1. Institute for Quantum Engineering, Science and Technology (iQUEST) and Department of Physics, University of California, Santa Barbara, California 93106, USA
2. Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow, G12 8QQ, UK
3. Present address: Toshiba Research Europe Limited, Cambridge Research Laboratory, 260 Cambridge Science Park, Milton Road, Cambridge, CB4 0WE, UK.

Correspondence to: Correspondence and requests for materials should be addressed to M.S.S. (e-mail: Email: sherwin@physics.ucsb.edu).

Quantum bits^{1, 2, 3, 4, 5} (qubits) are the fundamental building blocks of quantum information processors, such as quantum computers⁶. A qubit comprises a pair of well characterized quantum states that can in principle be manipulated quickly compared to the time it takes them to decohere by coupling to their environment⁷. Much remains to be understood about the manipulation and decoherence of semiconductor qubits. Here we show that hydrogen-atom-like motional states of electrons bound to donor impurities in currently available semiconductors can serve as model qubits. We use intense pulses⁸ of terahertz radiation to induce coherent, damped Rabi oscillations^{9, 10} in the population of two low-lying states of donor impurities in GaAs^{11, 12, 13}. Our observations demonstrate that a quantum-confined extrinsic electron in a semiconductor can be coherently manipulated like an atomic electron, even while sharing space with 10⁵ atoms in its semiconductor host. We anticipate that this model system will be useful for measuring intrinsic decoherence processes, and for testing both simple and complex manipulations of semiconductor qubits.