Optimal operation points for ultrafast, highly coherent Ge hole spin-orbit qubits

TL;DR

This paper demonstrates that Ge hole spin qubits have optimal operation points where they are both fast and long-lived, due to symmetry and spin-orbit effects, challenging the idea that speed reduces coherence.

Contribution

It identifies and explains the existence of optimal electric field points for Ge hole spin qubits that maximize coherence and speed, leveraging crystal symmetry and spin-orbit interactions.

Findings

Dephasing rate vanishes at optimal points.

Maximized electron dipole spin resonance strength.

Relaxation is reduced at small magnetic fields.

Abstract

Strong spin-orbit interactions make hole quantum dots central to the quest for electrical spin qubit manipulation enabling fast, low-power, scalable quantum computation. Yet it is important to establish to what extent spin-orbit coupling exposes qubits to electrical noise, facilitating decoherence. Here, taking Ge as an example, we show that group IV gate-defined hole spin qubits generically exhibit optimal operation points, defined by the top gate electric field, at which they are both fast and long-lived: the dephasing rate vanishes to first order in electric field noise along all directions in space, the electron dipole spin resonance strength is maximised, while relaxation is drastically reduced at small magnetic fields. The existence of optimal operation points is traced to group IV crystal symmetry and properties of the Rashba spin-orbit interaction unique to spin-3/2 systems. Our results overturn the conventional wisdom that fast operation implies reduced lifetimes, and suggest group IV hole spin qubits as ideal platforms for ultra-fast, highly coherent scalable quantum computing.