Periodic Reporting for period 3 - Topo2DEG (Topological states in superconducting two-dimensional electron gases)
Período documentado: 2022-03-01 hasta 2023-08-31
I will experimentally investigate hybrid superconductor/semiconductor devices for realizing novel topological states of matter, with interest both in fundamental physics and quantum computing applications. Common denominator of the proposed experiments is a regime where the characteristic energy scales of the system, namely Fermi energy, spin orbit interaction correction, superconducting gap and Zeeman splitting are comparable to each other, resulting in unique and mostly uncharted physical territories. Differently from the most widespread use of semiconductor nanowires coupled to superconductors, I will employ novel hybrid two-dimensional electron gases (2DEGs) where the superconductor is grown in-situ and matched to the semiconductor lattice. This novel system was mainly developed by the team I supervise, during the last two years. Compared to the conventional nanowire-based approach, hybrid 2DEGs are readily available, characterized by very low disorder and more amenable to complex sample designs. The work will focus on: 1) Taking full advantage of the planar geometry to study spatial and non-local properties of individual Majorana wires, as well as branched geometries. These experiments will constitute critical tests to establish if the commonly observed zero bias peaks are indeed associated with Majorana modes and pave the way to complex networks of interacting Majorana wires, a requirement for quantum computing. 2) Studying topological phenomena in multi-terminal Josephson junctions (JJs), with particular emphasis on tuning the superconducting phase difference across electrodes pairs. Topological JJs offer a new and possibly advantageous path forward to create and manipulate Majorana modes not explored up to date, including the possibility to reach the topological regime for vanishing small external magnetic fields, useful for applications. Success of the proposal will constitute a key step forward towards topological quantum computing.
Great part of the work has been devoted to the development of state-of-the-art InAs/Al heterostructures together with our collaborators from ETH Zurich (Prof. Wegscheider). Together, we screened over 26 wafers and successfully obtained very promising results, with electron mobilities over 60.000 cm^2/Vs. With such results, all the experiments envisioned in the project proposal can be realized. With such material we could realize nanowires and quantum dots which show clear superconducting properties, consistent with results available in literature. Unfortunately most of these experiments were performed in heterostructres grown about one year ago, and characterized by low quality. We plan to repeat such experiments on the new material in order to proceed with WP1 and WP2. WP3 is instead largely completed. We routinely realize planar Josephson junctions with integrated tunneling probes. Such devices feature a non-sinusoidal current-phase relation and allow for in-situ spectroscopy of the Andreev bound states. In an external magnetic field we observe the states forming a zero bias peak, as expected from a Majorana zero mode.
The work we are currently performing at IBM Zurich is paving the way towards a new generation of high-quality superconductor/semiconductor devices for the study of topological phenomena. With our new material platform, we can realize novel planar Josephson junctions with strong spin-orbit interaction and study them with a multitude of techniques as well as multi-terminal devices for non-local study of Majorana zero modes. Ultimately, understanding and controlling non-Abelian states constitutes a key advancement for using Majorana zero modes as tools for faster computing. Roadmaps in this field of research has already been articulated from one qubit gates all the way to error correction based quantum computing. We are excited to follow these promising paths, and to provide to the community the material platform that enables to realize topological devices as they emerge theoretically.