A two-qubit quantum gate is realized using electronic excited states in a single ion with an energy separation on the order of a terahertz times the Planck constant as a qubit. Two phase locked lasers are used to excite a stimulated Raman transition between two metastable states D3/2 and D5/2 separated by 1.82 THz in a single trapped 40Ca+ ion to construct a qubit, which is used as the target bit for the Cirac-Zoller two-qubit controlled NOT gate. Quantum dynamics conditioned on a motional qubit is clearly observed as a fringe reversal in Ramsey interferometry.
@article{arxiv.1001.4600,
title = {Quantum gate using qubit states separated by terahertz},
author = {K. Toyoda and S. Haze and R. Yamazaki and S. Urabe},
journal= {arXiv preprint arXiv:1001.4600},
year = {2015}
}