From nonreciprocal to charge-4e supercurrents in Ge-based Josephson devices with tunable harmonic content

TL;DR

This study demonstrates tunable harmonic content in Ge-based Josephson devices, enabling control over supercurrent properties and potential applications in quantum circuits with nonreciprocal and parity-protected functionalities.

Contribution

It reports the first experimental realization of gate-tunable harmonic-rich CPR in Ge-based Josephson devices, advancing superconducting quantum circuit technology.

Findings

Up to three harmonics in the CPR with gate-tunable amplitudes.

Control of Josephson diode behavior and $\pi$-periodic regimes via magnetic flux and gate voltage.

Observation of fractional Shapiro steps indicating double Cooper-pair transport.

Abstract

Hybrid superconductor(S)-semiconductor(Sm) devices bring a range of new functionalities into superconducting circuits. In particular, hybrid parity-protected qubits and Josephson diodes were recently proposed and experimentally demonstrated. Such devices leverage the non-sinusoidal character of the Josephson current-phase relation (CPR) in highly transparent S-Sm-S junctions. Here we report an experimental study of superconducting quantum-interference devices (SQUIDs) embedding Josephson field-effect transistors fabricated from a SiGe/Ge/SiGe heterostructure grown on a 200-mm silicon wafer. The single-junction CPR shows up to three harmonics with gate tunable amplitude. In the presence of microwave irradiation, the ratio of the first two dominant harmonics, corresponding to single and double Cooper-pair transport processes, is consistently reflected in relative weight of integer and half-integer Shapiro steps. A combination of magnetic-flux and gate-voltage control enables tuning the SQUID functionality from a nonreciprocal Josephson-diode regime with 27% asymmetry to a $\pi$-periodic Josephson regime suitable for the implementation of parity-protected superconducting qubits. These results illustrate the potential of Ge-based hybrid devices as versatile and scalable building blocks of novel superconducting quantum circuits.