Controlling linewidth of resonance fluorescence in the V-configuration of a four-level atom via quantum interference

The interaction of a four-level atom consisting of one excited state, two upper levels and one ground state is considered. We use one field to drive the transition between the excited state and the ground state, and simultaneously apply another field to couple two transitions from the upper-level doublet to the ground state. We investigate how to control the central linewidth of resonance fluorescence spectrum of the driven transition via quantum interference between two transitions from the upper doublet to the ground state of the atom. In the case of orthogonal dipole moments for the upper doublet and the ground state the widths of the central line and the sidebands of the resonance fluorescence spectrum are totally controlled by the spontaneous emission rate of the upper doublet when the applying field is sufficiently strong. When the dipole moments are parallel, the central linewidth can be much narrower than the linewidths of spontaneous emission of the excited state and of the upper doublet due to the quantum interference. The smaller the upper doublet splitting, the narrower the central linewidth. We also notice that when the upper levels are degenerate the resonance fluorescence spectrum may depend on the initial condition because of the quantum interference.