  We consider a double quantum dot system with two embedded and non-aligned
spin impurities to manipulate the magnitude and polarization of the electron
spin density. The device is attached to semi-infinite one-dimensional leads
which are treated exactly. We provide a real-time description of the electron
spin dynamics when a sequence of ultrafast voltage pulses acts on the device.
The numerical simulations are carried out using a spin generalized and modified
version of a recently proposed algorithm for the time propagation of open
systems [Phys. Rev. B {\bf 72}, 035308 (2005)]. Time-dependent spin
accumulations and spin currents are calculated during the entire operating
regime which includes spin injection and read-out processes. The full knowledge
of the electron dynamics allows us to engineer the transient responses and
improve the device performance. An approximate rate equation for the electron
spin is also derived and used to discuss the numerical results.
