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 u pper levels and one ground state is considered. We use one field to drive t he transition between the excited state and the ground state, and simultane ously apply another field to couple two transitions from the upper-level do ublet to the ground state. We investigate how to control the central linewi dth 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 d oublet and the ground state the widths of the central line and the sideband s of the resonance fluorescence spectrum are totally controlled by the spon taneous emission rate of the upper doublet when the applying field is suffi ciently strong. When the dipole moments are parallel, the central linewidth can be much narrower than the linewidths of spontaneous emission of the ex cited state and of the upper doublet due to the quantum interference. The s maller the upper doublet splitting, the narrower the central linewidth. We also notice that when the upper levels are degenerate the resonance fluores cence spectrum may depend on the initial condition because of the quantum i nterference.