Gate modulation of the hole singlet-triplet qubit frequency in germanium

TL;DR

This paper demonstrates that the frequency of a hole singlet-triplet qubit in germanium quantum dots can be strongly modulated by gate voltage, revealing high sensitivity of $g$-tensors to electrostatic confinement and strain.

Contribution

It shows that $g$-tensors in germanium quantum dots are highly tunable via gate voltages, enabling electric control of qubit properties and highlighting the role of strain in this tunability.

Findings

Qubit frequency strongly depends on barrier gate voltage.

$g$-factor can be increased by an order of magnitude with small voltage change.

Strain profile influences the $g$-tensor sensitivity.

Abstract

Spin qubits in germanium gate-defined quantum dots have made considerable progress within the last few years, partially due to their strong spin-orbit coupling and site-dependent $g$-tensors. While this characteristic of the $g$-factors removes the need for micromagnets and allows for the possibility of all-electric qubit control, relying on these $g$-tensors necessitates the need to understand their sensitivity to the confinement potential that defines the quantum dots. Here, we demonstrate a $S-T\_$ qubit whose frequency is a strong function of the voltage applied to the barrier gate shared by the quantum dots. We find a $g$-factor that can be approximately increased by an order of magnitude adjusting the barrier gate voltage only by 12 mV. We attribute the strong dependence to a variable strain profile in our device. This work not only reinforces previous findings that site-dependent $g$-tensors in germanium can be utilized for qubit manipulation, but reveals the sensitivity and tunability these $g$-tensors have to the electrostatic confinement of the quantum dot.