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Title: Laser spectroscopy of coherent quantum states in single quantum dots
Author: Brunner, Daniel
Awarding Body: Heriot-Watt University
Current Institution: Heriot-Watt University
Date of Award: 2010
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Laser spectroscopy was used for studying single charge-tunable InAs quantum dots (QD). The spectroscopy system consisted of a high resolution microscope combined with a solid immersion lens, a grating spectrometer and an in-situ detector to study the homodyne signal of the resonant laser and the QD. Low density QD samples were fabricated, which allowed spectral isolation of individual QDs. A modulation technique was used for noise rejection. Resonant absorption spectroscopy was used for directly probing transitions between ground and excited QD states. Lineshapes and signal strength were linked to life and coherence times of QD states. A theoretical model was developed combining coherent and non coherent processes in a master equation. Positively and negatively doped sample structures enabled spectroscopy of negatively, neutral and positively charged excitons. The relaxation time of hole spin ground states in a single QD was probed using resonant excitation in a magnetic eld parallel to the growth direction. Optical selection rules enable control over hole spin orientation. Hole spin relaxation times were studied from zero to ve Tesla, with relaxation times of di erent QDs ranging from 200 s to 1 ms. No signi cant in uence of the external magnetic eld on the hole spin relaxation time was found. A hole spin initialisation delity close to 100 % was achieved. Readout of resonantly created QD states was realised via a new microscope system. This dark eld microscope utilised spatial and polarisation ltering techniques to suppress the excitation laser by up to six orders of magnitude. Both ltering devices were included in the standard microscope, making it a highly practical and versatile system. Collected QD emission exceeded the resonant laser background by a factor of 100 for an unsaturated X1 transition. Pump-probe spectroscopy of the 3-level biexciton system was carried out, with the back scattered signal collected in re ection allowing spectral ltering via a grating spectrometer. The recorded probe spectrum revealed Autler-Townes splittings for high pump laser intensities, demonstrating the coherent superposition of QD exciton states. Swapping the pump probe geometry revealed weak quantum interferences. Spectroscopy of hole spin ground states in an in-plane magnetic eld created a coherent superposition of hole spin ground states via a -system. The resulting quantum interference between hole spin states resulted in the creation of a dark state. This experiment is known in quantum optics as coherent population trapping. The extracted lower bound of the hole spin coherence time was 1 s with greater than 40 % probability, demonstrating the enormous potential of hole spins in QDs for quantum information processing as well as for quantum optical experiments.
Supervisor: Warburton, Richard J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available