  Quantum optics is a field of research based on the quantum theory of light.
Here, we show that the classical theory of light can be equally effective in
explaining a cornerstone of quantum optics: the quantization of the free
radiation field. The quantization lies at the heart of quantum optics and has
never been obtained classically. Instead, we find it by taking into account the
degeneracy of the spherical harmonics that appear in multipole terms of the
ordinary Maxwell theory of the free electromagnetic field. In this context, the
number of energy quanta is determined by a finite countable set of spherical
harmonics of higher order than the fundamental (monopole). This one plays,
instead, the role of the electromagnetic vacuum that, contrary to the common
view, has its place in the classical theory of light.
