PhD defense: Zhenhai Sun

SNS Josephson Junction-Based Superconducting Qubits Revisited

Gate-tunable transmons (gatemons) are exotic superconducting qubits based on superconductorsemiconductor hybrid (SNS) Josephson junctions. The development of gatemons holds promise for exploring the rich mesoscopic physics of SNS junctions and building new types of superconducting circuits. However, to date, gatemons are limited by low relaxation times (T1), unreliability of the qubit frequency with gate voltage, and instability in time. This thesis revisits these three fundamental limitations, aiming to enhance the performance of gatemons.

First, we explore shadowed nanowire SNS junctions as a potential solution for improving gatemon relaxation times. Two types of nanowires: InAsSb/Al and InAs/Al, have been investigated. We start with DC characterization for each type, where a clear gate-tunable, reliable, and stable critical current is observed. A standard cQED measurement procedure is then performed on gatemons fabricated with shadowed SNS junctions. We show an electrostatically controllable qubit frequency and coherent manipulation of the qubit states. However, the relaxation times of these qubits are measured to be below 1 μs. Further, we conduct a comparison experiment where gatemons were fabricated with shadowed and etched junctions on the same chip and the nanowires are from the same InAs/Al nanowires batch. Two out of three etched gatemons show 2.77 μs and 4.28 μs relaxation times, respectively. We conclude that the shadowed SNSjunction technique is not a direct solution to increase gatemons relaxation times.

Next, we present a detailed analysis of the gatemon relaxation mechanisms, where internal loss, spontaneous emission, and the Purcell effect are included. We investigate three types of qubit devices to model various loss mechanisms: state-of-the-art transmons, transmons incorporating a carefully optimized gatemon design, and gatemons. In this experiment, the transmons utilize Manhattan-style SIS Josephson junctions, while the gatemons consist of etched full-shell nanowire-based SNS Josephson junctions. Our state-of-the-art transmons show around 100 μs relaxation time, approaching the best transmon T1 reported in the literature. The transmons with the optimized gatemon design exhibit around 70 μs relaxation time, whereas our best measured gatemon qubit T1 with the same design is around 10 μs. The significant relaxation time difference between the transmons with the gatemon design and the gatemons strongly indicates the existence of intrinsic loss channels yet to be discovered in the SNS Josephson junctions.

Finally, we develop characterization methods to characterize the three aforementioned gatemon fundamentals. By deploying these characterization protocols to our gatemons, we observe a strong frequency-dependent behavior of the qubit stability and reliability. The gatemons exhibit better reliability and stability at the high frequency regime. Moreover, we achieve highly reliable tuning of the qubit frequency with below 1MHz precision over a several GHz range, along with improved stability.

Together, these findings constitute a significant advance in the design, characterization protocol, and performance of gatemons and indicate potential mesoscopic loss mechanisms in SNS junctions that are yet to be fully understood.