Quantum dots and superconductivity in Ge-Si nanowires

Abstract

A quantum computer requires a quantum-mechanical two-level system with coherent control over its eigenstates. We investigate two different routes leading to quantum computational devices using Ge-Si core-shell nanowires in which holes are confined in one dimension: Normal-state quantum dots for spin qubits and proximity-induced su- perconductivity for Majorana fermions. The nanowires have an extremely low defect density resulting in high mobilities and the ability to form intentional quantum dots of several lengths up to half a micron. The predicted strong direct-Rashba spin-orbit coupling allows the hole spin state to be controlled using electric-dipole spin-resonance and is at the same time a requirement for obtaining Majorana zero modes. In a single quantum dot the g-factor is highly anisotropic with respect to the nanowire and electric field axis, which enables the tuning of the electric-dipole spin-resonance frequency. For a double quantum dot in Pauli-spin blockade we observe highly anistropic leakage currents and identify spin-flip cotunnelling and spin-orbit interaction as the main sources of spin relaxation. Spin-lifetimes can therefore be optimised by carefully choosing the magnitude and direction of the magnetic field. Highly transparent superconducting aluminium contacts to the nanowires are obtained by using a straightforward annealing procedure. We observe a Josephson current with a near ideal IcRn product and the device shows Shapiro steps when exposed to microwaves. We thus confirm our device is a Josephson junction. Near depletion, we see a strongly coupled few-hole quantum dot supporting a supercurrent through single-particle levels, an important step towards realising Majorana fermions. We find an additional superconducting phase with a lower critical temperature than of aluminium, most likely consisting of an Al/Si_x /Ge_y alloy. Near depletion, we observe a very hard induced superconducting gap, proof of a highly homogeneous superconductor-nanowire interface and indicating few in-gap states, which are detrimental for Majorana zero modes. We believe the hard gap, as well as the high interface transparencies, are the result of the presence of the superconducting alloy.