Coherent optically driven semiconductor quantum dot systems are proposed for the implementation of quantum information processing devices. Fundamental quantum computation units, the quantum bits (qubits), are realized and optically manipulated in the exciton and spin bases. In this thesis, consecutive phase-sensitive rotations of a single qubit and their importance in the measurement of the full physical density matrix have been demonstrated experimentally in the exciton system in neutral GaAs/Al0:3Ga0:7As interface fluctuation quantum dots. The experimental work done on the exciton system serves as a foundation for consequent work on the long coherence lifetime spin-based qubits in charged quantum dots. In the spin-based system in the charged GaAs/Al0:3Ga0:7As interface fluctuation quantum dots, we demonstrate the selective excitation from the two spin ground states to the charged exciton (trion) state through phase sensitive control of the spin coherence in an ensemble of three-level spin-trion Lambda systems. This controlled excitation utilized the optically coupled and decoupled states of the electron spin in the presence of coherent transient excitation of the spin ground states in singly charged quantum dots. This progress lays the ground work for achieving complete ultrafast spin rotations.
Semiconductor quantum dots (QDs) are ideal candidates for quantum information processing, due to their strong resemblance to atomic systems. This paper will show that an optically driven approach is an attractive scheme for quantum computing with QDs. Results will be presented from the experiments with the optical Bloch vector qubit in neutral QDs, demonstrating phase‐locked single qubit rotations and two‐qubit universal control‐NOT logic gates. We will conclude by discussing how the optical approach can be extended to control the electron spin qubit in charged QDs.