Four Atomic Optical Energy Levels as a Two Qubit Quantum Computer   Register
  It is proposed to use four atomic optical energy levels as a two qubit quantum register. A single Pr3+ atom in a monocrystal LaF3 subjected to resonant laser irradiation is used as an example to illustrate the implementation of the universal set of quantum gates. The equilibrium state of this physical system is a desirable input state for quantum computation and therefore there is no need for its special preparation procedure. 
Vladimir L. Ermakov, Alexander R. Kessel and Vitaly V. Samartsev