addition: $$\begin{array}{rl}{\log }_2(a)+{\log }_2(b)\ge 6& ={\log }_2(ab)\ge 6\\ & \Rightarrow 2^{lo{g}_2(ab)}\ge 2^6\\ & =ab\ge 64\end{array}$$ Due to the Quadratic Optimization or the AM-GM Inequality, the least value is obtained when $a=b$. Therefore, $a=b=8\Rightarrow a+b=\boxed{16}$