Control and Dispersive Readout of Si/SiGe~Quantum Dots Using Atomic Force Microscopy

Abstract

Silicon has the potential to be a leading platform for hosting a large-scale quantum processor. Recently, milestones achieved in two-qubit gate fidelities and the demonstration of cavity-mediated coupling between spins have strengthened the case for using Si/SiGe heterostructures. The essential part of any spin-qubit operation is spin readout, usually realized through all-electrical spin-to-charge conversion. The search for scalable readout solutions has recently led to the development of the in-situ dispersive technique, where the same gate is used for the formation and readout of the quantum dot. The combination of high-quality accumulation mode Si/SiGe and in-situ readout may make possible a novel scanning gate technique - a movable quantum dot (QD). In traditional gate-defined devices, the QD is induced strictly under the gates and cannot be reconfigured. However, using the tip of an atomic force microscope (AFM) as a movable plunger gate with in-situ readout can break this paradigm. The tip-induced quantum dot can enable experiments like wafer-scale valley spectroscopy and shuttling of the charge and spin in arbitrary directions. This thesis demonstrates the necessary ingredients for creating a movable quantum dot, including coupling the AFM tip to the 2DEG and performing the tip-based in-situ dispersive charge readout. We first performed classic scanning gate microscopy experiments and demonstrated that the tip can drive photon-assisted charge-qubit transitions. To increase the dot-to-tip coupling, we introduced the floating sub-micron gates, which are disconnected from room electronics. The small size of these gates allowed us to study single electron retention of the locked charge, providing insight into interlayer dynamics in overlapping architecture, which was previously inaccessible. Finally, we demonstrated tip-based dispersive charge readout with a signal-to-noise ratio comparable to the conventional state-of-the-art Si devices but without the parasitic background charge accumulation.