Resonance Fluorescence of a Two-Level Atom in a Feedback Loop

The resonance fluorescence spectrum of a two-level atom in a classical light field, whose phase is changed by a feedback system by pi after the detection of each spontaneous photon, has been calculated. The spectrum resembles the well-known Rautian-Mollow triplet. However, the sidebands become sharply asymmetric at a nonzero frequency detuning of the light field from the atomic resonance transition. In addition, the spectrum retains the pronounced triplet shape even beyond the secular approximation, when the sidebands of the Rautian-Mollow structure become unresolved in the absence of the feedback.