We study finite-time Landau-Zener transitions at a singlet-triplet level crossing in a GaAs double quantum dot, both experimentally and theoretically. Sweeps across the anticrossing in the high driving speed limit result in oscillations with a small
visibility. Here we demonstrate how to increase the oscillation visibility while keeping sweep times shorter than T2* using a tailored pulse with a detuning dependent level velocity. Our results show an improvement of a factor ~2.9 for the oscillation visibility. In particular, we were able to obtain a visibility of ~0.5 for Stuckelberg oscillations, which demonstrates the creation of an equally weighted superposition of the qubit states.
