One of the most interesting subfields of quantum optics is cavity QED - where microcavities impose nontrivial boundary conditions on the quantized electromagnetic field and alter the matter-light interactions of quantum electrodynamics. One of the first predictions of this theory was the modification of atomic spontaneous emission rates, through the use of microcavities to alter the optical density of modes from its free- space value. This phenomenon is often known as the Purcell effect. Since this prediction many theoretical analyses as well as experimental confirmations of this effect have been performed. Since the imposition of nontrivial boundary conditions modifies the electromagnetic Green's function, the Feynman diagrams for any QED process must be altered in a cavity, giving physical results that differ from those in free space.
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