Optimising germanium hole spin qubits with a room-temperature magnet

Abstract Germanium spin qubits offer high-fidelity control via strong spin-orbit interaction, but their magnetic field sensitivity typically requires bulky superconducting vector magnets that limit scalability. This study investigates replacing these with an external permanent magnet located outside the cryostat to reclaim internal sample space. By operating in a hybrid mode (internal and external magnets), we fine-tuned the field to an in-plane orientation, achieving a dephasing time $${T}_{2}^{* }=13\,\upmu {{{\rm{s}}}}$$ T 2 * = 13 μ s and single-qubit Clifford gate fidelities exceeding 99.9%. Remarkably, we demonstrate qubit operation even with the internal superconducting magnet deactivated. In this regime, we observed extended coherence times of $${T}_{2}^{* }=31\,\upmu {{{\rm{s}}}}$$ T 2 * = 31 μ s and $${T}_{2}^{{{{\rm{H}}}}}=266\,\upmu {{{\rm{s}}}}$$ T 2 H = 266 μ s , on a natural Germanium heterostructure. These results demonstrate that room-temperature magnets allow for high qubit performance. This approach facilitates scaling by freeing cryogenic space for essential control circuitry and wiring, paving the way for large-scale quantum processors.