# QED photon vs photon as particle

1. Aug 6, 2009

### neworder1

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.

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 = $$\psi (r, t)$$)?