Response of a two-level atom to a narrow-bandwidth squeezed-vacuum excitation - art. no. 033811

Using the coupled-system approach we calculate the optical spectra of the f luorescence and transmitted fields of a two-level atom driven by a squeezed vacuum of bandwidths smaller than the natural atomic linewidth. We find th at in this regime of squeezing bandwidths the spectra exhibit unique featur es, such as a hole burning and a three-peak structure, which do not appear for a broadband excitation. We show that the features are unique to the qua ntum nature of the driving squeezed vacuum field and donor appear when the atom is driven by a classically squeezed field. We find that a quantum sque ezed-vacuum field produces squeezing in the emitted fluorescence field whic h appears only in the squeezing spectrum while there is no squeezing in the total field. We also discuss a nonresonant excitation and find that depend ing on the squeezing bandwidth there is a peak or a hole in the spectrum at a frequency corresponding to a three-wave-mixing process. The hole appears only for a broadband excitation and results from the strong correlations b etween squeezed-vacuum photons.