  We compare the predictions of the fundamentally motivated minimal coupling
($\hat{\boldsymbol{p}}\cdot\hat{\boldsymbol{A}}$) and the ubiquitous dipole
coupling ($\hat{\boldsymbol{x}}\cdot\hat{\boldsymbol{E}}$) in the light-matter
interaction. By studying the light-matter interaction for hydrogen-like atoms
we find that the dipole approximation cannot be a-priori justified to analyze
the physics of vacuum excitations (a very important phenomenon in relativistic
quantum information) since a dominant wavelength is absent in those problems,
no matter how small (as compared to any frequency scale) the atom is.
Remarkably, we show that the dipole approximation in those regimes can still be
valid as long as the interaction time is longer than the light-crossing time of
the atoms, which is a very reasonable assumption. We also highlight some of the
subtleties that one has to be careful with when working with the explicitly
non-gauge invariant nature of the minimal coupling and we compare it with the
explicitly gauge invariant dipole coupling.
