Sub-Half-Wavelength Atom Localization via Coherent Control of Autler--Townes Spontaneous Spectrum

We show that it is possible to localize an atom in a half-wavelength region
by relaxing the strict condition that the atom is prepared in a specific excited state as in the recently proposed scheme Phys. Rev. A 65(2002)043819. In particular, we consider a four-level atom, for which a weak exciting field transfers population from the ground state to the excited state and three control fields (one standing-wave field while two travelling-wave fields) couple the excited state and two auxiliary states. By tuning the exciting field and by varying the collective phase of the control fields, the atom is localized in one of the two half-wavelength regions with 50% detecting probability. The main advantage of the scheme is the experimental accessibility and controllability.