Resonance fluorescence quenching and spectral line narrowing via quantum interference in a four-level atom driven by two coherent fields

The resonance fluorescence from a four-level atom driven by two coherent fields is studied. If the dipole moments between the two upper levels and each of the two lower levels are parallel, the spontaneous emission paths from the upper levels to the lower levels will have strongly or even completely destructive quantum interference. We show that this quantum interference will result in a complete inhibition of the fluorescence when one of the driving fields is tuned at the middle point of the upper levels. The quantum interference will lead to the dependence of atomic populations at steady state on the initial condition, when both of the driving fields are tuned to the middle point of the upper levels. When the dipole moments are close to but not exactly parallel, or none of the driving fields is tuned at the middle point of the upper levels and there is a small difference between detunings of the two driving fields to the uppermost level, ultranarrow lines at the spectral center appear because of the destructive quantum interference.