Quantum interference between decay processes from two upper levels, which are coupled by the same vacuum modes to a lower level, have been investigated and its effects on the spontaneous emission spectrum have been studied. The interference can result in spectral narrowing and a black dark line in the spectrum. The population in the upper levels is not a simple exponential decay due to the interference.

• Received 24 October 1994

DOI:https://doi.org/10.1103/PhysRevA.52.710