  Well controlled nonlinear interactions between light field pulses and single
atoms could be used to implement optical quantum information technologies based
on qubits encoded in superpositions of coherent states of light. Here, we
investigate the transformation of a coherent light field input at a single atom
sufficiently far from resonance to limit the decoherence effects associated
with random excitations of the atom. The conditions for suppressing
multi-photon scattering to implement arbitrarily large shifts of the coherent
light amplitude without decoherence are studied. It is shown that quantum
controlled coherent shifts can be achieved by sufficiently long coherent
pulses, indicating the possibility of generating superpositions of coherent
states with large amplitude differences. The dominant multi-photon scattering
effect associated with four wave mixing is also identified, and the spectral
and temporal characteristics of the entangled photon pairs generated by this
process are discussed.

Quantum Physics