[{"type":"dissertation","degree_awarded":"PhD","OA_place":"publisher","keyword":["Quantum computing","Variational Quantum Algorithms","Optimization"],"publication_identifier":{"issn":["2663-337X"]},"status":"public","ec_funded":1,"year":"2024","oa":1,"file":[{"file_size":"14218691","content_type":"application/zip","checksum":"6f45273d04f4418bc2adc018baed0525","file_name":"Raimel_Thesis-Final.zip","date_updated":"2024-07-10T11:34:09Z","date_created":"2024-07-09T09:21:44Z","relation":"source_file","access_level":"closed","creator":"rmedinar","file_id":"17212"},{"date_updated":"2024-07-17T09:23:24Z","file_name":"Raimel_Thesis-20_pdfa.pdf","content_type":"application/pdf","file_size":11253627,"checksum":"6724a95bec772dbabc0111b9f08a805e","date_created":"2024-07-17T09:23:24Z","relation":"main_file","access_level":"open_access","creator":"rmedinar","success":1,"file_id":"17275"}],"language":[{"iso":"eng"}],"tmp":{"name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)","short":"CC BY (4.0)","image":"/images/cc_by.png","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode"},"doi":"10.15479/at:ista:17208","has_accepted_license":"1","related_material":{"record":[{"status":"public","id":"10545","relation":"part_of_dissertation"},{"id":"10067","status":"public","relation":"part_of_dissertation"},{"id":"17222","status":"public","relation":"part_of_dissertation"},{"id":"13125","status":"public","relation":"part_of_dissertation"},{"relation":"part_of_dissertation","status":"public","id":"11471"}]},"page":"133","month":"07","department":[{"_id":"GradSch"},{"_id":"MaSe"}],"alternative_title":["ISTA Thesis"],"day":"09","_id":"17208","oa_version":"Published Version","article_processing_charge":"No","supervisor":[{"orcid":"0000-0002-2399-5827","first_name":"Maksym","last_name":"Serbyn","id":"47809E7E-F248-11E8-B48F-1D18A9856A87","full_name":"Serbyn, Maksym"}],"citation":{"ieee":"R. A. Medina Ramos, “Exploring the optimization landscape of variational quantum algorithms,” Institute of Science and Technology Austria, 2024.","short":"R.A. Medina Ramos, Exploring the Optimization Landscape of Variational Quantum Algorithms, Institute of Science and Technology Austria, 2024.","mla":"Medina Ramos, Raimel A. <i>Exploring the Optimization Landscape of Variational Quantum Algorithms</i>. Institute of Science and Technology Austria, 2024, doi:<a href=\"https://doi.org/10.15479/at:ista:17208\">10.15479/at:ista:17208</a>.","ista":"Medina Ramos RA. 2024. Exploring the optimization landscape of variational quantum algorithms. Institute of Science and Technology Austria.","chicago":"Medina Ramos, Raimel A. “Exploring the Optimization Landscape of Variational Quantum Algorithms.” Institute of Science and Technology Austria, 2024. <a href=\"https://doi.org/10.15479/at:ista:17208\">https://doi.org/10.15479/at:ista:17208</a>.","ama":"Medina Ramos RA. Exploring the optimization landscape of variational quantum algorithms. 2024. doi:<a href=\"https://doi.org/10.15479/at:ista:17208\">10.15479/at:ista:17208</a>","apa":"Medina Ramos, R. A. (2024). <i>Exploring the optimization landscape of variational quantum algorithms</i>. Institute of Science and Technology Austria. <a href=\"https://doi.org/10.15479/at:ista:17208\">https://doi.org/10.15479/at:ista:17208</a>"},"ddc":["539"],"acknowledged_ssus":[{"_id":"ScienComp"}],"publisher":"Institute of Science and Technology Austria","user_id":"ba8df636-2132-11f1-aed0-ed93e2281fdd","publication_status":"published","file_date_updated":"2024-07-17T09:23:24Z","date_published":"2024-07-09T00:00:00Z","corr_author":"1","project":[{"call_identifier":"H2020","name":"Non-Ergodic Quantum Matter: Universality, Dynamics and Control","grant_number":"850899","_id":"23841C26-32DE-11EA-91FC-C7463DDC885E"}],"date_created":"2024-07-09T09:14:24Z","title":"Exploring the optimization landscape of variational quantum algorithms","author":[{"orcid":"0000-0002-5383-2869","last_name":"Medina Ramos","first_name":"Raimel A","id":"CE680B90-D85A-11E9-B684-C920E6697425","full_name":"Medina Ramos, Raimel A"}],"date_updated":"2026-04-07T12:43:22Z","abstract":[{"lang":"eng","text":"Can current quantum computers provide a speedup over their classical counterparts for some kinds of problems? In this thesis, with a focus on ground state search/preparation, we address some of the challenges that both quantum annealing and variational quantum algorithms suffer from, hindering any possible practical speedup in comparison to the best classical counterparts. \r\n\r\nIn the first part of the thesis, we study the performance of quantum annealing for solving a particular combinatorial optimization problem called 3-XOR satisfability (3-XORSAT). The classical problem is mapped into a ground state search of a 3-local classical Hamiltonian $H_C$. We consider how modifying the initial problem, by adding more interaction terms to the corresponding Hamiltonian, leads to the emergence of a first-order phase transition during the annealing process. This phenomenon causes the total annealing duration, $T$, required to prepare the ground state of $H_C$ with a high probability to increase exponentially with the size of the problem. Our findings indicate that with the growing complexity of problem instances, the likelihood of encountering first-order phase transitions also increases, making quantum annealing an impractical solution for these types of combinatorial optimization problems.\r\n\r\nIn the second part, we focus on the problem of barren plateaus in generic variational quantum algorithms. Barren plateaus correspond to flat regions in the parameter space where the gradient of the cost function is zero in expectation, and with the variance decaying exponentially with the system size, thus obstructing an efficient parameter optimization.  We propose an algorithm to circumvent Barren Plateaus by monitoring the entanglement entropy of k-local reduced density matrices, alongside a method for estimating entanglement entropy via classical shadow tomography. We illustrate the approach with the paradigmatic example of the variational quantum eigensolver, and show that our algorithm effectively avoids barren plateaus in the initialization as well as during the optimization stage. \r\n\r\nLastly, in the last two Chapters of this thesis, we focus on the quantum approximate optimization algorithm (QAOA), originally introduced as an algorithm for solving generic combinatorial optimization problems in near-term quantum devices. Specifically, we focus on how to develop rigorous initialization strategies with guarantee improvement. Our motivation for this study lies in that for random initialization, the optimization typically leads to local minima with poor performance. Our main result corresponds to the analytical construction of index-1 saddle points or transition states, stationary points with a single direction of descent, as a tool for systematically exploring the QAOA optimization landscape. This leads us to propose a novel greedy parameter initialization strategy that guarantees for the energy to decrease with an increasing number of circuit layers. Furthermore, with precise estimates for the negative Hessian eigenvalue and its eigenvector, we establish a lower bound for energy improvement following a QAOA iteration."}]},{"user_id":"ba8df636-2132-11f1-aed0-ed93e2281fdd","publication_status":"published","file_date_updated":"2023-03-23T16:43:14Z","date_published":"2023-03-21T00:00:00Z","corr_author":"1","project":[{"call_identifier":"H2020","name":"Non-Ergodic Quantum Matter: Universality, Dynamics and Control","grant_number":"850899","_id":"23841C26-32DE-11EA-91FC-C7463DDC885E"}],"date_created":"2023-03-17T13:30:48Z","author":[{"full_name":"Brighi, Pietro","id":"4115AF5C-F248-11E8-B48F-1D18A9856A87","orcid":"0000-0002-7969-2729","last_name":"Brighi","first_name":"Pietro"}],"title":"Ergodicity breaking in disordered and kinetically constrained quantum many-body systems","date_updated":"2026-04-07T13:26:32Z","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."}],"department":[{"_id":"GradSch"},{"_id":"MaSe"}],"alternative_title":["ISTA Thesis"],"day":"21","_id":"12732","oa_version":"Published Version","article_processing_charge":"No","supervisor":[{"orcid":"0000-0002-2399-5827","last_name":"Serbyn","first_name":"Maksym","full_name":"Serbyn, Maksym","id":"47809E7E-F248-11E8-B48F-1D18A9856A87"}],"citation":{"ama":"Brighi P. Ergodicity breaking in disordered and kinetically constrained quantum many-body systems. 2023. doi:<a href=\"https://doi.org/10.15479/at:ista:12732\">10.15479/at:ista:12732</a>","apa":"Brighi, P. (2023). <i>Ergodicity breaking in disordered and kinetically constrained quantum many-body systems</i>. Institute of Science and Technology Austria. <a href=\"https://doi.org/10.15479/at:ista:12732\">https://doi.org/10.15479/at:ista:12732</a>","chicago":"Brighi, Pietro. “Ergodicity Breaking in Disordered and Kinetically Constrained Quantum Many-Body Systems.” Institute of Science and Technology Austria, 2023. <a href=\"https://doi.org/10.15479/at:ista:12732\">https://doi.org/10.15479/at:ista:12732</a>.","ista":"Brighi P. 2023. Ergodicity breaking in disordered and kinetically constrained quantum many-body systems. Institute of Science and Technology Austria.","ieee":"P. Brighi, “Ergodicity breaking in disordered and kinetically constrained quantum many-body systems,” Institute of Science and Technology Austria, 2023.","mla":"Brighi, Pietro. <i>Ergodicity Breaking in Disordered and Kinetically Constrained Quantum Many-Body Systems</i>. Institute of Science and Technology Austria, 2023, doi:<a href=\"https://doi.org/10.15479/at:ista:12732\">10.15479/at:ista:12732</a>.","short":"P. Brighi, Ergodicity Breaking in Disordered and Kinetically Constrained Quantum Many-Body Systems, Institute of Science and Technology Austria, 2023."},"ddc":["530"],"acknowledged_ssus":[{"_id":"ScienComp"}],"publisher":"Institute of Science and Technology Austria","year":"2023","oa":1,"file":[{"checksum":"5d2de651ef9449c1b8dc27148ca74777","file_size":42167561,"content_type":"application/zip","file_name":"Thesis_sub_PBrighi.zip","date_updated":"2023-03-23T16:42:56Z","date_created":"2023-03-23T16:42:56Z","relation":"source_file","access_level":"closed","creator":"pbrighi","file_id":"12753"},{"access_level":"open_access","success":1,"creator":"pbrighi","file_id":"12754","file_size":13977000,"checksum":"7caa153d4a5b0873a79358787d2dfe1e","content_type":"application/pdf","file_name":"Thesis_PBrighi.pdf","date_updated":"2023-03-23T16:43:14Z","date_created":"2023-03-23T16:43:14Z","relation":"main_file"}],"language":[{"iso":"eng"}],"tmp":{"legal_code_url":"https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode","image":"/images/cc_by_nc_sa.png","name":"Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)","short":"CC BY-NC-SA (4.0)"},"doi":"10.15479/at:ista:12732","page":"158","related_material":{"record":[{"relation":"part_of_dissertation","id":"12750","status":"public"},{"relation":"part_of_dissertation","id":"11470","status":"public"},{"status":"public","id":"8308","relation":"part_of_dissertation"},{"relation":"part_of_dissertation","status":"public","id":"11469"}]},"has_accepted_license":"1","license":"https://creativecommons.org/licenses/by-nc-sa/4.0/","month":"03","type":"dissertation","degree_awarded":"PhD","OA_place":"publisher","publication_identifier":{"issn":["2663-337X"]},"status":"public","ec_funded":1},{"language":[{"iso":"eng"}],"file":[{"content_type":"application/pdf","file_size":11947523,"checksum":"068fd3570506ec42b2faa390de784bc4","date_updated":"2024-11-30T23:30:03Z","file_name":"PhD_Thesis.pdf","date_created":"2023-11-30T15:53:10Z","relation":"main_file","access_level":"open_access","creator":"ssack","embargo":"2024-11-30","file_id":"14635"},{"access_level":"closed","creator":"ssack","file_id":"14636","content_type":"application/zip","file_size":18422964,"checksum":"0fa3bc0d108aed0ac59d2c6beef2220a","date_updated":"2024-11-30T23:30:03Z","file_name":"PhD Thesis (1).zip","embargo_to":"open_access","date_created":"2023-11-30T15:54:11Z","relation":"source_file"}],"tmp":{"legal_code_url":"https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode","image":"/images/cc_by_nc_sa.png","name":"Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)","short":"CC BY-NC-SA (4.0)"},"year":"2023","oa":1,"doi":"10.15479/at:ista:14622","page":"142","related_material":{"record":[{"status":"public","id":"13125","relation":"part_of_dissertation"},{"status":"public","id":"11471","relation":"part_of_dissertation"},{"status":"public","id":"9760","relation":"part_of_dissertation"}]},"has_accepted_license":"1","month":"11","type":"dissertation","degree_awarded":"PhD","OA_place":"publisher","status":"public","ec_funded":1,"publication_identifier":{"issn":["2663-337X"]},"file_date_updated":"2024-11-30T23:30:03Z","user_id":"ba8df636-2132-11f1-aed0-ed93e2281fdd","publication_status":"published","corr_author":"1","date_published":"2023-11-30T00:00:00Z","date_created":"2023-11-28T10:58:13Z","author":[{"last_name":"Sack","first_name":"Stefan","orcid":"0000-0001-5400-8508","full_name":"Sack, Stefan","id":"dd622248-f6e0-11ea-865d-ce382a1c81a5"}],"title":"Improving variational quantum algorithms : Innovative initialization techniques and extensions to qudit systems","project":[{"name":"IMB PhD Nomination Fellowship - Stefan Sack","_id":"bd660c93-d553-11ed-ba76-fb0fb6f49c0d"},{"_id":"23841C26-32DE-11EA-91FC-C7463DDC885E","call_identifier":"H2020","grant_number":"850899","name":"Non-Ergodic Quantum Matter: Universality, Dynamics and Control"}],"abstract":[{"lang":"eng","text":"This Ph.D. thesis presents a detailed investigation into Variational Quantum Algorithms\r\n(VQAs), a promising class of quantum algorithms that are well suited for near-term quantum\r\ncomputation due to their moderate hardware requirements and resilience to noise. Our\r\nprimary focus lies on two particular types of VQAs: the Quantum Approximate Optimization\r\nAlgorithm (QAOA), used for solving binary optimization problems, and the Variational Quantum\r\nEigensolver (VQE), utilized for finding ground states of quantum many-body systems.\r\nIn the first part of the thesis, we examine the issue of effective parameter initialization for\r\nthe QAOA. The work demonstrates that random initialization of the QAOA often leads to\r\nconvergence in local minima with sub-optimal performance. To mitigate this issue, we propose\r\nan initialization of QAOA parameters based on the Trotterized Quantum Annealing (TQA).\r\nWe show that TQA initialization leads to the same performance as the best of an exponentially\r\nscaling number of random initializations.\r\nThe second study introduces Transition States (TS), stationary points with a single direction\r\nof descent, as a tool for systematically exploring the QAOA optimization landscape. This\r\nleads us to propose a novel greedy parameter initialization strategy that guarantees for the\r\nenergy to decrease with increasing number of circuit layers.\r\nIn the third section, we extend the QAOA to qudit systems, which are higher-dimensional\r\ngeneralizations of qubits. This chapter provides theoretical insights and practical strategies for\r\nleveraging the increased computational power of qudits in the context of quantum optimization\r\nalgorithms and suggests a quantum circuit for implementing the algorithm on an ion trap\r\nquantum computer.\r\nFinally, we propose an algorithm to avoid “barren plateaus”, regions in parameter space with\r\nvanishing gradients that obstruct efficient parameter optimization. This novel approach relies\r\non defining a notion of weak barren plateaus based on the entropies of local reduced density\r\nmatrices and showcases how these can be efficiently quantified using shadow tomography.\r\nTo illustrate the approach we employ the strategy in the VQE and show that it allows to\r\nsuccessfully avoid barren plateaus in the initialization and throughout the optimization.\r\nTaken together, this thesis greatly enhances our understanding of parameter initialization and\r\noptimization in VQAs, expands the scope of QAOA to higher-dimensional quantum systems,\r\nand presents a method to address the challenge of barren plateaus using the VQE. These\r\ninsights are instrumental in advancing the field of near-term quantum computation."}],"date_updated":"2026-04-07T13:53:47Z","alternative_title":["ISTA Thesis"],"department":[{"_id":"GradSch"},{"_id":"MaSe"}],"_id":"14622","oa_version":"Published Version","article_processing_charge":"No","day":"30","ddc":["530"],"citation":{"chicago":"Sack, Stefan. “Improving Variational Quantum Algorithms : Innovative Initialization Techniques and Extensions to Qudit Systems.” Institute of Science and Technology Austria, 2023. <a href=\"https://doi.org/10.15479/at:ista:14622\">https://doi.org/10.15479/at:ista:14622</a>.","apa":"Sack, S. (2023). <i>Improving variational quantum algorithms : Innovative initialization techniques and extensions to qudit systems</i>. Institute of Science and Technology Austria. <a href=\"https://doi.org/10.15479/at:ista:14622\">https://doi.org/10.15479/at:ista:14622</a>","ama":"Sack S. Improving variational quantum algorithms : Innovative initialization techniques and extensions to qudit systems. 2023. doi:<a href=\"https://doi.org/10.15479/at:ista:14622\">10.15479/at:ista:14622</a>","short":"S. Sack, Improving Variational Quantum Algorithms : Innovative Initialization Techniques and Extensions to Qudit Systems, Institute of Science and Technology Austria, 2023.","mla":"Sack, Stefan. <i>Improving Variational Quantum Algorithms : Innovative Initialization Techniques and Extensions to Qudit Systems</i>. Institute of Science and Technology Austria, 2023, doi:<a href=\"https://doi.org/10.15479/at:ista:14622\">10.15479/at:ista:14622</a>.","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."},"supervisor":[{"full_name":"Serbyn, Maksym","id":"47809E7E-F248-11E8-B48F-1D18A9856A87","last_name":"Serbyn","first_name":"Maksym","orcid":"0000-0002-2399-5827"}],"publisher":"Institute of Science and Technology Austria"}]