Two-level atom-field interaction: Exact master equations for non-Markoviandynamics, decoherence, and relaxation - art. no. 033821

We perform a first-principles derivation of the general master equation to study the non-Markovian dynamics of a two-level atom (2LA) interacting with an electromagnetic field (EMF). We use the influence functional method, wh ich can incorporate the full back reaction of the field on the atom, while adopting Grassmannian variables for the 2LA and the coherent-state represen tation for the EMF. We find exact master equations for the cases of a free quantum field and a cavity field in the vacuum. In response to the search f or mechanisms to preserve maximal coherence in quantum computations in ion trap prototypes, we apply these equations to analyze the decoherence of a 2 LA in an EMF, and find that decoherence time is close to relaxation time. T his is at variance with the claims by authors who studied the same system b ut used a different coupling model. We explain the source of difference and argue that, contrary to common belief, the EMF, when resonantly coupled to an atom, does not decohere it as efficiently as a bath does on a quantum B rownian particle. The master equations for non-Markovian dynamics derived h ere are expected to be useful for exploring new regimes of 2LAEMF interacti on, which is becoming physically important experimentally.