  We review the progress and main challenges in implementing large-scale
quantum computing by optical control of electron spins in quantum dots (QDs).
Relevant systems include self-assembled QDs of III-V or II-VI compound
semiconductors (such as InGaAs and CdSe), monolayer fluctuation QDs in compound
semiconductor quantum wells, and impurity centers in solids such as P-donors in
silicon and nitrogen-vacancy centers in diamond. The decoherence of the
electron spin qubits is discussed and various schemes for countering the
decoherence problem are reviewed. We put forward designs of local nodes
consisting of a few qubits which can be individually addressed and controlled.
Remotely separated local nodes are connected by photonic structures
(microcavities and waveguides) to form a large-scale distributed quantum system
or a quantum network. The operation of the quantum network consists of optical
control of a single electron spin, coupling of two spins in a local nodes,
optically controlled quantum interfacing between stationary spin qubits in QDs
and flying photon qubits in waveguides, rapid initialization of spin qubits,
and qubit-specific single-shot non-demolition quantum measurement. The rapid
qubit initialization may be realized by selectively enhancing certain entropy
dumping channels via phonon or photon baths. The single-shot quantum
measurement may be in-situ implemented through the integrated photonic network.
The relevance of quantum non-demolition measurement to large-scale quantum
computation is discussed. To illustrate the feasibility and demand, the
resources are estimated for the benchmark problem of factorizing 15 with Shor's
algorithm.
