Question about quantum of excitations in QED

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About photons, in qed, I have not understood if there is difference between "the excitation modes of the EM field" or "the energy difference between these excitation modes". Sorry if I make a mess with these concepts.

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[UltrafastPED]

There may be many excitation modes; the different modes represent different energies.

 

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There may be many excitation modes; the different modes represent different energies.

So every photon is a single excitation mode or what?

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[UltrafastPED]

Photons are the quantization of the modes of the electromagnetic field.

 

[dextercioby]

The quantized modes of the free e-m field (in vacuum) are 1-1 related to the eigenstates of the number operator.

 

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Thanks to both for the answeres.

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