The isolation of qubits from decoherence is crucial to the prospect of building revolutionary quantum devices. This work is devoted to an optical study of the decoherence on spin qubits in self-assembled quantum dots. This thesis contributes towards a complete understanding of quantum decoherence, of which highlighted discoveries include bypassing the spectral diffusion in neutral quantum dot emission lines; observing for the first time the self-polarization phenomenon of nuclear spins, via the resonance-locking effect on a negatively charged quantum dot; and revealing the limiting factors on hole spin dephasing, by measuring polarization correlations on a positively charged quantum dot.

Three studies were conducted using two different spectroscopy techniques. For the first study, the spectral diffusion of emission line due to random electrostatic fluctuations was revealed, by scanning a neutral quantum dot transition across the laser resonance. Exciting the quantum dot resonantly bypassed this problem, paving the way for an on-demand antibunched source that generates narrow-band photons. For the second study, evidences supporting the spontaneous self-polarization of nuclear spins were observed for the first time, since it was predicted nearly four decades ago by M. Dyankonov and V.I. Perel. The self-polarization phenomenon is a remarkable demonstration of dynamic nuclear spin polarization since it manifests without the ground state electron being spin-polarized. In the last study, factors limiting the hole spin lifetime was inferred from measuring polarization correlation of successively emitted photons from a positively charged quantum dot. Evidences support a strong dependence on the carrier repopulation rate and the single electron spin dephasing in the upper state, due to the Overhauser field. In combination with the observation of spontaneous nuclear polarization, this opens the possibility of an electron spin sensor, which can indirectly probe the nuclear field.