Demonstration of conditional quantum phase shift between ions in a solid
  Due to their potential for long coherence times, dopant ions have long been considered promising candidates for scalable solid state quantum computing. However, the demonstration of two qubit operation has proven to be problematic, largely due to the difficulty of addressing closely spaced ions. Here we use optically active ions and optical frequency addressing to demonstrate a conditional phase shift between two qubits. 
J. J. Longdell, M. J. Sellars, N. B. Manson