Gate- and flux-tunable sin(2$\varphi$) Josephson element with proximitized Ge-based junctions

TL;DR

This paper demonstrates a gate- and flux-tunable Josephson element with a nearly perfect sin(2ϕ) current-phase relation, achieved using Ge-based junctions, which could enable more coherent superconducting qubits.

Contribution

It introduces a novel Josephson circuit element with dominant charge-4e supercurrent, realized via a SQUID with Ge-based JoFETs, exploiting multi-harmonic CPR for quantum computing applications.

Findings

Achieved >95% sin(2ϕ) component in supercurrent.

Demonstrated gate and flux control of the CPR.

Realized a nearly π-periodic Josephson element.

Abstract

Hybrid superconductor-semiconductor Josephson field-effect transistors (JoFETs) function as Josephson junctions with a gate-tunable critical current. Additionally, they can feature a non-sinusoidal current-phase relation (CPR) containing multiple harmonics of the superconducting phase difference, a so-far underutilized property. In this work, we exploit this multi-harmonicity to create a Josephson circuit element with an almost perfectly $\pi$-periodic CPR, indicative of a largely dominant charge-4e supercurrent transport. Such a Josephson element was recently proposed as the basic building block of a protected superconducting qubit. Here, it is realized using a superconducting quantum interference device (SQUID) with low-inductance aluminum arms and two nominally identical JoFETs. The latter are fabricated from a SiGe/Ge/SiGe quantum-well heterostructure embedding a high-mobility two-dimensional hole gas. By carefully adjusting the JoFET gate voltages and finely tuning the magnetic flux through the SQUID close to half a flux quantum, we achieve a regime where the $\sin(2\varphi)$ component accounts for more than \SI{95}{\percent} of the total supercurrent. This result demonstrates a new promising route for the realization of superconducting qubits with enhanced coherence properties.