Talk with prof. Dimi Culcer

Title: Charge noise and inhomogeneous strain effects on the operation of Ge hole spin qubits

Abstract: Ge hole quantum dots (QDs) are actively studied with the goal of establishing all-electrical quantum information processing platforms. However, their coherence properties are incompletely understood, particularly their interaction with background charge fluctuations.

I will discuss the dephasing rate of a planar hole qubit under the influence of random telegraph noise (RTN) and 1/f noise, studying defects lying in the plane of the dot as well as above the dot, and introduce symmetry arguments and a simplified Schrieffer-Wolff picture to elucidate the interaction between the qubit and arbitrary RTN sources and their dependence on system parameters.

Our main findings are as follows: (i) we demonstrate that the defect potential induces dephasing already at the level of the 2D hole gas, identifying terms that are independent of the specifics of in-plane confinement; (ii) we find that, at the low gate fields used in experiments, the dephasing rate does not change significantly with the gate field; (iii) the dephasing rate is minimized when the charge defect is in the plane and the magnetic field is out of the plane, but increases considerably when the magnetic field is in the plane, particularly along the axis connecting the charge defect and the axis perpendicular to it; (iv) the orbital magnetic terms dominate dephasing in all configurations, except when the defect and magnetic field are both in the plane, in this case, both the Zeeman and orbital magnetic terms play significant roles; (v) we observe an anisotropy in the dephasing rate as a function of the azimuthal magnetic field angle, which is especially striking for in-plane defects.

Many of our results can be explained qualitatively by the theory of invariants, which I will discuss if time allows.

In the second part of the talk I will discuss the effects of random alloy disorder and gate-induced strain on the operation of planar Ge hole spin qubits, including their coherence, tying in with the picture of dephasing introduce in the first part.

We use the atomistic valence force field (VFF) method to compute the strain due to random alloy disorder, and thermal expansion models in COMSOL Multiphysics to obtain the strain from a realistic gate-stack of planar hole quantum dot confinement.

Our hybrid approach to realistic device modeling suggests that strain inhomogeneity due to both random alloy disorder and gate-induced strain make a strong contribution to the linear-k Dresselhaus spin-orbit coupling, which eventually dominates hole spin EDSR; and there exist specific in-plane orientations of the global magnetic field and the microwave drive for maximum EDSR Rabi frequency.

The current model including strain inhomogeneity accurately predicts the EDSR Rabi frequency to be 100 MHz for typical electric and magnetic fields in experiments, which represents at least an order of magnitude improvement in accuracy over phenomenological models assuming uniform uniaxial strain. State-of-the-art atomistic tight binding calculations via nano-electronic modeling (NEMO3D) are in agreement with the k.p description.