  One-dimensional quantum optical models usually rest on the intuition of large
scale separation or frozen dynamics associated with the different spatial
dimensions, for example when studying quasi one-dimensional atomic dynamics,
potentially resulting in the violation of $3+1D$ Maxwell's theory. Here, we
provide a rigorous foundation for this approximation by means of the
light-matter interaction. We show how the quantized electromagnetic field can
be decomposed $-$ exactly $-$ into an infinite number of subfields living on a
lower dimensional subspace and containing the entirety of the spectrum when
studying axially symmetric setups, such as with an optical fiber, a laser beam
or a waveguide. The dimensional reduction approximation then corresponds to a
truncation in the number of such subfields that in turn, when considering the
interaction with for instance an atom, corresponds to a modification to the
atomic spatial profile. We explore under what conditions the standard approach
is justified and when corrections are necessary in order to account for the
dynamics due to the neglected spatial dimensions. In particular we will examine
what role vacuum fluctuations and structured laser modes play in the validity
of the approximation.
