A fully quantum-mechanical description of the spontaneous emission fro
m an excited two-level atom placed in front of a two-slit interferomet
er is given. Global modes of the electromagnetic field in a two slit s
ystem are derived within the Kirchhoff-Huygens diffraction approximati
on, serving as a base for the field quantisation. The standard Fermi's
golden rule, supplemented by a factor coming from the nontrivial mode
structure caused by the presence of the two-slit interferometer, is u
sed to show that interference results from the position-dependent coup
ling between the atom and different field modes of the system.