PhD Defense: Anders Kringhøj

Exploring the Semiconducting Josephson Junction of Nanowire-based Superconducting Qubits

This thesis investigates superconducting qubits based on proximitized InAs/Al nanowires. These qubits consist of semiconducting Josephson junctions, and present a gate tunable derivative of the transmon qubit. Beyond the gateable nature, this new qubit (the gatemon) exhibits fundamentally different characteristics depending on operating regime, which is the main focus of this thesis. 

First, a systematic investigation of gatemon anharmonicity is presented. Here, we observe a deviation from the traditional transmon result. To explain this, we derive a simple model yielding information about the transmission properties of the semiconducting Josephson junction. In conclusion we find that the junction is dominated by 1-3 conduction channels with at least one channel reaching transmission probabilities greater than 0.9 certain gate voltages, in clear contrast to the sinusoidal energy phase relations that describe conventional transmon junctions.

Next, we present a new gatemon design, where a semiconducting region is operated as a field-effect-transistor to allow transport through the gatemon device without introducing a new dominant relaxation source. In addition, we demonstrate clear correlation between transport and transitional circuit quantum electrodynamics qubit measurements. In this geometry, for certain gate voltage, we observe resonant features in the qubit spectrum, both in transport and qubit measurements. Across the resonances, we carefully map the charge dispersion, which, at resonance, shows clear suppression orders of magnitude beyond what is traditionally expected. We explain this by an almost perfectly transmitting conduction channel, which renormalizes the charge of the superconducting island. This is in quantitative agreement with a developed resonant tunneling model, where the large transmission is achieved by a resonant level with nearly symmetric tunnel barriers.

Finally, we demonstrate compatibility with operation in large magnetic fields and the destructive Little-Parks regime. As we enter the first lobe of the oscillating qubit spectrum, we observe the emergence of additional coherent energy transitions. We explain these as transitions between Andreev states, which experience a path-dependent phase difference across the Josephson junction due to the phase twists associated with the Little-Parks effect. These observations are in qualitative agreement with numerical junction model.