Froning, Florian N. M.. Hole Spin Qubits in Ge/Si Core/Shell Nanowires. 2021, Doctoral Thesis, University of Basel, Faculty of Science.

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Abstract
Spins in semiconductor quantum dots are among the most promising candidates for the realization of a scalable quantum bit (qubit), the basic building block of a quantum computer. Quantum dots in the common semiconductors Si and Ge profit from the compatibility with industrial microelectronic technologies, a small footprint, and thermal stability. Moreover, the low amount of isotopes with nuclear spin is beneficial for the qubit coherence. The rich physics in the valence band of Ge gives rise to particular properties which make holes attractive for the implementation of hole spin qubits. In particular, the strong spinorbit interaction, termed direct Rashba spinorbit interaction, that arises in onedimensional Ge/Si core/shell nanowires due to the admixture of heavy hole and light hole states is promising for very fast qubit gates and allelectrical qubit control. In order to implement a spin qubit, a large degree of control over quantum dots and the spins confined in it is essential. We demonstrate the formation of single, double and triple quantum dots in Ge/Si core/shell nanowires. In a single quantum dot, we observe indications for single hole occupation. Furthermore, the transport through a double quantum dot at an effective (1,1)(0,2) charge transition is governed by Pauli spin blockade, which leads to current rectification. In presence of spinorbit interaction, the blockade is lifted at finite magnetic field and leads to a leakage current. The study of the leakage current as a function of external magnetic field and double quantum dot detuning yields information about the dominant lifting mechanisms. Here, we observe pronounced orbital effects and a renormalization of the gfactor which arises in presence of strong spinorbit interaction. A spectroscopic model accounts for all these effects and allows to extract a spinorbit interaction length of lSO = 65nm in a Ge/Si core/shell nanowire quantum dot. Finally, spinorbit interaction is used to drive electric dipole spin resonance of a hole spin qubit in a Ge/Si core/shell nanowire. We demonstrate coherent Rabi oscillations and twoaxis single qubit control. Important qubit parameters such as the Rabi frequency and the gfactor can be tuned over a wide range by changing the gate voltages. This tunability arises from the electric field dependent spinorbit interaction in Ge/Si core/shell nanowires. In an optimal configuration, the Rabi frequency increases to 435 MHz at a drive frequency of fMW = 3.4 GHz, thus almost entering the strong driving regime. The results shown here demonstrate the suitability of Ge/Si core/shell nanowires to implement a hole spin qubit which can be electrically switched between a control state, enabling fast qubit gates, and an idle state, prolonging qubit coherence.
Advisors:  Zumbühl, Dominik M and Katsaros, Georgios and Veldhorst, Menno 

Faculties and Departments:  05 Faculty of Science > Departement Physik > Physik > Experimentalphysik Quantenphysik (Zumbühl) 
Item Type:  Thesis 
Thesis Subtype:  Doctoral Thesis 
Thesis no:  14123 
Thesis status:  Complete 
Number of Pages:  219 
Language:  English 
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Last Modified:  08 Jul 2021 12:54 
Deposited On:  08 Jul 2021 12:54 
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