Degenerate cascade fluorescence: Optical spectral-line narrowing via a single microwave cavity

For a three-level atom, two nondegenerate (even microwave and optical) electric dipole transitions are usually allowed; for either of these, the fluorescence spectra are well-described in terms of spontaneous transitions from a triplet of dressed sublevels to an adjacent lower-lying triplet. When the three dressed sublevels are equally spaced from each other, a remarkable feature known as degenerate cascade fluorescence takes place, which displays a five-peaked structure. We show that a single cavity can make all the spectral lines extremely narrow, whether they arise from cavity-coupled or cavity-free transitions. This effect is based on intrinsic cascade lasing feedback and makes it possible to use a single microwave cavity (even a bad cavity) to narrow the spectral lines in the optical frequency regime.