  We show that a scalable photonic crystal nanocavity array, in which single
embedded quantum dots are coherently interacting, can perform as an universal
single-operation quantum gate. In a passive system, the optical analogue of
electromagnetically-induced-transparency is observed. The presence of a single
two-level system in the array dramatically controls the spectral lineshapes.
When each cavity couples with a two-level system, our scheme achieves two-qubit
gate operations with high fidelity and low photon loss, even in the bad cavity
limit and with non-ideal detuning and decoherence.
