Optimising germanium hole spin qubits with a room-temperature magnet
Cécile X. Yu (TU Delft - Business Development Hospitality, TU Delft - QuTech Advanced Research Centre, Kavli institute of nanoscience Delft)
Barnaby van Straaten (TU Delft - QuTech Advanced Research Centre, Kavli institute of nanoscience Delft, TU Delft - QCD/Veldhorst Lab)
Alexander S. Ivlev (TU Delft - QCD/Veldhorst Lab, Kavli institute of nanoscience Delft, TU Delft - QuTech Advanced Research Centre)
Valentin John (TU Delft - QuTech Advanced Research Centre, Kavli institute of nanoscience Delft, TU Delft - QCD/Veldhorst Lab)
Damien R. Crielaard (Kavli institute of nanoscience Delft, TU Delft - QuTech Advanced Research Centre, TU Delft - QCD/Veldhorst Lab)
Stefan D. Oosterhout (TU Delft - Applied Sciences, TNO)
Lucas E.A. Stehouwer (TU Delft - Business Development Start-ups, Kavli institute of nanoscience Delft)
Francesco Borsoi (TU Delft - QuTech Advanced Research Centre, Kavli institute of nanoscience Delft, TU Delft - QCD/Veldhorst Lab)
Giordano Scappucci (Kavli institute of nanoscience Delft, TU Delft - QuTech Advanced Research Centre, TU Delft - Electrical Engineering, Mathematics and Computer Science, TU Delft - QCD/Scappucci Lab)
Menno Veldhorst (TU Delft - Applied Sciences, Kavli institute of nanoscience Delft, TU Delft - QCD/Veldhorst Lab, TU Delft - QuTech Advanced Research Centre)
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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 T2*=13μs 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 T2*=31μs and T2H=266μs, 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.
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