Feasibility study of HTS flux qubit read-out using aluminium dc-SQUID

Abstract

This thesis for the degree of Master of Science studies and tests the feasibility of HTS flux qubit read-out using an aluminium dc-SQUID. The feasibility is studied on a device consisting of an YBCO dc-SQUID surrounded by an aluminium dc-SQUID separated by an intermediate layer. The intermediate layer acts as insulator between the YBCO and the aluminium and as capping layer for the underlying YBCO. Each part of the device has been studied within this project. Aluminium dc-SQUIDs for flux qubit read-out have been fabricated using electron beam lithography and shadow evaporation. A typical SQUID had a critical current of 56 nA and was highly under-damped. The SQUIDs were characterized below 300 mK. An intermediate layer consisting of amorphous STO, deposited at room temperature, and thermally evaporated silicon dioxide has been studied. The roughness, peak-to-valley, was found to be approximately 10 nm and the resistance was 9 k$\Omega$ for an 360 nm thick layer of silicon dioxide and 12 k$\Omega$ for a 500 nm thick layer at 4.2 K. YBCO SQUIDs were fabricated on STO 0$^o$/30$^o$ bi-crystals using electron beam lithography and ion milling through a mask of amorphous carbon. The properties of the grain boundary junctions in the SQUIDs were unchanged when a multi-layer of amorphous STO and silicon dioxide was deposited on top of them. A SQUID that consists of 3 $\mu$m wide junctions typically had a critical current of 1.5 mA. It was found that the insulating properties, the adhesion and the smoothness of the intermediate layer has to be improved in order to get the proposed device working.