We reconsider the problem of the spontaneous emission of light by an excited atomic state. We scrutinize the survival probability of this excited state for very short times, in the so-called Zeno regime, for which we show that the dynamics is dictated by a coherent --in phase -- response of the on-shell and off-shell vacuum modes. We also develop a perturbative approach in order to interpolate between different temporal regimes: the Zeno, golden rule (linear) and Wigner-Weisskopf (exponential) regimes. We compare results obtained with the $\hat{ \mathbf{ E}}\cdot\hat{ \mathbf{ x}}$ and $\hat{ \mathbf{ A}}\cdot\hat{ \mathbf{ p}}$ interaction Hamiltonian,s, using successively the dipole approximation and the exact coupling.
Categories: quant-ph