--- _id: '14622' alternative_title: - ISTA Thesis article_processing_charge: No author: - first_name: Stefan full_name: Sack, Stefan id: dd622248-f6e0-11ea-865d-ce382a1c81a5 last_name: Sack orcid: 0000-0001-5400-8508 citation: ama: 'Sack S. Improving variational quantum algorithms: Innovative initialization techniques and extensions to qudit systems. 2023. doi:10.15479/at:ista:14622' apa: 'Sack, S. (2023). Improving variational quantum algorithms: Innovative initialization techniques and extensions to qudit systems. Institute of Science and Technology Austria. https://doi.org/10.15479/at:ista:14622' chicago: 'Sack, Stefan. “Improving Variational Quantum Algorithms: Innovative Initialization Techniques and Extensions to Qudit Systems.” Institute of Science and Technology Austria, 2023. https://doi.org/10.15479/at:ista:14622.' ieee: 'S. Sack, “Improving variational quantum algorithms: Innovative initialization techniques and extensions to qudit systems,” Institute of Science and Technology Austria, 2023.' ista: 'Sack S. 2023. Improving variational quantum algorithms: Innovative initialization techniques and extensions to qudit systems. Institute of Science and Technology Austria.' mla: 'Sack, Stefan. Improving Variational Quantum Algorithms: Innovative Initialization Techniques and Extensions to Qudit Systems. Institute of Science and Technology Austria, 2023, doi:10.15479/at:ista:14622.' short: 'S. Sack, Improving Variational Quantum Algorithms: Innovative Initialization Techniques and Extensions to Qudit Systems, Institute of Science and Technology Austria, 2023.' date_created: 2023-11-28T10:58:13Z date_published: 2023-11-30T00:00:00Z date_updated: 2023-12-13T14:47:25Z day: '30' ddc: - '530' degree_awarded: PhD department: - _id: GradSch - _id: MaSe doi: 10.15479/at:ista:14622 ec_funded: 1 file: - access_level: closed checksum: 068fd3570506ec42b2faa390de784bc4 content_type: application/pdf creator: ssack date_created: 2023-11-30T15:53:10Z date_updated: 2023-12-01T11:10:46Z embargo: 2024-11-30 embargo_to: open_access file_id: '14635' file_name: PhD_Thesis.pdf file_size: 11947523 relation: main_file - access_level: closed checksum: 0fa3bc0d108aed0ac59d2c6beef2220a content_type: application/zip creator: ssack date_created: 2023-11-30T15:54:11Z date_updated: 2023-12-01T11:10:46Z file_id: '14636' file_name: PhD Thesis (1).zip file_size: 18422964 relation: source_file file_date_updated: 2023-12-01T11:10:46Z has_accepted_license: '1' language: - iso: eng month: '11' oa_version: Published Version page: '142' project: - _id: bd660c93-d553-11ed-ba76-fb0fb6f49c0d name: Quantum_Quantum Circuits and Software_Variational quantum algorithms on NISQ devices - _id: 23841C26-32DE-11EA-91FC-C7463DDC885E call_identifier: H2020 grant_number: '850899' name: 'Non-Ergodic Quantum Matter: Universality, Dynamics and Control' publication_identifier: issn: - 2663 - 337X publication_status: published publisher: Institute of Science and Technology Austria related_material: record: - id: '11471' relation: part_of_dissertation status: public - id: '13125' relation: part_of_dissertation status: public - id: '9760' relation: part_of_dissertation status: public status: public supervisor: - first_name: Maksym full_name: Serbyn, Maksym id: 47809E7E-F248-11E8-B48F-1D18A9856A87 last_name: Serbyn orcid: 0000-0002-2399-5827 title: 'Improving variational quantum algorithms: Innovative initialization techniques and extensions to qudit systems' tmp: image: /images/cc_by_nc_sa.png legal_code_url: https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode name: Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) short: CC BY-NC-SA (4.0) type: dissertation user_id: 8b945eb4-e2f2-11eb-945a-df72226e66a9 year: '2023' ... --- _id: '12732' abstract: - lang: eng text: "Nonergodic systems, whose out-of-equilibrium dynamics fail to thermalize, provide a fascinating research direction both for fundamental reasons and for application in state of the art quantum devices.\r\nGoing beyond the description of statistical mechanics, ergodicity breaking yields a new paradigm in quantum many-body physics, introducing novel phases of matter with no counterpart at equilibrium.\r\nIn this Thesis, we address different open questions in the field, focusing on disorder-induced many-body localization (MBL) and on weak ergodicity breaking in kinetically constrained models.\r\nIn particular, we contribute to the debate about transport in kinetically constrained models, studying the effect of $U(1)$ conservation and inversion-symmetry breaking in a family of quantum East models.\r\nUsing tensor network techniques, we analyze the dynamics of large MBL systems beyond the limit of exact numerical methods.\r\nIn this setting, we approach the debated topic of the coexistence of localized and thermal eigenstates separated by energy thresholds known as many-body mobility edges.\r\nInspired by recent experiments, our work further investigates the localization of a small bath induced by the coupling to a large localized chain, the so-called MBL proximity effect.\r\n\r\nIn the first Chapter, we introduce a family of particle-conserving kinetically constrained models, inspired by the quantum East model.\r\nThe system we study features strong inversion-symmetry breaking, due to the nature of the correlated hopping.\r\nWe show that these models host so-called quantum Hilbert space fragmentation, consisting of disconnected subsectors in an entangled basis, and further provide an analytical description of this phenomenon.\r\nWe further probe its effect on dynamics of simple product states, showing revivals in fidelity and local observalbes.\r\nThe study of dynamics within the largest subsector reveals an anomalous transient superdiffusive behavior crossing over to slow logarithmic dynamics at later times.\r\nThis work suggests that particle conserving constrained models with inversion-symmetry breaking realize new universality classes of dynamics and invite their further theoretical and experimental studies.\r\n\r\nNext, we use kinetic constraints and disorder to design a model with many-body mobility edges in particle density.\r\nThis feature allows to study the dynamics of localized and thermal states in large systems beyond the limitations of previous studies.\r\nThe time-evolution shows typical signatures of localization at small densities, replaced by thermal behavior at larger densities.\r\nOur results provide evidence in favor of the stability of many-body mobility edges, which was recently challenged by a theoretical argument.\r\nTo support our findings, we probe the mechanism proposed as a cause of delocalization in many-body localized systems with mobility edges suggesting its ineffectiveness in the model studied.\r\n\r\nIn the last Chapter of this Thesis, we address the topic of many-body localization proximity effect.\r\nWe study a model inspired by recent experiments, featuring Anderson localized coupled to a small bath of free hard-core bosons.\r\nThe interaction among the two particle species results in non-trivial dynamics, which we probe using tensor network techniques.\r\nOur simulations show convincing evidence of many-body localization proximity effect when the bath is composed by a single free particle and interactions are strong.\r\nWe furthter observe an anomalous entanglement dynamics, which we explain through a phenomenological theory.\r\nFinally, we extract highly excited eigenstates of large systems, providing supplementary evidence in favor of our findings." acknowledged_ssus: - _id: ScienComp alternative_title: - ISTA Thesis article_processing_charge: No author: - first_name: Pietro full_name: Brighi, Pietro id: 4115AF5C-F248-11E8-B48F-1D18A9856A87 last_name: Brighi orcid: 0000-0002-7969-2729 citation: ama: Brighi P. Ergodicity breaking in disordered and kinetically constrained quantum many-body systems. 2023. doi:10.15479/at:ista:12732 apa: Brighi, P. (2023). Ergodicity breaking in disordered and kinetically constrained quantum many-body systems. Institute of Science and Technology Austria. https://doi.org/10.15479/at:ista:12732 chicago: Brighi, Pietro. “Ergodicity Breaking in Disordered and Kinetically Constrained Quantum Many-Body Systems.” Institute of Science and Technology Austria, 2023. https://doi.org/10.15479/at:ista:12732. ieee: P. Brighi, “Ergodicity breaking in disordered and kinetically constrained quantum many-body systems,” Institute of Science and Technology Austria, 2023. ista: Brighi P. 2023. Ergodicity breaking in disordered and kinetically constrained quantum many-body systems. Institute of Science and Technology Austria. mla: Brighi, Pietro. Ergodicity Breaking in Disordered and Kinetically Constrained Quantum Many-Body Systems. Institute of Science and Technology Austria, 2023, doi:10.15479/at:ista:12732. short: P. Brighi, Ergodicity Breaking in Disordered and Kinetically Constrained Quantum Many-Body Systems, Institute of Science and Technology Austria, 2023. date_created: 2023-03-17T13:30:48Z date_published: 2023-03-21T00:00:00Z date_updated: 2023-09-20T10:44:12Z day: '21' ddc: - '530' degree_awarded: PhD department: - _id: GradSch - _id: MaSe doi: 10.15479/at:ista:12732 ec_funded: 1 file: - access_level: closed checksum: 5d2de651ef9449c1b8dc27148ca74777 content_type: application/zip creator: pbrighi date_created: 2023-03-23T16:42:56Z date_updated: 2023-03-23T16:42:56Z file_id: '12753' file_name: Thesis_sub_PBrighi.zip file_size: 42167561 relation: source_file - access_level: open_access checksum: 7caa153d4a5b0873a79358787d2dfe1e content_type: application/pdf creator: pbrighi date_created: 2023-03-23T16:43:14Z date_updated: 2023-03-23T16:43:14Z file_id: '12754' file_name: Thesis_PBrighi.pdf file_size: 13977000 relation: main_file success: 1 file_date_updated: 2023-03-23T16:43:14Z has_accepted_license: '1' language: - iso: eng month: '03' oa: 1 oa_version: None page: '158' project: - _id: 23841C26-32DE-11EA-91FC-C7463DDC885E call_identifier: H2020 grant_number: '850899' name: 'Non-Ergodic Quantum Matter: Universality, Dynamics and Control' publication_identifier: issn: - 2663-337X publication_status: published publisher: Institute of Science and Technology Austria related_material: record: - id: '11470' relation: part_of_dissertation status: public - id: '8308' relation: part_of_dissertation status: public - id: '11469' relation: part_of_dissertation status: public - id: '12750' relation: part_of_dissertation status: public status: public supervisor: - first_name: Maksym full_name: Serbyn, Maksym id: 47809E7E-F248-11E8-B48F-1D18A9856A87 last_name: Serbyn orcid: 0000-0002-2399-5827 title: Ergodicity breaking in disordered and kinetically constrained quantum many-body systems tmp: image: /images/cc_by_nc_sa.png legal_code_url: https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode name: Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) short: CC BY-NC-SA (4.0) type: dissertation user_id: 8b945eb4-e2f2-11eb-945a-df72226e66a9 year: '2023' ...