Cryogen-Free Scanning Probe Microscopy in Silicon Device Physics at milli-Kelvin temperatures

Abstract

Spin qubits housed in silicon quantum dots are rapidly emerging as a viable quantum computing platform. In recent years, there have been single- and two-qubit gate demonstrations, which showed high fidelities to the extent that implementing quantum error correction codes appears to be within reach. With the already existing industrial manufacturing infrastructure, semiconductor spin qubits are well poised to bring us ever closer to making quantum computing in everyday application a reality. There do stand a few technical obstacles in the way, however. One prominent issue has been the splitting of the two lowest lying valley states in the silicon band structure. The valley states, when their energy is comparable to the Zeeman splitting, have shown to often lead to spin-relaxation hotspots, which significantly increases the spin relaxation rate. In addition, the valley splittings have shown to vary greatly not only among different wafers but also at different locations across the same wafer. Having a scanning gate microscope to provide spatially resolved data could be key to tackling this problem. In this thesis, we report our progress toward building a scanning gate system probing silicon quantum devices. We have built and operated a custom scanning probe microscope to show that the biasing of the tip of the scanning probe could play the role of a plunger gate even on devices with overlapping gate structure. Given the scanning probe sensor being a quartz crystal tuning fork and the richness of the physics one could probe with a superconducting resonator, we have fabricated and characterized microwave resonators on a quartz crystal to show the microwave resonators can potentially be housed on the resonator to directly port all our existing microwave measurement techniques. We have also etched a quartz crystal in the shape of a tuning fork to show the feasibility of designing and fabricating a custom tuning fork for future microwave impedance measurements.