FPO - Seong Woo Oh - "Cryogen-Free Scanning Probe Microscopy in Silicon Device Physics at milli-Kelvin temperatures"

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. In silicon qubit devices, the splitting of the lowest lying valley states has shown to often interfere with the spin-based two level system by inhibiting proper spin initialization and spin dependent tunneling readout routines. The valley states, when their energy is comparable to the Zeeman splitting, have also shown to often lead to spin-relaxation hotspots, which significantly increases the spin relaxation rate. Valley physics has been somewhat of a mystery, as the energy splitting reported has differed widely from device to device (25 − 300 µeV). Given the high cost of device fabrication and valley physics measurement techniques limited to using on-chip gates, high throughput data acquisition with spatial resolution has been difficult to achieve. To tackle this problem, there has been a theory proposal of using a scanning probe tip, which consists of a metallic gate wire and a microwave tank circuit, to induce and dispersively characterize a quantum dot in a non-invasive manner. This measurement approach would give us the ability to rapidly acquire spatially resolved valley splitting data. Using the tip of a scanning probe to influence the energy potential of a device has been demonstrated in the past, albeit not on Si/SiGe devices. Given the screening effect of the overlapping gate structure the Petta lab is well known for, it was uncertain whether the technique would be applicable to Si/SiGe devices with overlapping iii gates. 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. We have shown the technical feasibility of the the proposal’s vision of the scanning probe tip. Given the scanning probe sensor being a quartz crystal tuning fork, 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.