  The ability to perform high-precision one- and two-qubit operations is
sufficient for universal quantum computation. For the Loss-DiVincenzo proposal
to use single electron spins confned to quantum dots as qubits, it is therefore
sufficient to analyze only single- and coupled double-dot structures, since the
strong Heisenberg exchange coupling between spins in this proposal falls off
exponentially with distance and long-ranged dipolar coupling mechanisms can be
made significantly weaker. This scalability of the Loss-DiVincenzo design is
both a practical necessity for eventual applications of multi-qubit quantum
computing and a great conceptual advantage, making analysis of the relevant
components relatively transparent and systematic. We review the Loss-DiVincenzo
proposal for quantum-dot-confned electron spin qubits, and survey the current
state of experiment and theory regarding the relevant single- and double-
quantum dots, with a brief look at some related alternative schemes for quantum
computing.
