Locally tunable inAs nanowire quantum dots for cooper pair splitting

Quantum entanglement has applications in quantum cryptography and quantum computing as the main and speed-enhancing ingredient to quantum algorithms. A possible way to obtain spin-entangled electrons is by a Cooper pair splitter. In these devices electrons are extracted from a central superconducting contact into two parallel, spatially separated quantum dots. The first implementation of a Cooper pair splitter with InAs nanowires led to the detection of correlated electrical currents through the two quantum dots, evidencing that the Cooper pairs separate. The separation rate, i.e. efficiency was, however, low. Tunable quantum dots can help to boost the efficiency as high as it is needed for the detection of the entanglement. This work presents experiments on such InAs devices with a series of bottom gates providing the tunability. First, the formation of quantum dots in single crystalline InAs nanowires is investigated. Applying negative voltages to the individual bottom gates induces tunnel barriers in these InAs nanowires.In the so formed quantum dots, the electron g-factor is extracted from the splitting of Kondo resonances and is found to vary continuously in the range between |g∗|=5 and 15. The measurement of the g-factor anisotropy in these quantum dots reveals that the orientation of the g-tensor is random with respect to the orientation of the nanowire, i.e. the crystal structure. The main findings of this thesis are obtained from Cooper pair splitting devices. Transitions between positive conductance correlation in the quantum dots due to Cooper pair splitting and negative correlations due to quantum dot dynamics are induced by gate potential changes. Using a semi-classical rate equation model, the experimental findings is shown to be consistent with the manipulation of the weight of the different local and non-local quantum transport processes. Moreover, bias voltage dependence is measured and analysed, and it is found that a larger bias voltage can increase the splitting signal in accordance with theory. The preliminary results even indicate that Cooper pair splitting is possible at bias voltages larger than the superconducting gap. Thus, the present thesis shows that it is possible to in-situ optimize the efficiency of a Cooper pair splitter, thereby adding an important step to the construction of a source of entangled electrons.