Ge/Si Core-Shell Nanowires coupled to Superconductor

A fellowship for an experimental PhD thesis work is available in the Nano- and Quantum Electronics group at the Department of Physics of the University of Basel: www.nanoelectronics.ch

The project is motivated by the recent discovery of spectral features at zero energy in tunneling spectroscopy of semiconducting nanowires proximitized by a superconductor. These “zero-bias anomalies” show features in agreement with Majorana bound states (MBSs), expected to appear in topological superconductors. A one-dimension topological superconductor can be engineered in semiconducting nanowires (NWs) with strong spin-orbit inter-action (SOI) coupled to a “normal” BCS-type superconductor in magnetic field. To unravel the emergence of MBSs in single and coupled NWs, we develop new probes with which the proximity-induced bound states can be quantified. Our approach is based on measuring both DC and AC transport, the latter also at GHz frequencies using reflectometry. We can also study the microwave radiation in the GHz domain emitted by the quantum device so-called Josephson radiation.

Most of the work in this area has been performed with III-V semiconductors, e.g. with InAs and InSb. In this project we focus instead on holes in Ge/Si core-shell nanowires (NWs). The use of holes in Ge/Si is motivated by a recent prediction that these holes have a large SOI which is tunable. The challenge of the project will be to induce in addition a pairing potential into the valence band by the proximity effect from a superconductor. We will then perform transport experiments and assess the subgap states. Double NWs that are coupled by a superconductor and make up for a “box-qubit” will also be considered.

This fellowship is part of a European FET-open project entitled TOPOLOGICALLY PROTECTED AND SCALABLE QUANTUM BITS: TOPSQUAD. The consortium consists of 4 academic partners: TU-Twente, TU-Eindhoven, IST Vienna and the University of Basel, as well as two startup companies. See also: TOPSQUAD_News

Left: Ge/Si core-shell nanowire (NW) device with bottom-gates to define quantum dots. Middle: Illustration of a NW-SQUID coupled to a \lambda/4 transmission-line resonator for GHz state readout. Right: proposal of using TWO NWs coupled by a superconductor (top), and (bottom) a first devices with a pair of NWs.

We are looking for a highly motivated student (preferably a physicist) who is keen to explore fundamental aspects of quantum devices. You will design and fabricate your own devices using tailored nanowires and state-of-the-art micro- and nanofabrication.

All PhD fellows are expected to work in a team and collaborate with other PhD and postdoctoral fellows as well as bachelor. Start 1st of January 2020 or earlier. Duration 3-4 years. Requirement: you need to have a profound understanding of quantum and solid-state physics as it is taught in a physics curriculum.

To apply, please email to Christian.Schoenenberger@unibas.ch your short curriculum vitae including names and contact info of referees and scanned copies of grades. Please add a short statement (few lines only) on your motivation and your education / background in quantum and solid-state physics.