Masters Thesis defence by Christian Starup – University of Copenhagen

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Masters Thesis defence by Christian Starup

This thesis will focus on the optical control of the spin of a positively charged InAs Quantum Dot embedded in a GaAs Nanobeam Waveguide. A protocol to generate N-photon GHZ states is used as the main motivator, leading to demonstrations of several optical control techniques including spin-pumping, Rabi oscillations, Raman transitions and Ramsey interferometry. All of these techniques will be explained briefly theoretically, and then carried out experimentally. From this, a very high spin state initialization with fidelity F>=99.87% is shown, and both optical Rabi oscillations and ground state Raman transitions are demonstrated successfully. During preparation for the Raman transitions, Raster scans of laser polarization is shown to be an effective way of determining laser polarization at the location of the quantum dot to optimize Raman oscillations.
With these techniques in hand, Ramsey interferometry is carried out to reveal spin coherence time T_2^* 24ns, about 10 times that of previously examined negatively charged quantum dots. Additionally, the lifetime of above-band generated hole states is examined and found to exceed 700ns. This work also features simulations of the branching ratio of decay for different dipole orientations and positions within the waveguide. These simulations are sought verified by frequency filtering the emitted photons to determine collection rates from either decay path. It is shown that the experimentally found collection rates does not accurately reflect the branching ratio of the quantum dot, possibly because of the multi-mode nature of the waveguide making collection biased towards favouring certain modes over others.