Quantitative condition is necessary in guaranteeing the validity of the adiabatic approximation.

The quantitative condition has been widely used in the practical applications of the adiabatic theorem. However, it had never been proved to be sufficient or necessary before. It was only recently found that the quantitative condition is insufficient, but whether it is necessary remains unresolved. In this Letter, we prove that the quantitative condition is necessary in guaranteeing the validity of the adiabatic approximation.