Abstract
We demonstrate sub-Planck structure experimentally in spectrograms via frequency resolved optical gating (FROG) measurement of ultrashort pulses. Through the correspondence between classical time-frequency transform limit and quantum uncertainty in phase space, superposition of quantum states is realized with four ultrashort pulses for the first time. Comparisons between theoretical results and experimental measurements show the variations in these sub-Planck structures, due to different time delays and imperfect Gaussian profiles.