Hybrid quantum dots in InAs nanowires

In this thesis measurements performed on InAs nanowire based hybrid quantum dots are presented. The term "hybrid" indicates that the nanowires (NWs) are contacted with superconducting and ferromagnetic materials besides normal metal leads. This allows to inject the different electron correlations of these materials and exploit their interplay in a nanoscale object. The "nanoscale objects" are the quantum dots (QDs) formed in the NW segment between the contacts. There, the geometrical confinement of the electron wavefunction in all three spatial orientations gives rise to quantized energy levels, allowing to investigate transport at the single electron level.
All of the presented experiments are focusing on essential building blocks in the field of solid-state quantum computation with electrons. The first part concentrates on the investigation of the level dependence of the g-factor of InAs NW quantum dots. The g value is the key parameter for spin manipulation at the single electron level. Thus, its tunability and control is highly desirable for experiments where different spin states need to be addressed individually (e.g. qubit control).
Furthermore, the transport in InAs NW QD structures is explored that are contacted with a ferromagnetic and a superconducting lead. The proximity-induced exchange field (Martinek et al., Phys. Rev. Lett. 91, 247202 (2003)) in the QDs is analyzed in the Kondo regime. In addition, it is shown that a superconducting contact can serve as spectroscopy tool for the visualization of the ferromagnetic exchange field in the absence of Kondo correlations. The magnetic proximity effect is promising for the use of efficient spin filtering, necessary for the detection of entanglement in split Cooper pairs.
The study of Cooper pair splitting in a two-quantum dot Y-junction is the central part of this thesis. Cooper pairs are electron pairs in an s-wave superconductor, which occur naturally in a spin-singlet state (Bardeen et al., Phys. Rev. 108, 1175-1204 (1957)). Therefore, extracting Cooper pairs from the superconductor is a possible source of electrically controllable electron-electron entanglement. The experiment follows closely the proposal by Recher et al. (Recher et al., Phys. Rev. B 63, 165314 (2001)), who suggest to make use of the Coulomb interaction in quantum dots, for the controlled generation of spatially separated entangled Einstein-Podolsky-Rosen (EPR) pairs (Einstein et al., Phys. Rev. 47, 0777-0780 (1935)). Besides the interest to study fundamental quantum mechanical properties, such entangled pairs could e.g. be used for entanglement distribution to synchronize quantum circuits or to investigate non-classical correlations in other materials. The first realization of an electrically tunable Cooper pair splitter is presented. In addition, experiments to optimize the splitting efficiency are discussed followed by a detailed outlook for future experiments.
The main part of this work is closed by preliminary results on a Cooper pair splitting experiment at finite bias. It seems that if the device is operated at finite bias, the relative rates of Cooper pair splitting and elastic cotunneling can be tuned by exploiting the gate dependence of the level energy and the density of states at the relevant energies.