We investigate coherent time evolution of a charge qubit in a semiconductor double quantum dot. A high-speed voltage pulse controls the energy bias and strength of the decoherence of the system. The qubit is effectively isolated from the electrodes in the Coulomb blockade regime, while it is affected by dissipative tunneling processes in the single-electron tunneling regime. We discuss the importance of the controlled decoherence by means of density matrix simulations. (C) 2004 American Vacuum Society.