  The performance of a quantum computation system is investigated, with qubits
represented by magnetic impurities in coupled quantum dots filled with two
electrons. Magnetic impurities are electrically manipulated by electrons. The
dominant noise source is the electron mediated indirect coupling between
magnetic impurities and host spin bath. As a result of the electron mediated
coupling, both noise properties and the time needed for elementary gate
operations, depend on controllable system parameters, such as size and geometry
of the quantum dot, and external electric and magnetic fields. We find that the
maximum number of quantum operations per coherence time for magnetic impurities
increases as electron spin singlet triplet energy gap decreases. The advantage
of magnetic impurities over electrons for weak coupling and large magnetic
fields will be illustrated.
