Strong Charge-Photon Coupling in Planar Germanium Enabled by Granular Aluminium Superinductors

TL;DR

This paper demonstrates strong charge-photon coupling in a planar germanium system using granular aluminium superinductors, enabled by a novel in situ measurement technique for controlling inductance during fabrication.

Contribution

It introduces a method to reliably fabricate high-impedance granular aluminium resonators with controlled inductance, enabling strong coupling with germanium quantum dots.

Findings

Achieved charge-photon coupling rate of 566 MHz.

Reproducible fabrication of high-impedance circuits exceeding 13 kΩ.

Enabled in situ control of kinetic inductance during film deposition.

Abstract

High kinetic inductance superconductors are gaining increasing interest for the realisation of qubits, amplifiers and detectors. Moreover, thanks to their high impedance, quantum buses made of such materials enable large zero-point fluctuations of the voltage, boosting the coupling rates to spin and charge qubits. However, fully exploiting the potential of disordered or granular superconductors is challenging, as their inductance and, therefore, impedance at high values are difficult to control. Here we have integrated a granular aluminium resonator, having a characteristic impedance exceeding the resistance quantum, with a germanium double quantum dot and demonstrate strong charge-photon coupling with a rate of $g_\text{c}/2\pi= (566 \pm 2)$ MHz. This was achieved due to the realisation of a wireless ohmmeter, which allows \emph{in situ} measurements during film deposition and, therefore, control of the kinetic inductance of granular aluminium films. Reproducible fabrication of circuits with impedances (inductances) exceeding 13 k$\Omega$ (1 nH per square) is now possible. This broadly applicable method opens the path for novel qubits and high-fidelity, long-distance two-qubit gates.