The emission spectrum of the bichromatically excited resonance fluorescence of a single atomic particle, i.e. of a permanently trapped and cooled Ba + ion has been recorded. It shows up to five peaks, two of them partially resolved, in agreement with fluorescence spectra of three-level atoms calculated from sets of parameters separately measured from recorded excitation spectra of the integrated fluorescence.

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We analyse a pump-probe set-up for detecting the optical Stark effect on twolevel atoms with a quantized pump field in terms of a density matrix approach (DMA). The approximation used reveals a method to calculate the probe absorption spectra without high computational effort. It is even applicable for large numbers of atoms where the exact calculation is not practicable. The probe spectra in the DMA reflect the photon statistics of the pump field and deviate only slightly from exact results for relevant experimental parameters.

The optical Stark effect in a pump-probe setup is expected to show interesting additional features if the quantization of the pump field becomes important. A major effect is that the lineshape of a Stark-shifted resonance is strongly modified when squeezing the pump field. Furthermore, a probe gain is predicted here which does not appear in a semiclassical treatment. It appears for pump detunings considerably lower than the mean Rabi frequency. The nonclassical gain in the optical Stark effect is investigated on an ensemble of two-level systems (TLSs). A density matrix approach is presented which allows the accurate calculation of the probe absorption. The precise treatment of correlations between pairs of TLSs is crucial to explain details of the lineshape.

We theoretically analyze a pump–probe experiment to detect the optical Stark effect on impurity‐bound excitons taking the quantization of the pump field into account. A density matrix approach allows the calculation of the probe absorption spectra with high accuracy. For bound excitons with low inhomogeneous broadening of several tens of μeV, the photon statistics of the pump field has a considerable influence on the lineshape of the Stark‐shifted resonance. The effect should be clearly observable for broad photon number distributions and low pump detuning.