[{"file":[{"file_name":"Raimel_Thesis-Final.zip","file_id":"17212","date_updated":"2024-07-10T11:34:09Z","date_created":"2024-07-09T09:21:44Z","access_level":"closed","file_size":"14218691","relation":"source_file","creator":"rmedinar","content_type":"application/zip","checksum":"6f45273d04f4418bc2adc018baed0525"},{"access_level":"open_access","date_created":"2024-07-17T09:23:24Z","relation":"main_file","file_size":11253627,"content_type":"application/pdf","checksum":"6724a95bec772dbabc0111b9f08a805e","creator":"rmedinar","success":1,"file_id":"17275","file_name":"Raimel_Thesis-20_pdfa.pdf","date_updated":"2024-07-17T09:23:24Z"}],"publication_status":"published","keyword":["Quantum computing","Variational Quantum Algorithms","Optimization"],"date_created":"2024-07-09T09:14:24Z","ddc":["539"],"oa":1,"ec_funded":1,"day":"09","_id":"17208","title":"Exploring the optimization landscape of variational quantum algorithms","acknowledged_ssus":[{"_id":"ScienComp"}],"publication_identifier":{"issn":["2663-337X"]},"year":"2024","corr_author":"1","publisher":"Institute of Science and Technology Austria","date_updated":"2026-04-07T12:43:22Z","user_id":"ba8df636-2132-11f1-aed0-ed93e2281fdd","month":"07","doi":"10.15479/at:ista:17208","author":[{"last_name":"Medina Ramos","full_name":"Medina Ramos, Raimel A","id":"CE680B90-D85A-11E9-B684-C920E6697425","orcid":"0000-0002-5383-2869","first_name":"Raimel A"}],"citation":{"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>","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>.","ieee":"R. A. Medina Ramos, “Exploring the optimization landscape of variational quantum algorithms,” Institute of Science and Technology Austria, 2024.","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>","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>.","ista":"Medina Ramos RA. 2024. Exploring the optimization landscape of variational quantum algorithms. Institute of Science and Technology Austria."},"related_material":{"record":[{"status":"public","id":"10545","relation":"part_of_dissertation"},{"relation":"part_of_dissertation","id":"10067","status":"public"},{"status":"public","relation":"part_of_dissertation","id":"17222"},{"status":"public","relation":"part_of_dissertation","id":"13125"},{"relation":"part_of_dissertation","id":"11471","status":"public"}]},"tmp":{"legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","short":"CC BY (4.0)","image":"/images/cc_by.png","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"alternative_title":["ISTA Thesis"],"status":"public","type":"dissertation","oa_version":"Published Version","license":"https://creativecommons.org/licenses/by/4.0/","language":[{"iso":"eng"}],"page":"133","file_date_updated":"2024-07-17T09:23:24Z","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."}],"degree_awarded":"PhD","article_processing_charge":"No","supervisor":[{"orcid":"0000-0002-2399-5827","first_name":"Maksym","full_name":"Serbyn, Maksym","id":"47809E7E-F248-11E8-B48F-1D18A9856A87","last_name":"Serbyn"}],"project":[{"name":"Non-Ergodic Quantum Matter: Universality, Dynamics and Control","grant_number":"850899","_id":"23841C26-32DE-11EA-91FC-C7463DDC885E","call_identifier":"H2020"}],"OA_place":"publisher","department":[{"_id":"GradSch"},{"_id":"MaSe"}],"has_accepted_license":"1","date_published":"2024-07-09T00:00:00Z"},{"keyword":["quantum computing","superinductor","quantum metrology"],"file":[{"date_created":"2021-08-16T09:33:21Z","access_level":"closed","file_size":151387283,"relation":"source_file","content_type":"application/x-zip-compressed","checksum":"3cd1986efde5121d7581f6fcf9090da8","creator":"mperuzzo","date_updated":"2021-09-06T08:39:47Z","file_id":"9924","file_name":"GeometricSuperinductorsForCQED.zip"},{"file_id":"9939","file_name":"GeometricSuperinductorsAndTheirApplicationsIncQED-1b.pdf","date_updated":"2021-09-06T08:39:47Z","date_created":"2021-08-18T14:20:06Z","access_level":"open_access","file_size":17596344,"relation":"main_file","checksum":"50928c621cdf0775d7a5906b9dc8602c","content_type":"application/pdf","creator":"mperuzzo"},{"file_name":"GeometricSuperinductorsAndTheirApplicationsIncQED-2b.pdf","file_id":"9940","date_updated":"2021-09-06T08:39:47Z","creator":"mperuzzo","content_type":"application/pdf","checksum":"37f486aa1b622fe44af00d627ec13f6c","date_created":"2021-08-18T14:20:09Z","access_level":"closed","file_size":17592425,"relation":"other","description":"Extra copy of the thesis as PDF/A-2b"}],"publication_status":"published","_id":"9920","day":"19","ddc":["539"],"date_created":"2021-08-16T09:44:09Z","oa":1,"publisher":"Institute of Science and Technology Austria","date_updated":"2026-04-15T06:43:02Z","year":"2021","corr_author":"1","publication_identifier":{"isbn":["978-3-99078-013-8"],"issn":["2663-337X"]},"acknowledged_ssus":[{"_id":"NanoFab"},{"_id":"M-Shop"}],"title":"Geometric superinductors and their applications in circuit quantum electrodynamics","citation":{"chicago":"Peruzzo, Matilda. “Geometric Superinductors and Their Applications in Circuit Quantum Electrodynamics.” Institute of Science and Technology Austria, 2021. <a href=\"https://doi.org/10.15479/at:ista:9920\">https://doi.org/10.15479/at:ista:9920</a>.","ama":"Peruzzo M. Geometric superinductors and their applications in circuit quantum electrodynamics. 2021. doi:<a href=\"https://doi.org/10.15479/at:ista:9920\">10.15479/at:ista:9920</a>","ista":"Peruzzo M. 2021. Geometric superinductors and their applications in circuit quantum electrodynamics. Institute of Science and Technology Austria.","short":"M. Peruzzo, Geometric Superinductors and Their Applications in Circuit Quantum Electrodynamics, Institute of Science and Technology Austria, 2021.","apa":"Peruzzo, M. (2021). <i>Geometric superinductors and their applications in circuit quantum electrodynamics</i>. Institute of Science and Technology Austria. <a href=\"https://doi.org/10.15479/at:ista:9920\">https://doi.org/10.15479/at:ista:9920</a>","ieee":"M. Peruzzo, “Geometric superinductors and their applications in circuit quantum electrodynamics,” Institute of Science and Technology Austria, 2021.","mla":"Peruzzo, Matilda. <i>Geometric Superinductors and Their Applications in Circuit Quantum Electrodynamics</i>. Institute of Science and Technology Austria, 2021, doi:<a href=\"https://doi.org/10.15479/at:ista:9920\">10.15479/at:ista:9920</a>."},"doi":"10.15479/at:ista:9920","author":[{"orcid":"0000-0002-3415-4628","first_name":"Matilda","full_name":"Peruzzo, Matilda","id":"3F920B30-F248-11E8-B48F-1D18A9856A87","last_name":"Peruzzo"}],"month":"08","user_id":"ba8df636-2132-11f1-aed0-ed93e2281fdd","type":"dissertation","status":"public","related_material":{"record":[{"status":"public","id":"9928","relation":"part_of_dissertation"},{"status":"public","id":"8755","relation":"part_of_dissertation"}]},"alternative_title":["ISTA Thesis"],"degree_awarded":"PhD","article_processing_charge":"No","language":[{"iso":"eng"}],"page":"149","oa_version":"Published Version","file_date_updated":"2021-09-06T08:39:47Z","abstract":[{"text":"This work is concerned with two fascinating circuit quantum electrodynamics components, the Josephson junction and the geometric superinductor, and the interesting experiments that can be done by combining the two. The Josephson junction has revolutionized the field of superconducting circuits as a non-linear dissipation-less circuit element and is used in almost all superconducting qubit implementations since the 90s. On the other hand, the superinductor is a relatively new circuit element introduced as a key component of the fluxonium qubit in 2009. This is an inductor with characteristic impedance larger than the resistance quantum and self-resonance frequency in the GHz regime. The combination of these two elements can occur in two fundamental ways: in parallel and in series. When connected in parallel the two create the fluxonium qubit, a loop with large inductance and a rich energy spectrum reliant on quantum tunneling. On the other hand placing the two elements in series aids with the measurement of the IV curve of a single Josephson junction in a high impedance environment. In this limit theory predicts that the junction will behave as its dual element: the phase-slip junction. While the Josephson junction acts as a non-linear inductor the phase-slip junction has the behavior of a non-linear capacitance and can be used to measure new Josephson junction phenomena, namely Coulomb blockade of Cooper pairs and phase-locked Bloch oscillations. The latter experiment allows for a direct link between frequency and current which is an elusive connection in quantum metrology. This work introduces the geometric superinductor, a superconducting circuit element where the high inductance is due to the geometry rather than the material properties of the superconductor, realized from a highly miniaturized superconducting planar coil. These structures will be described and characterized as resonators and qubit inductors and progress towards the measurement of phase-locked Bloch oscillations will be presented.","lang":"eng"}],"OA_place":"publisher","supervisor":[{"first_name":"Johannes M","orcid":"0000-0001-8112-028X","id":"4B591CBA-F248-11E8-B48F-1D18A9856A87","full_name":"Fink, Johannes M","last_name":"Fink"}],"has_accepted_license":"1","date_published":"2021-08-19T00:00:00Z","department":[{"_id":"GradSch"},{"_id":"JoFi"}]},{"title":"Singlet-Triplet qubits and spin-orbit interaction in 2-dimensional Ge hole gases","acknowledged_ssus":[{"_id":"M-Shop"},{"_id":"NanoFab"}],"publication_identifier":{"issn":["2663-337X"]},"corr_author":"1","year":"2021","date_updated":"2026-04-08T07:12:19Z","publisher":"Institute of Science and Technology Austria","user_id":"ba8df636-2132-11f1-aed0-ed93e2281fdd","month":"10","doi":"10.15479/at:ista:10058","author":[{"full_name":"Jirovec, Daniel","id":"4C473F58-F248-11E8-B48F-1D18A9856A87","last_name":"Jirovec","first_name":"Daniel","orcid":"0000-0002-7197-4801"}],"citation":{"ieee":"D. Jirovec, “Singlet-Triplet qubits and spin-orbit interaction in 2-dimensional Ge hole gases,” Institute of Science and Technology Austria, 2021.","mla":"Jirovec, Daniel. <i>Singlet-Triplet Qubits and Spin-Orbit Interaction in 2-Dimensional Ge Hole Gases</i>. Institute of Science and Technology Austria, 2021, doi:<a href=\"https://doi.org/10.15479/at:ista:10058\">10.15479/at:ista:10058</a>.","short":"D. Jirovec, Singlet-Triplet Qubits and Spin-Orbit Interaction in 2-Dimensional Ge Hole Gases, Institute of Science and Technology Austria, 2021.","apa":"Jirovec, D. (2021). <i>Singlet-Triplet qubits and spin-orbit interaction in 2-dimensional Ge hole gases</i>. Institute of Science and Technology Austria. <a href=\"https://doi.org/10.15479/at:ista:10058\">https://doi.org/10.15479/at:ista:10058</a>","ista":"Jirovec D. 2021. Singlet-Triplet qubits and spin-orbit interaction in 2-dimensional Ge hole gases. Institute of Science and Technology Austria.","chicago":"Jirovec, Daniel. “Singlet-Triplet Qubits and Spin-Orbit Interaction in 2-Dimensional Ge Hole Gases.” Institute of Science and Technology Austria, 2021. <a href=\"https://doi.org/10.15479/at:ista:10058\">https://doi.org/10.15479/at:ista:10058</a>.","ama":"Jirovec D. Singlet-Triplet qubits and spin-orbit interaction in 2-dimensional Ge hole gases. 2021. doi:<a href=\"https://doi.org/10.15479/at:ista:10058\">10.15479/at:ista:10058</a>"},"publication_status":"published","file":[{"date_updated":"2022-12-20T23:30:07Z","file_name":"PHD_Thesis_Jirovec_Source.zip","file_id":"10061","creator":"djirovec","checksum":"ad6bcb24083ed7c02baaf1885c9ea3d5","content_type":"application/x-zip-compressed","access_level":"closed","date_created":"2021-09-30T14:29:14Z","file_size":32397600,"relation":"source_file","embargo_to":"open_access"},{"relation":"main_file","file_size":26910829,"date_created":"2021-10-05T07:56:49Z","access_level":"open_access","embargo":"2022-10-06","creator":"djirovec","content_type":"application/pdf","checksum":"5fbe08d4f66d1153e04c47971538fae8","file_id":"10087","file_name":"PHD_Thesis_pdfa2b_1.pdf","date_updated":"2022-12-20T23:30:07Z"}],"keyword":["qubits","quantum computing","holes"],"oa":1,"date_created":"2021-09-30T07:53:49Z","ddc":["621","539"],"day":"05","_id":"10058","supervisor":[{"last_name":"Katsaros","id":"38DB5788-F248-11E8-B48F-1D18A9856A87","full_name":"Katsaros, Georgios","first_name":"Georgios","orcid":"0000-0001-8342-202X"}],"project":[{"grant_number":"P30207","name":"Hole spin orbit qubits in Ge quantum wells","call_identifier":"FWF","_id":"2641CE5E-B435-11E9-9278-68D0E5697425"}],"OA_place":"publisher","department":[{"_id":"GradSch"},{"_id":"GeKa"}],"acknowledgement":"The author gratefully acknowledges support by the Austrian Science Fund (FWF), grants No P30207, and the Nomis foundation.","date_published":"2021-10-05T00:00:00Z","has_accepted_license":"1","alternative_title":["ISTA Thesis"],"related_material":{"record":[{"status":"public","id":"10066","relation":"part_of_dissertation"},{"status":"public","id":"10065","relation":"part_of_dissertation"},{"relation":"part_of_dissertation","id":"8831","status":"public"},{"status":"public","relation":"part_of_dissertation","id":"8909"},{"status":"public","id":"5816","relation":"part_of_dissertation"}]},"tmp":{"legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","short":"CC BY (4.0)","image":"/images/cc_by.png","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"status":"public","type":"dissertation","file_date_updated":"2022-12-20T23:30:07Z","abstract":[{"text":"Quantum information and computation has become a vast field paved with opportunities for researchers and investors. As large multinational companies and international funds are heavily investing in quantum technologies it is still a question which platform is best suited for the task of realizing a scalable quantum processor. In this work we investigate hole spins in Ge quantum wells. These hold great promise as they possess several favorable properties: a small effective mass, a strong spin-orbit coupling, long relaxation time and an inherent immunity to hyperfine noise. All these characteristics helped Ge hole spin qubits to evolve from a single qubit to a fully entangled four qubit processor in only 3 years. Here, we investigated a qubit approach leveraging the large out-of-plane g-factors of heavy hole states in Ge quantum dots. We found this qubit to be reproducibly operable at extremely low magnetic field and at large speeds while maintaining coherence. This was possible because large differences of g-factors in adjacent dots can be achieved in the out-of-plane direction. In the in-plane direction the small g-factors, on the other hand, can be altered very effectively by the confinement potentials. Here, we found that this can even lead to a sign change of the g-factors. The resulting g-factor difference alters the dynamics of the system drastically and produces effects typically attributed to a spin-orbit induced spin-flip term.  The investigations carried out in this thesis give further insights into the possibilities of holes in Ge and reveal new physical properties that need to be considered when designing future spin qubit experiments.","lang":"eng"}],"oa_version":"Published Version","page":"151","language":[{"iso":"eng"}],"article_processing_charge":"No","degree_awarded":"PhD"}]