Coherent and ultra-low-power EDSR with a flopping-mode spin qubit in germanium

Abstract: Hole spin qubits in semiconductor quantum dots (QDs) enable high-fidelity all-electric control, but conventional electric dipole spin resonance (EDSR) can require substantial rf drive power at the low magnetic fields that are favorable for qubit coherence and readout. In planar Ge hole spin qubits, this can reach -27 dBm at the device, posing challenges for scalable architectures due to heating and crosstalk. Here, we demonstrate a flopping-mode (FM) qubit in Ge, where a single spin is delocalized in a double QD, combining first-order protection against charge noise with exceptionally efficient electric driving. By mapping out coherence sweet-spots as a function of magnetic field orientation we achieve $T_2^*= 1.4μ\mathrm{s}$, $T_2^{{\mathrm{Hahn}}=} 11.5 μ\mathrm{s}$, $$T^{φ, \mathrm{CPMG32}}<em>2= 130 μ\mathrm{s}$$, and $T_1= 226 μ\mathrm{s}$, and a single-qubit gate fidelity of up to 99.76$\%$ for a gate time $t</em>{Xπ} = 88$ ns. Importantly, these results are obtained at a nearly in-plane magnetic field of 5 mT using only -52 dBm drive power at the device. We further find that qubit relaxation in this regime is consistent with a two-photon Orbach process, providing a route for further optimization. Our results demonstrate that FM-EDSR supports ultra-low-power, high-fidelity single-qubit operations, improvements that could benefit scalable hole-spin-based architectures and hybrid spin-photon interfaces.