  Exchange-coupled singlet-triplet spin qubits in two gate-defined double
quantum dots are considered theoretically. Using charge density operators to
describe the double-dot orbital states, we calculate the Coulomb couplings
between the qubits, and identify optimal bias points for single- and two-qubit
operations, as well as convenient idle positions. The same intuitive
formulation is used to derive dephasing rates of these qubits due to the
fluctuating charge environment, thereby providing the main considerations for a
quantum computation architecture that is within current experimental
capabilities.
