Single three-level atoms for quantum information

A single three-level atom in interaction with a single longitudinal mode of a high-Q cavity is used to quantum engineer the intracavity field. in our proposal the quantum bit states correspond to the vacuum and single photon Fock states of each of two circular polarization states of the longitudinal mode. We show that for particular velocities of the three-level atom crossing the cavity it is possible to implement in one single step a two-qubit quantum phase gate between the two circular polarization states. Fidelity against several decoherence mechanisms such as atomic velocity fluctuations or the presence of a weak magnetic field along the cavity axis is analyzed.