I'm trying to get a definitive and clear answer to the question of what a photon actually is. Textbooks seem to give quite vague explanations, all starting with Einstein's idea that a quanta is a form of light which has a frequency associated with it, finite energy, and is localised in space. The frequency is said to be that of electromagnetic waves. Does it only have a single frequency, or can it have a group? Is the frequency that of the electric field, or the magnetic field, or both, or is it not possible to say? Is it always localised in space, or does it actually disperse, or is either option valid? Can we write down a wavefunction for a photon, or give a set of conditions that wavefunctions must satisfy in order to be considered a photon?
Photons seem to be one of the foundation ideas of quantum mechanics, so I am concerned that without a clear definition or set of concrete examples, the basis for understanding quantum experiments is a little fuzzy.
Remember I'm looking for a clear definition! On the other hand if you think there is no clear definition, I would also like to know.
EDIT: Although this question is assumed to be the same as asking "what is a quantum of light?", according to Lamb's Anti-Photon paper:
"G.N.Lewis, in 1926, coined the word 'photon' to describe something completely different from the Einstein light quantum."
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