  Controlling coherent interaction at avoided crossings is at the heart of
quantum information processing. The regime between sudden switches and
adiabatic transitions is characterized by quantum superpositions that enable
interference experiments. Here, we implement periodic passages at intermediate
speed in a GaAs-based two-electron charge qubit and observe
Landau-Zener-St\"uckelberg-Majorana (LZSM) quantum interference of the
resulting superposition state. We demonstrate that LZSM interferometry is a
viable and very general tool to not only study qubit properties but beyond to
decipher decoherence caused by complex environmental influences. Our scheme is
based on straightforward steady state experiments. The coherence time of our
two-electron charge qubit is limited by electron-phonon interaction. It is much
longer than previously reported for similar structures.
