Master's Defense: Casper Wied

This thesis investigates superconducting qubits using proximitised InAs nanowires with shadow defined tantalum junctions on an all tantalum control layer. The gate-tunable transmon or gatemon is based on the Superconductor-Normal-Superconductor (SNS) Josephson Junction and has been lagging behind in T1 in comparison to the state of the art transmon.

One possible avenue of improvement is exploring novel material platforms.

First, to explore the DC transport properties of the tantalum SNS junction and a proof of principle on the gate-tunability, a simple aluminium on silicon circuit was fabricated.

The nanowires were patched onto the circuit, ensuring good ohmic contact using argon ion milling. This thesis includes a guide to calibrating this technique.

The current biasing measurements showed a critical current of ∼20nA, which was gatetunable, albeit in a limited range. From this the theoretical qubit frequency was determined to lie in 2-5GHz range, due to the weak tunability. Additionally the tuning of the critical current showed a symmetric behaviour at positive and negative gate voltages. Lastly the junction showed clear hysterical behaviour in accordance with the RCSJ model.

Moreover, a tantalum dry-etching protocol was developed to ensure consistent high precision etching. It was shown that this recipe could be used to do a 20nm counter etch, ensuring that the nanowire can float above the gate.

Tantalum resonators and all tantalum gatemons were fabricated with this dry etching protocol.

The internal quality factor was show to be 0.4-0.5 x 10^6. The gatemon had a limited gate-tunable range and similar frequencies (3.50GHz and 2.75GHz) to the ones extrapolated from the DC experiments. The second device could be coherently controlled and a Rabi oscillation was measured and used to extract a pi-pulse. The Rabi oscillation showed an increasing dampening effect, likely from excitation to higher levels. Repeated T1 measurements were done demonstrating a lifetime nearing 2μs.