| Empowered by the exponential growth rate by parallel processing on multiple quantum bits simultaneously, quantum computing algorithms can solve some problems, which require an impossibly long time and large machines based on classical computing, within a much shorter time and smaller machines. Semiconductor quantum dots have been proposed for implementation of quantum computing algorithms due to their compatibility with the semiconductor fabrication infrastructure. To implement the optically driven quantum dot quantum computing algorithms, optical properties of the electronic and spin states in quantum dots need be studied. In this thesis, coherent optical properties of spin states in charged quantum dots are studied by optical absorption measurements on electrically charged single quantum dots and spin noise measurements on n-GaAs.;In the optical absorption measurements on electrically charged single quantum dots, experimental techniques including differential reflection and voltage modulation are studied to measure voltage dependent absorption of single quantum dots, which is comparable or below the apparent noise level in laser spectroscopy measurements. A differential reflection technique is developed for mapping out voltage dependent absorption of single quantum dots for dots where transmission measurements are not possible. The voltage dependent absorption of single quantum dots shows the quantum confined Stark effect. Based on the quantum confined Stark effect, a voltage modulation technique is studied to obtain the voltage dependent absorption with high speed and high signal-noise-ratio. The absorption of a single quantum dot is measured by both differential reflection and voltage modulation consistently.;In the spin noise measurements on n-GaAs, optical effects including laser energy and intensity on spin relaxation time are studied. Spin noise from the electron spin relaxation is measured to study the electron spin relaxation time with an advantage of not disturbing the system under study. Optical effects on spin noise power and width of n-GaAs are studied. With a two-level system model with Lorentzian line shape and saturation behavior, the laser energy and intensity dependent spin noise shows optical excitation induced spin relaxation due to optically excited impurity ionization. |
