Hybrid circuits on planar Germanium

Thesis

Abstract

Acknowledgements

About the Author

List of Collaborators and Publications

Table of Contents

List of Figures

List of Tables

1 Motivation

2 Theoretical background

3 Measurement setup

4 Hybrid Josephson junctions

5 CPW resonators on SiGe substrate

6 Towards Ge gatemons

7 Ge Gatemons

8 Outlook and Conclusions

Bibliography

A Additional resonator data

B Effect of gate material on JJs

C Reference measurements on Si transmons

D Rabi anomalies

E Fabrication recipes

The new era of Ge has opened up new possibilities in quantum computing. The maturity of Ge spin qubits is unquestioned, while hybrid semiconductor-superconductor Ge circuits are on track to enter the game. Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for such hybrid quantum circuits. In this thesis, we present a gatemon fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.