Low-frequency spin qubit detuning noise in highly purified $^{28}$Si/SiGe

TL;DR

This study identifies charge noise as the main source of low-frequency detuning noise in highly purified $^{28}$Si/SiGe spin qubits, highlighting the importance of charge noise management and magnetic field design for improving qubit fidelity.

Contribution

The paper pinpoints charge noise as the dominant low-frequency detuning noise source in $^{28}$Si/SiGe qubits with embedded nanomagnets, providing insights for enhancing qubit performance.

Findings

Charge noise explains the transition from 1/f^2 to 1/f PSD dependence.

Spin dephasing time T2* remains unaffected by $^{73}$Ge nuclei due to frozen dynamics.

Design of magnetic field gradients and charge noise reduction are crucial for fidelity improvements.

Abstract

The manipulation fidelity of a single electron qubit gate-confined in a $^{28}$Si/SiGe quantum dot has recently been drastically improved by nuclear isotope purification. Here, we identify the dominant source for low-frequency qubit detuning noise in a device with an embedded nanomagnet, a remaining $^{29}$Si concentration of only 60$\,$ppm in the strained $^{28}$Si quantum well layer and a spin echo decay time $T_2^{\text{echo}}=128\,\mu$s. The power spectral density (PSD) of the charge noise explains both the observed transition of a $1/f^2$- to a $1/f$-dependence of the detuning noise PSD as well as the observation of a decreasing time-ensemble spin dephasing time from $T_2^* \approx 20\,\mu$s with increasing measurement time over several hours. Despite their strong hyperfine contact interaction, the few $^{73}$Ge nuclei overlapping with the quantum dot in the barrier do not limit $T_2^*$, as their dynamics is frozen on a few hours measurement scale. We conclude that charge noise and the design of the gradient magnetic field is the key to further improve the qubit fidelity.