Quasi-classically, a photon is often considered as a particle with some momentum travelling across the space - for example, when describing experimental setups like Mach-Zender interferometer we often talk as if the photon was actually a particle moving along some possible paths, i.e. we treat it just like any other particle, with a wavefunction giving its position probability amplitude.(adsbygoogle = window.adsbygoogle || []).push({});

Now, in quantum electrodynamics a photon is simply an excitation of the vacuum state, that is, an eigenstate of the total photon number operator [tex]\int a_{k}a_{k}^{\dagger}[/tex]. Physically, how do you connect this Fock representation picture (photon = [tex]\int c(k)a_{k}a_{k}^{\dagger}\vert 0 \rangle dk[/tex]) to the "photon as a wavepacket" picture (photon = [tex]\psi (r, t)[/tex])?

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# QED photon vs photon as particle

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