Ultrafast hole spin qubit with gate-tunable spin-orbit switch functionality.

Quantum computers promise to execute complex tasks exponentially faster than any possible classical computer, and thus spur breakthroughs in quantum chemistry, material science and machine learning. However, quantum computers require fast and selective control of large numbers of individual qubits while maintaining coherence. Qubits based on hole spins in one-dimensional germanium/silicon nanostructures are predicted to experience an exceptionally strong yet electrically tunable spin–orbit interaction, which allows us to optimize qubit performance by switching between distinct modes of ultrafast manipulation, long coherence and individual addressability. Here we used millivolt gate voltage changes to tune the Rabi frequency of a hole spin qubit in a germanium/silicon nanowire from 31 to 219 MHz, its driven coherence time between 7 and 59 ns, and its Lande g-factor from 0.83 to 1.27. We thus demonstrated spin–orbit switch functionality, with on/off ratios of roughly seven, which could be further increased through improved gate design. Finally, we used this control to optimize our qubit further and approach the strong driving regime, with spin-flipping times as short as ~1 ns. Quantum computing requires fast and selective control of a large number of individual qubits while maintaining coherence, which is hard to achieve concomitantly. All-electrical operation of a hole spin qubit in a Ge/Si nanowire demonstrates the principle of switching from a mode of selective and fast control to idling with increased coherence.