Characterisation and optimisation of superconducting microwave resonators

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dc.contributor.authorBoehme, Thijs
dc.contributor.departmentChalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskapsv
dc.contributor.departmentChalmers University of Technology / Department of Microtechnology and Nanoscienceen
dc.date.accessioned2019-07-03T13:55:45Z
dc.date.available2019-07-03T13:55:45Z
dc.date.issued2016
dc.description.abstractIn this master's thesis, I present the design, fabrication, and characterisation of superconducting microwave resonators. These resonators are quarter-wavelength coplanar waveguide resonators, operating at resonance frequencies between 4 GHz and 8 GHz. This kind of resonator is a key component in several applications, ranging from quantum computation devices to parametric amplifiers. Minimising the losses (achieving high internal quality factors) in these resonators is important to maximise performance of the applications they are used for. During the thesis work, we have studied several possible changes to improve the resonator quality. Fabrication of the resonators was done in the Nanofabrication Laboratory at Chalmers University of Technology, and consist of the following steps: cleaning a sapphire or silicon wafer, depositing a layer of superconducting niobium, photolithography, and etching the waveguide. We then measured the resonators in a cryogen-free dilution refrigerator, at a temperature of 10 mK. The measurements consisted of a power sweep, in order to quantify the quality factor at high and low excitation powers. As an example, the first resonator we fabricated had a quality factor of 4.3 x 10^4 at high power (about 10^5 photons in the resonator), and quality factor of 3.7 x 10^4 at the single-photon level. We then investigated different loss mechanisms that we expect to negatively affect the quality factor, and how to reduce this loss. Firstly, to reduce radiation losses and magnetic vortex losses, we reduced the lateral dimensions of the resonator. Secondly, as two-level system (TLS) losses are believed to mainly reside at the surfaces of the materials and interfaces between them, we etched into the substrate to selectively move a part of the TLS away from the resonator. We also studied the effect of the meandering of the resonator on the quality factor, as well as using two different etching gasses. By gathering all this data, we were then able to compare the different combinations to see the effect of the changing parameters. In conclusion, the reduced lateral dimensions and the removal of part of the substrate indeed reduce the losses, as expected from the theory. We also found interesting behaviour with regards to the meandering, namely that having a meandered structure negatively affects the quality factor. Combining all results, we can give a guideline for the fabrication of resonators in the future, which gave us a quality factor up to 4.9 x 10^5 at high excitation power, and 3.0 x 10^5 at the single photon level.
dc.identifier.urihttps://hdl.handle.net/20.500.12380/238574
dc.language.isoeng
dc.setspec.uppsokPhysicsChemistryMaths
dc.subjectSupraledning
dc.subjectLågtemperaturfysik
dc.subjectElektronik
dc.subjectNanoteknik
dc.subjectNanovetenskap och nanoteknik
dc.subjectSuperconductivity
dc.subjectLow temperature physics
dc.subjectElectronics
dc.subjectNano Technology
dc.subjectNanoscience & Nanotechnology
dc.titleCharacterisation and optimisation of superconducting microwave resonators
dc.type.degreeExamensarbete för masterexamensv
dc.type.degreeMaster Thesisen
dc.type.uppsokH
local.programmeNanotechnology (MPNAT), MSc
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