The $\omega$-SQUIPT: phase-engineering of Josephson topological materials

TL;DR

This paper reports the first realization of a three-terminal Josephson interferometer that demonstrates topologically protected phase transitions between gapped and gapless states, paving the way for engineering artificial topological superconductors.

Contribution

It introduces a novel three-terminal Josephson device that exhibits topologically protected phase transitions, enabling phase engineering of artificial topological materials.

Findings

Observation of phase-controlled transitions between gapped and gapless states.

Identification of topological nature of these states via superconducting phase windings.

Demonstration of topologically protected states in a nanoscale Josephson junction.

Abstract

Multi-terminal superconducting Josephson junctions based on the proximity effect offer the bright opportunity to tailor non trivial quantum states in nanoscale weak-links. These structures can realize exotic topologies in multidimensions as, for example, artificial topological superconductors able to support Majorana bound states, and pave the way to emerging quantum technologies and future quantum information schemes. Here, we report the first realization of a three-terminal Josephson interferometer based on a proximized nanosized weak-link. Our tunneling spectroscopy measurements reveal transitions between gapped (i.e., insulating) and gapless (i.e., conducting) states, those being controlled by the phase configuration of the three superconducting leads connected to the junction. We demonstrate the $topological$ nature of these transitions: a gapless state necessarily occurs between two gapped states of different topological index, very much like the interface between two insulators of different topology is necessarily conducting. The topological numbers characterizing such gapped states are given by superconducting phase windings over the two loops forming the Josephson interferometer. Since these gapped states cannot be transformed to one another continuously withouth passing through a gapless condition, these are topologically $protected$. Our observation of the gapless state is pivotal for enabling phase engineering of more sophisticated artificial topological materials realizing Weyl points or the anomalous Josephson effect.