Contact transparency for proximity induced superconductivity in 2D topological insulators for qubit fabrication

Proximity induced superconductivity in Bi_xSb_2-xTe₃  using s-wave superconductors is a promising route to developing Majorana based qubits as this hybrid system will behave as a spinless superconductor. The bismuth antimony telluride (BST) has no bulk carriers so a gap in the induced superconductivity would provide a platform for long lived Majorana parity qubits. The strength of the induced superconductivity is strongly dependent on the contact transparency between the TI and the superconductor. We have developed an analytic solution for normal state current flow from the superconducting metal for two geometries, circular and rectangular. Modeling the current flow into the 2D topological material we obtain the junction conductance and contact transparency. The devices are arrays of niobium islands in contact with the BST that produce arrays of Josephson junctions with supercurrents flowing in the proximitized BST. The experimental verification of induced superconductivity is obtained by measuring the differential conductance of the array which exhibits a finite two dimensional supercurrent. Optimizing contacts should lead to stronger proximity induced superconductivity in the BST surface state.