{"id":{"repo_id":"uiuc","oai_identifier":"oai:www.ideals.illinois.edu:2142/78283"},"canonical_url":"https://search.dev.ndltd.org/etd/uiuc/oai:www.ideals.illinois.edu:2142/78283","repository":{"repo_id":"uiuc","name":"University of Illinois - Urbana-Champaign","base_url":"https://www.ideals.illinois.edu/oai-pmh"},"display":{"title":"Low frequency critical current noise and two level system defects in Josephson junctions","abstract":"The critical current in a Josephson junction is known to exhibit a 1/f low frequency noise. Implemented as a superconducting qubit, this low frequency noise can lead to decoherence. While the 1/f noise has been known to arise from an ensemble of two level systems connected to the tunnel barrier, the precise microscopic nature of these TLSs remain a mystery. In this thesis we will present measurements of the 1/f low frequency noise in the critical current and tunneling resistance of Al-AlOx-Al Josephson junctions. Measurements in a wide range of resistively shunted and unshunted junctions confirm the equality of critical current and tunneling resistance noise. That is the critical current fluctuation corresponds to fluctuations of the tunneling resistance. In not too small Al-AlOx-Al junctions we have found that the fractional power spectral density scales linearly with temperature. We confirmed that the 1/f power spectrum is the result of a large number of two level systems modulating the tunneling resistance. At small junction areas and low temperatures, the number of thermally active TLSs is insufficient to integrate out a featureless 1/f spectral shape. By analyzing the spectral variance in small junction areas, we have been able to deduce the TLS defect density, n ~ 2.53 per micrometer squared per Kelvin spread in the TLS energy per factor e in the TLS lifetimes. This density is consistent with the density of tunneling TLSs found in glassy insulators, as well as the density deduced from coherent TLSs interacting at qubit frequencies. The deduced TLS density combined with the magnitude of the 1/f power spectral density in large area junctions, gives an average TLS effective area, A ~ 0.3 nanometer squared. In ultra small tunnel junctions, we have studied the time-domain dynamics of isolated TLSs. We have found a TLS whose dynamics is described by the quantum tunneling between the two localized wells, and a one-phonon absorption/emission switching rate. From the quantum limiting rate and the WKB approximation, we estimated that the TLS has a mass and tunneling distance product consistent with an atomic mass tunneling through crystal lattice distances. At higher temperatures TLSs have been found that obey a simple thermal activation dynamics. By analyzing the TLS response to an external electric field, we have deduced that the TLS electric dipole is in the order of, P ~ 1 electron-Angstrom, consistent with the TLS having the charge of one electron tunneling through a disorder potential of distances, d ~ 1 Angstrom.","abstract_html":"The critical current in a Josephson junction is known to exhibit a 1/f low frequency noise. Implemented as a superconducting qubit, this low frequency noise can lead to decoherence. While the 1/f noise has been known to arise from an ensemble of two level systems connected to the tunnel barrier, the precise microscopic nature of these TLSs remain a mystery. In this thesis we will present measurements of the 1/f low frequency noise in the critical current and tunneling resistance of Al-AlOx-Al Josephson junctions. Measurements in a wide range of resistively shunted and unshunted junctions confirm the equality of critical current and tunneling resistance noise. That is the critical current fluctuation corresponds to fluctuations of the tunneling resistance. In not too small Al-AlOx-Al junctions we have found that the fractional power spectral density scales linearly with temperature. We confirmed that the 1/f power spectrum is the result of a large number of two level systems modulating the tunneling resistance. At small junction areas and low temperatures, the number of thermally active TLSs is insufficient to integrate out a featureless 1/f spectral shape. By analyzing the spectral variance in small junction areas, we have been able to deduce the TLS defect density, n ~ 2.53 per micrometer squared per Kelvin spread in the TLS energy per factor e in the TLS lifetimes. This density is consistent with the density of tunneling TLSs found in glassy insulators, as well as the density deduced from coherent TLSs interacting at qubit frequencies. The deduced TLS density combined with the magnitude of the 1/f power spectral density in large area junctions, gives an average TLS effective area, A ~ 0.3 nanometer squared. In ultra small tunnel junctions, we have studied the time-domain dynamics of isolated TLSs. We have found a TLS whose dynamics is described by the quantum tunneling between the two localized wells, and a one-phonon absorption/emission switching rate. From the quantum limiting rate and the WKB approximation, we estimated that the TLS has a mass and tunneling distance product consistent with an atomic mass tunneling through crystal lattice distances. At higher temperatures TLSs have been found that obey a simple thermal activation dynamics. By analyzing the TLS response to an external electric field, we have deduced that the TLS electric dipole is in the order of, P ~ 1 electron-Angstrom, consistent with the TLS having the charge of one electron tunneling through a disorder potential of distances, d ~ 1 Angstrom.","abstract_has_math":false,"creators":["Nugroho, Christopher"],"institution":"University of Illinois at Urbana-Champaign","degree_name":"Ph.D.","degree_level":"Dissertation","degree_discipline":"Physics","degree_department":null,"school":null,"contributors":["Van Harlingen, Dale J.","Eckstein, James N.","Weissman, Michael B.","Neubauer, Mark"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2015,"date_issued":"2015-07-22T22:15:23Z","date_published":"2015-07-22T22:15:23Z","updated_at":"2026-07-22T22:26:11Z","subjects":["quantum bits","superconducting qubits","individual two level systems","decoherence","dephasing","qubit","resistance noise","critical current noise","noise","tunneling two level systems","Two level systems"],"languages":[],"rights":["Copyright 2014 Christopher Daniel Nugroho"],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"http://hdl.handle.net/2142/78283","outbound_label":"Handle","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Van Harlingen, Dale J.","Eckstein, James N.","Weissman, Michael B.","Neubauer, Mark"]},{"key":"dc:creator","label":"Author","values":["Nugroho, Christopher"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date","label":"Dc Date","values":["2015-07-22T22:15:23Z","2015-05","2015-01-06","2015-5"]},{"key":"dc:type","label":"Dc Type","values":["text"]},{"key":"thesis:degree_discipline","label":"Discipline","values":["Physics"]},{"key":"thesis:degree_level","label":"Degree Level","values":["Dissertation"]},{"key":"thesis:degree_name","label":"Degree Name","values":["Ph.D."]},{"key":"thesis:institution_name","label":"Thesis Institution Name","values":["University of Illinois at Urbana-Champaign"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["quantum bits","superconducting qubits","individual two level systems","decoherence","dephasing","qubit","resistance noise","critical current noise","noise","tunneling two level systems","Two level systems"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:rights","label":"Dc Rights","values":["Copyright 2014 Christopher Daniel Nugroho"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["http://hdl.handle.net/2142/78283"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["The critical current in a Josephson junction is known to exhibit a 1/f low frequency noise. Implemented as a superconducting qubit, this low frequency noise can lead to decoherence. While the 1/f noise has been known to arise from an ensemble of two level systems connected to the tunnel barrier, the precise microscopic nature of these TLSs remain a mystery. In this thesis we will present measurements of the 1/f low frequency noise in the critical current and tunneling resistance of Al-AlOx-Al Josephson junctions. Measurements in a wide range of resistively shunted and unshunted junctions confirm the equality of critical current and tunneling resistance noise. That is the critical current fluctuation corresponds to fluctuations of the tunneling resistance. In not too small Al-AlOx-Al junctions we have found that the fractional power spectral density scales linearly with temperature. We confirmed that the 1/f power spectrum is the result of a large number of two level systems modulating the tunneling resistance. At small junction areas and low temperatures, the number of thermally active TLSs is insufficient to integrate out a featureless 1/f spectral shape. By analyzing the spectral variance in small junction areas, we have been able to deduce the TLS defect density, n ~ 2.53 per micrometer squared per Kelvin spread in the TLS energy per factor e in the TLS lifetimes. This density is consistent with the density of tunneling TLSs found in glassy insulators, as well as the density deduced from coherent TLSs interacting at qubit frequencies. The deduced TLS density combined with the magnitude of the 1/f power spectral density in large area junctions, gives an average TLS effective area, A ~ 0.3 nanometer squared. In ultra small tunnel junctions, we have studied the time-domain dynamics of isolated TLSs. We have found a TLS whose dynamics is described by the quantum tunneling between the two localized wells, and a one-phonon absorption/emission switching rate. From the quantum limiting rate and the WKB approximation, we estimated that the TLS has a mass and tunneling distance product consistent with an atomic mass tunneling through crystal lattice distances. At higher temperatures TLSs have been found that obey a simple thermal activation dynamics. By analyzing the TLS response to an external electric field, we have deduced that the TLS electric dipole is in the order of, P ~ 1 electron-Angstrom, consistent with the TLS having the charge of one electron tunneling through a disorder potential of distances, d ~ 1 Angstrom.","Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2015-07-22 without embargo terms","The student, Christopher Nugroho, accepted the attached license on 2014-12-11 at 16:56.","The student, Christopher Nugroho, submitted this Dissertation for approval on 2015-01-06 at 00:49.","This Dissertation was approved for publication on 2015-01-06 at 13:47.","DSpace SAF Submission Ingestion Package generated from Vireo submission #7652 on 2015-07-22 at 10:29:21","Made available in DSpace on 2015-07-22T22:15:23Z (GMT). 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While the 1/f noise has been known to arise from an ensemble of two level systems connected to the tunnel barrier, the precise microscopic nature of these TLSs remain a mystery. In this thesis we will present measurements of the 1/f low frequency noise in the critical current and tunneling resistance of Al-AlOx-Al Josephson junctions. Measurements in a wide range of resistively shunted and unshunted junctions confirm the equality of critical current and tunneling resistance noise. That is the critical current fluctuation corresponds to fluctuations of the tunneling resistance. In not too small Al-AlOx-Al junctions we have found that the fractional power spectral density scales linearly with temperature. We confirmed that the 1/f power spectrum is the result of a large number of two level systems modulating the tunneling resistance. At small junction areas and low temperatures, the number of thermally active TLSs is insufficient to integrate out a featureless 1/f spectral shape. By analyzing the spectral variance in small junction areas, we have been able to deduce the TLS defect density, n ~ 2.53 per micrometer squared per Kelvin spread in the TLS energy per factor e in the TLS lifetimes. This density is consistent with the density of tunneling TLSs found in glassy insulators, as well as the density deduced from coherent TLSs interacting at qubit frequencies. The deduced TLS density combined with the magnitude of the 1/f power spectral density in large area junctions, gives an average TLS effective area, A ~ 0.3 nanometer squared. In ultra small tunnel junctions, we have studied the time-domain dynamics of isolated TLSs. We have found a TLS whose dynamics is described by the quantum tunneling between the two localized wells, and a one-phonon absorption/emission switching rate. From the quantum limiting rate and the WKB approximation, we estimated that the TLS has a mass and tunneling distance product consistent with an atomic mass tunneling through crystal lattice distances. At higher temperatures TLSs have been found that obey a simple thermal activation dynamics. By analyzing the TLS response to an external electric field, we have deduced that the TLS electric dipole is in the order of, P ~ 1 electron-Angstrom, consistent with the TLS having the charge of one electron tunneling through a disorder potential of distances, d ~ 1 Angstrom.","Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2015-07-22 without embargo terms","The student, Christopher Nugroho, accepted the attached license on 2014-12-11 at 16:56.","The student, Christopher Nugroho, submitted this Dissertation for approval on 2015-01-06 at 00:49.","This Dissertation was approved for publication on 2015-01-06 at 13:47.","DSpace SAF Submission Ingestion Package generated from Vireo submission #7652 on 2015-07-22 at 10:29:21","Made available in DSpace on 2015-07-22T22:15:23Z (GMT). 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