Quantum-information processing in decoherence-free subspace with low-Q cavities

We consider the input-output process with low-Q cavities confining atoms for quantum-information processing, based on a general expression for reflection coefficient of the single photon. Focusing on the qubit encoding in decoherence-free subspace (DFS), we propose entanglement generation, universal quantum computing, and multiqubit controlled phase flip, using individual photon input and output from the cavities. Because of the DFS and no requirement for high-Q cavities, our schemes are not only immune to dephasing but also feasible with currently available technology.