  Electrons bound to shallow donors in GaAs have orbital energy levels
analogous to those of the hydrogen atom. The polarization selection rules for
optical transitions between the states analogous to the 1s and 2p states of
hydrogen in a magnetic field are verified using Terahertz (THz) radiation from
the UCSB Free Electron Laser. A polarization-selective coherent manipulation of
the quantum states is demonstrated and the relevance to quantum information
processing schemes is discussed.
