Nuclear Spin Phenomena in Optically Active Semiconductor Quantum Dots

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HILDMANN, Julia, 2014. Nuclear Spin Phenomena in Optically Active Semiconductor Quantum Dots [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Hildmann2014Nucle-26928, title={Nuclear Spin Phenomena in Optically Active Semiconductor Quantum Dots}, year={2014}, author={Hildmann, Julia}, address={Konstanz}, school={Universität Konstanz} }

Nuclear Spin Phenomena in Optically Active Semiconductor Quantum Dots 2014 2014-03-17T09:18:53Z 2014-03-17T09:18:53Z The motivating background for this thesis lies in the recent development in<br />the coherent optical control of electron spin states in semiconductor quan-<br />tum dots. One possible and attractive application for these achievements is a<br />physical implementation of a quantum computer based on spin qubits (quan-<br />tum bit) in quantum dots. However, one of the obstacles in building such<br />quantum computer is decoherence caused by the interaction of the electron<br />spins with nuclear spins of the host material. In this thesis we theoretically<br />investigate the effects caused by the hyperfine interaction in optically active<br />semiconductor quantum dots.<br /><br /><br />In Chapter 2 we assume optically induced rotations of single electron spins in<br />semiconductor quantum dots. The optical control of the electron spin states<br />is considered to be conducted by means of Raman type optical transitions<br />between electron spin states. We investigate the influence of nuclear spins<br />on the performance of the single-qubit gates by incorporating the additional<br />effect of the Overhauser field into the electron spin dynamics. To calculate<br />the errors caused by the hyperfine interaction, we determine average fidelities<br />of rotations around characteristic axes in the Bloch sphere in the presence<br />of nuclear spins analytically with perturbation theory up to second order in<br />the Overhauser field. By applying numerical averaging over the nuclear field<br />distribution we find the average fidelity to the arbitrary orders of the hyper-<br />fine interaction.<br /><br /><br />In Chapter 3 we investigate the dynamics of spin qubits interacting by means<br />of an optical cavity. The qubits are represented by a single electron spin each<br />confined to a quantum dot. Knowing that electron spins in III-V semiconduc-<br />tor quantum dots are affected by the decoherence due to the hyperfine inter-<br />action with nuclear spins, we find that the interaction between two qubits is<br />consequently influenced by the Overhauser field. Assuming an unpolarizied<br />nuclear ensemble, we investigate the fidelities for two-qubit gates depending<br />on the Overhauser field. We include the hyperfine interaction perturbatively<br />to second order in our analytical results, and to arbitrary precision numeri<br />cally.<br /><br /><br />Quantum dots can be used not only as physical systems for qubits, but also<br />as sources for entangled photons. In chapter 4 we consider an emission of<br />entangled photons by spontaneous decay of the biexciton state to the ground<br />state. Due to electron-hole exchange interaction, the intermediate excitonic<br />states have a finite fine structure splitting. The electron spins of the excitons<br />also interact via hyperfine interaction with the nuclear spins. We study the<br />temporal and fine structure splitting dependance of the coherence measures<br />considering the effect of the nuclear magnetic field. We find that the hy<br />perfine interaction contributes considerably to the reduction of the photon<br />coherence.<br /><br /><br />One of the possibilities to minimize the decoherence effects of the hyperfine<br />interaction is to polarize the nuclear spins to 100%. However, up to date it<br />was not possible to reach such degree of the nuclear polarization in experi-<br />ments. In a recent experiment by Chekhovich et al. it was shown that high<br />nuclear spin polarization can be achieved in self-assembled quantum dots by<br />exploiting an optically forbidden transition between a heavy hole and a trion<br />state. A fully polarized state in this case was predicted by a classical rate<br />equation, but could not be obtained experimentally. Therefore we theoret-<br />ically investigate this problem with the help of a quantum master equation<br />in Chapter 5, and we show that a fully polarized state cannot be achieved<br />due to formation of a nuclear dark state. Moreover, we demonstrate that<br />the maximal degree of polarization depends on the form of the electron wave<br />function inside of the quantum dot. eng Hildmann, Julia terms-of-use Kernspin-Effekte in optisch aktiven Halbleiter-Quantenpunkten Hildmann, Julia

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