Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions

TL;DR

This paper reviews microwave experiments on HgTe-based topological Josephson junctions, highlighting the observation of the fractional Josephson effect linked to topologically protected states, advancing understanding of topological superconductivity.

Contribution

It reports experimental evidence of the fractional Josephson effect in HgTe topological insulator junctions using microwave techniques, providing insights into topological superconductivity.

Findings

Observation of signatures of fractional Josephson effect

Identification of topologically protected gapless Andreev bound states

Assessment of topological origin of experimental features

Abstract

The rise of topological phases of matter is strongly connected to their potential to host Majorana bound states, a powerful ingredient in the search for a robust, topologically protected, quantum information processing. In order to produce such states, a method of choice is to induce superconductivity in topological insulators. The engineering of the interplay between superconductivity and the electronic properties of a topological insulator is a challenging task and it is consequently very important to understand the physics of simple superconducting devices such as Josephson junctions, in which new topological properties are expected to emerge. In this article, we review recent experiments investigating topological superconductivity in topological insulators, using microwave excitation and detection techniques. More precisely, we have fabricated and studied topological Josephson junctions made of HgTe weak links in contact with two Al or Nb contacts. In such devices, we have observed two signatures of the fractional Josephson effect, which is expected to emerge from topologically-protected gapless Andreev bound states. We first recall the theoretical background on topological Josephson junctions, then move to the experimental observations. Then, we assess the topological origin of the observed features and conclude with an outlook towards more advanced microwave spectroscopy experiments, currently under development.