Towards a Cooper pair splitter in InAs nanowires with crystal-phase defined quantum dots

Abstract: Cooper pair splitting (CPS) is a process in which the two spin-entangled electrons of a Cooper pair in a superconductor are split into two spatially separated electrons. If the separated electrons are still entangled, CPS can be studied to increase the knowledge of non-locality in quantum systems. This thesis is a first step towards the realization of CPS in InAs nanowires with crystal-phase defined quantum dots. Previous studies with these nanowires have shown that it is possible to control the spin-ground state with the help of small electric and magnetic fields. If CPS also is possible in these nanowires, this may in the future lead to new opportunities for spin-resolved CPS, which opens a route towards a test of the Einstein-Podolsky-Rosen (EPR) paradox and Bell's inequality. The nanowires studied in this thesis had a shell of GaSb to improve contact alignment. Good contact alignment was important in this project since the superconducting contact must be placed very close to the quantum dots, in order for the superconductivity to be induced into the nanowire via the proximity effect. The shell of GaSb was removed prior to metal evaporation by a wet-etch in the developer MF319. To improve the quality of the InAs-Ti/Al contact interface, two different processing schemes were developed and compared. The quality of the contact interface became better if the whole GaSb shell was removed before any other process step than if the GaSb was removed in two separate steps. It was also found that water also can be used to etch GaSb. The search for experimental signatures of CPS was performed at mK temperatures in a dilution refrigerator. A CPS signal can potentially be very weak and difficult to isolate from a noisy background. Therefore, several comparisons between standard DC measurements and AC measurements with a lock-in amplifier were performed, where one research question was to determine which measurement technique is most suitable for detecting CPS. One important topic of study was finding out how a setup can generate non-local signals. This can occur in DC measurements due to a DC offset between amplifiers, as is shown in this work. The non-local signal will look like a CPS signal but can remain even when the contact is no longer superconducting. Some of the advantages of DC measurements are instead that they are, in general, faster and simpler to perform since they do not need significant settings optimization. AC measurements, on the other hand, require that settings such as reference frequency and root-mean square voltage are selected appropriately. These settings were most likely not well selected for the various measurements performed in this thesis since the AC measurements resulted in broader features and lower signals than the corresponding DC measurements. A conclusion from this thesis is that it can be difficult to make good AC measurements, but they have the advantage of eliminating the problem with DC offsets and should therefore be used when studying CPS to reduce the risk of other non-local signals. The ultimate goal of this thesis was to observe experimental signs of CPS, but no such signs could be confirmed. The primary reason for this was probably a too long distance between the quantum dots. The width of the superconducting contact of 400 nm was likely too wide compared to the coherence length in diffusive aluminum nanostructures which typically lie in the range of 100 to 200 nm.