We show that ultrasharp spectral lines, with widths 2 orders of magnitude below the natural width, may be produced in the resonance fluorescence of a V-type three-level atom excited by a single-mode laser field when the dipole moments are nearly parallel. The smaller the splitting of the excited doublet, the narrower the line. This effect is due to quantum interference between the two transition pathways.

• Received 18 April 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.3995