A fully quantum-mechanical description of the spontaneous emission from an
excited two-level atom placed in front of the two slit interferometer is gi
ven. Global modes of the electromagnetic field in a two slit system are der
ived within the Kirchhoff-Huygens diffraction approximation, serving as a b
ase for the field quantization. The standard Fermi's golden rule, supplemen
ted by a factor coming from the nontrivial mode structure caused by the pre
sence of the two slit interferometer, is used to show that interference res
ults from the position dependent coupling between the atom and different fi
eld modes of the system.