Rabi oscillations of a quantum dot exciton coupled to acoustic phonons: coherence and population readout

While the advanced coherent control of qubits is now routinely carried out in low-frequency (gigahertz) systems like single spins, it is far more challenging to achieve for two-level systems in the optical domain. This is because the latter evolve typically in the terahertz range, calling for tools of ultrafast, coherent, nonlinear optics. Using four-wave mixing microspectroscopy, we here measure the optically driven dynamics of a single exciton quantum state confined in a semiconductor quantum dot. In a combined experimental and theoretical approach, we reveal the intrinsic Rabi oscillation dynamics by monitoring both central exciton quantities, i.e., its occupation and the microscopic coherence, as resolved by the four-wave mixing technique. In the frequency domain, this oscillation generates the Autler–Townes splitting of the light-exciton dressed states, directly seen in the four-wave mixing spectra. We further demonstrate that the coupling to acoustic phonons strongly influences the four-wave mixing dynamics on the picosecond time scale, because it leads to transitions between the dressed states.