  We investigate the possibility to achieve scalable photonic quantum computing
by the giant optical circular birefringence induced by a quantum-dot spin in a
double-sided optical microcavity as a result of cavity quantum electrodynamics.
We construct a deterministic controlled-not gate on two photonic qubits by two
single-photon input-output processes and the readout on an electron-medium spin
confined in an optical resonant microcavity. This idea could be applied to
multi-qubit gates on photonic qubits and we give the quantum circuit for a
three-photon Toffoli gate. High fidelities and high efficiencies could be
achieved when the side leakage to the cavity loss rate is low. It is worth
pointing out that our devices work in both the strong and the weak coupling
regimes.
