Optimising germanium hole spin qubits with a room-temperature magnet

TL;DR

This study demonstrates that room-temperature permanent magnets can effectively control germanium hole spin qubits, achieving high fidelity and long coherence times, offering a scalable alternative to superconducting magnets for quantum computing.

Contribution

The paper introduces a hybrid magnetic field approach using external permanent magnets to optimize germanium spin qubits, enhancing scalability and performance.

Findings

Achieved a dephasing time T2* of 13 microseconds.

Obtained Hahn-echo times T2H of 88 microseconds.

Single-qubit gate fidelity exceeded 99.9%.

Abstract

Germanium spin qubits exhibit strong spin-orbit interaction, which allow for high-fidelity qubit control, but also provide a strong dependence on the magnetic field. Superconducting vector magnets are often used to minimize dephasing due to hyperfine interactions and to maximize spin control, but these compromise the sample space and thus challenge scalability. Here, we explore whether a permanent magnet outside the cryostat can be used as an alternative. Operating in a hybrid mode with an internal and external magnet, we find that we can fine-tune the magnetic field to an in-plane orientation. We obtain a qubit dephasing time T2*=13 microseconds, Hahn-echo times T2H=88 microseconds, and an average single-qubit Clifford gate fidelity above 99.9%, from which we conclude that room temperature magnets allow for high qubit performance. Furthermore, we probe the qubit resonance frequency using only the external magnet, with the internal superconducting magnet switched off. Our approach may be used to scale semiconductor qubits and use the increased sample space for the integration of cryogenic control circuitry and wiring to advance to large-scale quantum processors.