This thesis presents experiments carried out on a single InGaAs/GaAs quantum dot coupled with a photonic crystal cavity (H1). The single exciton qubit system is controlled by ultrafast optical pulses. Then, the resonance fluorescence (RF), which is proportional to the population of the quantum dot, is measured by a spectrometer. The two main results of the whole thesis are: i) measurement of the short exciton lifetime (22.7~ps); ii) achievement of full populated quantum dot through a few photons (on average approx. 2.5 photons). To measure such a short lifetime the Two Pulse Resonance Fluorescence technique was developed. This technique enables measurements with high time resolution. This required the development of the Differential Resonance Fluorescence technique. This technique is highly efficient in suppressing the laser scattered light, permitting measurements of the RF of the dot. These two main results are the consequence of cavity enhancement. A Purcell factor of 42 was measured. This is the largest Purcell factor reported so far for the weak coupling regime. This enhancement allows the recovery of the coherence of the QD, permitting the investigation of the quantum dot–cavity system as a near–ideal single photon source on–chip and on–demand. The cavity enhancement also affects the exciton–phonon interaction. The full monotonic phonon side band is here presented for first time. This quantum dot–cavity system also allows the control of the cavity scattered light from the quantum dot. This can be used as an ultrafast switcher.