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Title:Study of the effects of surface cladding on 3D transmon
Author(s):Yoscovits, Zachary
Director of Research:Eckstein, James N.
Doctoral Committee Chair(s):Bezryadin, Alexey
Doctoral Committee Member(s):Eckstein, James N.; Grosse Perdekamp, Matthias; Vishveshwara, Smitha
Department / Program:Physics
Discipline:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):qubits
superconductor
quantum computing
transmons
Abstract:Quantum computing is a very interesting field, due to the ability of quantum computers to solve many problems much faster than a classical computer. Superconducting qubits are electronic circuits composed of superconducting capacitors, inductors and Josephson junctions, which can implement a physical qubit. However they need improvements in their coherence time to create a viable quantum computing. In this work I study the effect on decoherence caused by two level systems in the native oxide that forms on the surface of qubit. To this end I fabricate 3D transmon qubits using materials grown my molecular beam epitaxy, to which a variety of different surface treatments had been applied. I began by fabricating qubits from niobium/aluminum oxide/niobium trilayers. To this end I developed a self-aligned process for fabricating sub-micron Josephson junctions. This process presented many challenges. During the development of this process, it became clear that niobium was an inferior material for fabricating qubits compared to aluminum. I then switched to making qubits from Aluminum. I began by studying the growth of aluminum on sapphire, and was able to achieve aluminum films with an RMS roughness of 0.2 nm by growing on c-plane sapphire that had been annealed in oxygen at 1100 °C and dipped in BOE. Next I fabricated 3D transmon qubits by adapting the standard shadowmask process for use with MBE. I fabricated qubits with a long in situ oxidation to fully passivate the surface before exposure to air. I also passivated the surface by means of growing co deposited aluminum oxide, and by grown aluminum nitride using a nitrogen plasma source. The coherence times of these qubits were compared to those a control sample that had been exposed to air immediately after growth. Overall it doesn’t appear that cladding the surface changes the coherence time much, however it is difficult to form conclusions with this small sample size. The coated samples appeared to have slightly longer coherence times than the untreated samples, with diffusive oxidation and coating with aluminum oxide providing the largest increase in T1 and diffusive oxidation providing the largest increase in T2.
Issue Date:2014-09-16
URI:http://hdl.handle.net/2142/50678
Rights Information:Copyright 2014 Zachary Yoscovits
Date Available in IDEALS:2014-09-16
Date Deposited:2014-08


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