Figure 2.

(a, b) Tuning the excitation laser power reveals Rabi rotations in the spectrally filtered single-photon detector counts of the X- (blue, upper panel) and XX- (green, lower panel) states up to 4π, confirming the coherent population of the three-level system. Solid lines correspond to fits and dashed red lines to the extracted envelope function from which preparation fidelities >80% are extracted. (c) The decay dynamics illustrate the cascaded emission under pulsed TPE, with the X-photons (blue line) appearing delayed with respect to the XX-photons (green). The XX-state decays ≈4× faster than the X-state. (d, e) The second-order autocorrelation measurement confirms the single-photon-nature of X- (blue) and XX- (green) emission.