Spin qubits are considered to be among the most promising candidates for building a quantum processor 1 . Group IV hole spin qubits have moved into the focus of interest due to the ease of operation and compatibility with Si technology 2;3;4;5;6 . In addition, Ge offers the option for monolithic superconductor-semiconductor integration. Here we demonstrate a hole spin qubit operating at fields below 10 mT, the critical field of Al, by exploiting the large out-ofplane hole g-factors in planar Ge and by encoding the qubit into the singlet-triplet states of a double quantum dot 7;8 . We observe electrically controlled X and Z-rotations with tunable frequencies exceeding 100 MHz and dephasing times of 1 µs which we extend beyond 15 µs with echo techniques. These results show that Ge hole singlet triplet qubits outperform their electronic Si and GaAs based counterparts in speed and dephasing time, respectively. In addition, their rotation frequency and coherence time are on par with Ge single spin qubits, but they can be operated at much lower fields underlining their potential for on chip integration with superconducting technologies.
Pauli spin blockade (PSB) has long been an important tool for spin read-out in double quantum dot (DQD) systems with interdot tunneling t. In this paper we show that the blockade is lifted if the two dots experience distinct effective magnetic fields caused by site-dependent g-tensors g L and g R for the left and right dot, and that this effect can be more pronounced than the leakage current due to the spin–orbit interaction (SOI) via spin-flip tunneling and the hyperfine interaction (HFI) of the electron spin with the host nuclear spins. Using analytical results obtained in special parameter regimes, we show that information about both the out-of-plane and in-plane g-factors of the dots can be inferred from characteristic features of the magneto-transport curve. For a symmetric DQD, we predict a pronounced maximum in the leakage current at the characteristic out-of-plane magnetic field B * = t / μ B g z L g z R which we term the g-tensor resonance of the system. Moreover, we extend the results to contain the effects of strong SOI and argue that in this more general case the leakage current carries information about the g-tensor components and SOI of the system.
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