  When a quantum particle traverses a rectangular potential created by a
quantum field both photon exchange and entanglement between particle and field
take place. We present analytic results for the transition amplitudes of any
possible photon exchange processes for an incoming plane wave and initial Fock,
thermal and coherent field states. We show that for coherent field states the
entanglement correlates the particle's position to the photon number in the
field instead of the particle's energy as usual. Besides entanglement
formation, remarkable differences to the classical field treatment also appear
with respect to the symmetry between photon emission and absorption, resonance
effects and if the field initially occupies the vacuum state.
