  Graphene nanoribbon quantum dot qubits have been proposed as promising
candidates for quantum computing applications to overcome the spin-decoherence
problems associated with typical semiconductor (e.g. GaAs) quantum dot qubits.
We perform theoretical studies of the electronic structures of graphene
nanoribbon quantum dots by solving the Dirac equation with appropriate boundary
conditions. We then evaluate the exchange splitting based on an unrestricted
Hartree-Fock method for the Dirac particles. The electronic wave function and
long-range exchange coupling due to the Klein tunneling and the Coulomb
interaction are calculated for various gate configurations. It is found that
the exchange coupling between qubits can be significantly enhanced by the Klein
tunneling effect. The implications of our results for practical qubit
construction and operation are discussed.
