Quantum signature in classical electrodynamics of the free radiation field

Abstract

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.