We demonstrate coherent control of two logical qubits encoded in a decoherence free subspace (DFS) of four dipolar-coupled protons in an NMR quantum information processor. A pseudo-pure fiducial state is created in the DFS, and a unitary logical qubit entangling operator evolves the system to a logical Bell state. The four-spin molecule is partially aligned by a liquid crystal solvent, which introduces strong dipolar couplings among the spins. Although the system Hamiltonian is never fully specified, we demonstrate high fidelity control over the logical degrees of freedom. In fact, the DFS encoding leads to higher fidelity control than is available in the full four-spin Hilbert space.
@article{arxiv.0705.4119,
title = {Fidelity enhancement by logical qubit encoding},
author = {Michael K. Henry and Chandrasekhar Ramanathan and Jonathan S. Hodges and Colm A. Ryan and Michael J. Ditty and Raymond Laflamme and David G. Cory},
journal= {arXiv preprint arXiv:0705.4119},
year = {2009}
}