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.In recent years there has been great interest in low dimensional semiconductors, motivated in large part by the potential to implement quantum information processing 1,2,3 in a solid-state system 4,5,6 . The development of methods for selectively addressing and coherently manipulating such quantum systems is essential to a successful implementation of any quantum information scheme. In this work we describe verification of the selection rules for orbital transitions of electrons bound to shallow donors in GaAs and implementation of selective coherent manipulations.The energy levels of shallow donor-bound electrons can be described by the Coulomb interaction between the electron and the positive charge at the donor site, modified by the effective mass of the electron (0.0665 m e ) and the dielectric constant of the material (12.56) 7,8 . This leads to electron energy levels analogous to those of the hydrogen atom, with a small correction for the actual donor species, called the central cell correction (0.110 meV for S donor) 9 . The orbital transitions are in the TeraHertz (THz) frequency regime (a few meV). In the presence of a magnetic field, additional free electron states arise, forming a continuum above each Landau level 8 . The hydrogenic states can be labelled with the usual notation: 1s, 2p + , 2p − , etc.The energy of the hydrogenic states can be tuned with an applied magnetic field to bring orbital transitions into resonance with fixed frequencies of radiation. Klaassen, et al. 8 and Stillman, et al. 10 , have used this technique to perform spectroscopy experiments. To aid in identifying excited states, they have assumed that transitions are governed by the usual hydrogen atom dipole selection rules. However, these experiments have not included control of the polarization state of the THz beam, and to our knowledge no work has yet verified that the hydrogen selection rules remain valid for hydrogenic donors in a semiconductor.The hydrogen selection rules require a σ + -polarized photon to conserve angular momentum and excite the 1s to 2p + transition; σ − -polarization is required for 1s to 2p − . Since the electron effective mass in GaAs is isotropic, the selection rules for the bare hydrogen atom might be expected to hold. However, symmetry in a semiconductor can easily be lost. In quantum dots, for example, elongation of the dot along specific crystal planes can break the dot symmetry and mix the polarization eigenstates of excitons 11 . In this work, we first verify the selection rules for the 1s to 2p ± transitions ...