RELAXATION TIME FLUCTUATIONS IN TRANSMONS WITH DIFFERENT SUPERCONDUCTING GAPS
| dc.contributor.advisor | Lobb, Christopher | en_US |
| dc.contributor.author | Li, Kungang | en_US |
| dc.contributor.department | Chemical Physics | en_US |
| dc.contributor.publisher | Digital Repository at the University of Maryland | en_US |
| dc.contributor.publisher | University of Maryland (College Park, Md.) | en_US |
| dc.date.accessioned | 2024-02-14T06:39:43Z | |
| dc.date.available | 2024-02-14T06:39:43Z | |
| dc.date.issued | 2023 | en_US |
| dc.description.abstract | In this thesis, I discuss the fabrication and measurement of Al/AlOx/Al transmons that have electrodes with different superconducting gaps. With gap-engineering, the tunneling of single quasiparticle from the low-gap side to the high-gap side can be suppressed, hence increasing the relaxation time T1. The best gap-engineered device showed T1 exceeding 300 μs. Large T1 fluctuations in my devices were also observed. I proposed a mechanism for exploring the T1 fluctuation data and discuss the possible underlying cause of the T1 fluctuations. I first discuss the theory of the loss in gap-engineered transmons, with a focus on the loss from non-equilibrium quasiparticles. The model yields the quasiparticle-induced loss in transmons and its dependence on temperature. I also discuss how multiple Andreev reflection (MAR) effects might alter these conclusions, leading to a further reduction in T1. I then describe the design, fabrication and basic characterization of the transmon chip SKD102, which features two transmons – one with thin-film electrodes of pure Al and another that had one electrode made from oxygen-doped Al. I next examined T1 vs temperature and how the T1 fluctuations depended on temperature. I compare my results to a simple model and find reasonable agreement in transmons on chip SKD102, KL103 and KL109, which had different electrode and layer configurations. Finally, I analyze T1 fluctuations in different devices and as a function of temperature and propose a model to explain this behavior. Over the different devices, the T1 fluctuation magnitude roughly scaled as T13/2. With increasing temperature, T1 decreases due to a higher density of thermally generated quasiparticles. In contrast, for an individual device measured from 20mK to 250 mK, the fluctuation magnitude appears to be proportional to T1. I present a model of quasiparticle dissipation channels that reproduces both of these observed scaling relationships. | en_US |
| dc.identifier | https://doi.org/10.13016/kni0-yeyn | |
| dc.identifier.uri | http://hdl.handle.net/1903/31734 | |
| dc.language.iso | en | en_US |
| dc.subject.pqcontrolled | Quantum physics | en_US |
| dc.subject.pquncontrolled | quantum computing | en_US |
| dc.subject.pquncontrolled | superconducting qubits | en_US |
| dc.subject.pquncontrolled | transmon | en_US |
| dc.title | RELAXATION TIME FLUCTUATIONS IN TRANSMONS WITH DIFFERENT SUPERCONDUCTING GAPS | en_US |
| dc.type | Dissertation | en_US |
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