QED photon vs photon as particle

[neworder1]

Quasi-classically, a photon is often considered as a particle with some momentum traveling 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.

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

 

[DrClaude]

Quasi-classically, a photon is often considered as a particle with some momentum traveling 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.

That picture is incorrect. See for instance
https://www.physicsforums.com/threads/why-photon-wave-function-does-not-exist.659614/https://www.physicsforums.com/threa...n-and-qed-the-strange-theory-of-light.812112/