[{"publication":"IEEE Transactions on Quantum Engineering","date_updated":"2025-01-27T15:06:15Z","file_date_updated":"2025-01-27T15:03:09Z","license":"https://creativecommons.org/licenses/by/4.0/","scopus_import":"1","status":"public","page":"1-14","day":"14","citation":{"mla":"Wurtz, Jonathan, et al. “Solving Nonnative Combinatorial Optimization Problems Using Hybrid Quantum–Classical Algorithms.” <i>IEEE Transactions on Quantum Engineering</i>, vol. 5, Institute of Electrical and Electronics Engineers , 2024, pp. 1–14, doi:<a href=\"https://doi.org/10.1109/tqe.2024.3443660\">10.1109/tqe.2024.3443660</a>.","short":"J. Wurtz, S. Sack, S.-T. Wang, IEEE Transactions on Quantum Engineering 5 (2024) 1–14.","ieee":"J. Wurtz, S. Sack, and S.-T. Wang, “Solving nonnative combinatorial optimization problems using hybrid quantum–classical algorithms,” <i>IEEE Transactions on Quantum Engineering</i>, vol. 5. Institute of Electrical and Electronics Engineers , pp. 1–14, 2024.","apa":"Wurtz, J., Sack, S., &#38; Wang, S.-T. (2024). Solving nonnative combinatorial optimization problems using hybrid quantum–classical algorithms. <i>IEEE Transactions on Quantum Engineering</i>. Institute of Electrical and Electronics Engineers . <a href=\"https://doi.org/10.1109/tqe.2024.3443660\">https://doi.org/10.1109/tqe.2024.3443660</a>","ama":"Wurtz J, Sack S, Wang S-T. Solving nonnative combinatorial optimization problems using hybrid quantum–classical algorithms. <i>IEEE Transactions on Quantum Engineering</i>. 2024;5:1-14. doi:<a href=\"https://doi.org/10.1109/tqe.2024.3443660\">10.1109/tqe.2024.3443660</a>","ista":"Wurtz J, Sack S, Wang S-T. 2024. Solving nonnative combinatorial optimization problems using hybrid quantum–classical algorithms. IEEE Transactions on Quantum Engineering. 5, 1–14.","chicago":"Wurtz, Jonathan, Stefan Sack, and Sheng-Tao Wang. “Solving Nonnative Combinatorial Optimization Problems Using Hybrid Quantum–Classical Algorithms.” <i>IEEE Transactions on Quantum Engineering</i>. Institute of Electrical and Electronics Engineers , 2024. <a href=\"https://doi.org/10.1109/tqe.2024.3443660\">https://doi.org/10.1109/tqe.2024.3443660</a>."},"has_accepted_license":"1","language":[{"iso":"eng"}],"_id":"18923","abstract":[{"text":"Combinatorial optimization is a challenging problem applicable in a wide range of fields from logistics to finance. Recently, quantum computing has been used to attempt to solve these problems using a range of algorithms, including parameterized quantum circuits, adiabatic protocols, and quantum annealing. These solutions typically have several challenges: 1) there is little to no performance gain over classical methods; 2) not all constraints and objectives may be efficiently encoded in the quantum ansatz; and 3) the solution domain of the objective function may not be the same as the bit strings of measurement outcomes. This work presents “nonnative hybrid algorithms”: a framework to overcome these challenges by integrating quantum and classical resources with a hybrid approach. By designing nonnative quantum variational anosatzes that inherit some but not all problem structure, measurement outcomes from the quantum computer can act as a resource to be used by classical routines to indirectly compute optimal solutions, partially overcoming the challenges of contemporary quantum optimization approaches. These methods are demonstrated using a publicly available neutral-atom quantum computer on two simple problems of Max k-Cut and maximum independent set. We find improvements in solution quality when comparing the hybrid algorithm to its “no quantum” version, a demonstration of a “comparative advantage.”","lang":"eng"}],"quality_controlled":"1","type":"journal_article","OA_place":"publisher","date_published":"2024-08-14T00:00:00Z","intvolume":"         5","user_id":"2DF688A6-F248-11E8-B48F-1D18A9856A87","ddc":["530"],"year":"2024","author":[{"first_name":"Jonathan","last_name":"Wurtz","full_name":"Wurtz, Jonathan"},{"full_name":"Sack, Stefan","last_name":"Sack","id":"dd622248-f6e0-11ea-865d-ce382a1c81a5","first_name":"Stefan","orcid":"0000-0001-5400-8508"},{"last_name":"Wang","full_name":"Wang, Sheng-Tao","first_name":"Sheng-Tao"}],"volume":5,"department":[{"_id":"MaSe"}],"month":"08","article_type":"original","oa":1,"doi":"10.1109/tqe.2024.3443660","acknowledgement":"The authors would like to thank Alexander Keesling, Maddie Cain, Nate Gemelke, and Phillip Weinberg for helpful discussions and Danylo Lykov who had early contributions to this work.\r\n10.13039/100000185-Defense Advanced Research Projects Agency Noisy Intermediate-Scale Quantum Devices (Grant Number: W911NF2010021), DARPA Small Business Technology Transfer program (Grant Number: 140D0422C0035).","article_processing_charge":"Yes (in subscription journal)","tmp":{"short":"CC BY (4.0)","image":"/images/cc_by.png","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"publisher":"Institute of Electrical and Electronics Engineers ","file":[{"access_level":"open_access","file_size":1753095,"date_created":"2025-01-27T15:03:09Z","content_type":"application/pdf","success":1,"file_name":"2024_IEEEQuantumComputing_Wurtz.pdf","checksum":"19b84e35cba05bde72bfe7e0b54c3e6c","relation":"main_file","creator":"dernst","file_id":"18924","date_updated":"2025-01-27T15:03:09Z"}],"publication_status":"published","title":"Solving nonnative combinatorial optimization problems using hybrid quantum–classical algorithms","date_created":"2025-01-27T15:00:44Z","OA_type":"hybrid","oa_version":"Published Version","publication_identifier":{"issn":["2689-1808"]}},{"_id":"15122","language":[{"iso":"eng"}],"abstract":[{"text":"Quantum computers are increasing in size and quality but are still very noisy. Error mitigation extends the size of the quantum circuits that noisy devices can meaningfully execute. However, state-of-the-art error mitigation methods are hard to implement and the limited qubit connectivity in superconducting qubit devices restricts most applications to the hardware's native topology. Here we show a quantum approximate optimization algorithm (QAOA) on nonplanar random regular graphs with up to 40 nodes enabled by a machine learning-based error mitigation. We use a swap network with careful decision-variable-to-qubit mapping and a feed-forward neural network to optimize a depth-two QAOA on up to 40 qubits. We observe a meaningful parameter optimization for the largest graph which requires running quantum circuits with 958 two-qubit gates. Our paper emphasizes the need to mitigate samples, and not only expectation values, in quantum approximate optimization. These results are a step towards executing quantum approximate optimization at a scale that is not classically simulable. Reaching such system sizes is key to properly understanding the true potential of heuristic algorithms like QAOA.","lang":"eng"}],"external_id":{"arxiv":["2307.14427"]},"type":"journal_article","quality_controlled":"1","has_accepted_license":"1","day":"01","citation":{"short":"S. Sack, D.J. Egger, Physical Review Research 6 (2024).","ieee":"S. Sack and D. J. Egger, “Large-scale quantum approximate optimization on nonplanar graphs with machine learning noise mitigation,” <i>Physical Review Research</i>, vol. 6, no. 1. American Physical Society, 2024.","mla":"Sack, Stefan, and Daniel J. Egger. “Large-Scale Quantum Approximate Optimization on Nonplanar Graphs with Machine Learning Noise Mitigation.” <i>Physical Review Research</i>, vol. 6, no. 1, 013223, American Physical Society, 2024, doi:<a href=\"https://doi.org/10.1103/PhysRevResearch.6.013223\">10.1103/PhysRevResearch.6.013223</a>.","apa":"Sack, S., &#38; Egger, D. J. (2024). Large-scale quantum approximate optimization on nonplanar graphs with machine learning noise mitigation. <i>Physical Review Research</i>. American Physical Society. <a href=\"https://doi.org/10.1103/PhysRevResearch.6.013223\">https://doi.org/10.1103/PhysRevResearch.6.013223</a>","ista":"Sack S, Egger DJ. 2024. Large-scale quantum approximate optimization on nonplanar graphs with machine learning noise mitigation. Physical Review Research. 6(1), 013223.","ama":"Sack S, Egger DJ. Large-scale quantum approximate optimization on nonplanar graphs with machine learning noise mitigation. <i>Physical Review Research</i>. 2024;6(1). doi:<a href=\"https://doi.org/10.1103/PhysRevResearch.6.013223\">10.1103/PhysRevResearch.6.013223</a>","chicago":"Sack, Stefan, and Daniel J. Egger. “Large-Scale Quantum Approximate Optimization on Nonplanar Graphs with Machine Learning Noise Mitigation.” <i>Physical Review Research</i>. American Physical Society, 2024. <a href=\"https://doi.org/10.1103/PhysRevResearch.6.013223\">https://doi.org/10.1103/PhysRevResearch.6.013223</a>."},"article_number":"013223","intvolume":"         6","year":"2024","user_id":"2DF688A6-F248-11E8-B48F-1D18A9856A87","ddc":["530"],"date_published":"2024-03-01T00:00:00Z","file_date_updated":"2024-03-19T07:16:38Z","date_updated":"2025-05-14T09:32:15Z","publication":"Physical Review Research","status":"public","issue":"1","DOAJ_listed":"1","scopus_import":"1","publication_status":"published","file":[{"success":1,"content_type":"application/pdf","date_created":"2024-03-19T07:16:38Z","file_size":2777593,"access_level":"open_access","creator":"dernst","date_updated":"2024-03-19T07:16:38Z","file_id":"15123","relation":"main_file","checksum":"274c9f1b15b3547a10a03f39e4ccc582","file_name":"2024_PhysicalReviewResearch_Sack.pdf"}],"arxiv":1,"publication_identifier":{"eissn":["2643-1564"]},"date_created":"2024-03-17T23:00:59Z","title":"Large-scale quantum approximate optimization on nonplanar graphs with machine learning noise mitigation","oa_version":"Published Version","acknowledgement":"S.H.S. acknowledges support from the IBM Ph.D. fellowship 2022 in quantum computing. The authors also thank M. Serbyn, R. Kueng, R. A. Medina, and S. Woerner for fruitful discussions.","doi":"10.1103/PhysRevResearch.6.013223","oa":1,"volume":6,"author":[{"orcid":"0000-0001-5400-8508","id":"dd622248-f6e0-11ea-865d-ce382a1c81a5","first_name":"Stefan","last_name":"Sack","full_name":"Sack, Stefan"},{"full_name":"Egger, Daniel J.","last_name":"Egger","first_name":"Daniel J."}],"project":[{"_id":"bd660c93-d553-11ed-ba76-fb0fb6f49c0d","name":"IMB PhD Nomination Fellowship - Stefan Sack"}],"article_type":"original","month":"03","department":[{"_id":"MaSe"}],"tmp":{"short":"CC BY (4.0)","image":"/images/cc_by.png","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"corr_author":"1","article_processing_charge":"Yes","publisher":"American Physical Society"},{"file_date_updated":"2024-11-30T23:30:03Z","date_updated":"2026-04-07T13:53:47Z","status":"public","page":"142","ec_funded":1,"license":"https://creativecommons.org/licenses/by-nc-sa/4.0/","language":[{"iso":"eng"}],"_id":"14622","abstract":[{"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.","lang":"eng"}],"type":"dissertation","has_accepted_license":"1","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>.","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>","ista":"Sack S. 2023. Improving variational quantum algorithms : Innovative initialization techniques and extensions to qudit systems. Institute of Science and Technology Austria.","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>","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>.","short":"S. Sack, Improving Variational Quantum Algorithms : Innovative Initialization Techniques and Extensions to Qudit Systems, Institute of Science and Technology Austria, 2023.","ieee":"S. Sack, “Improving variational quantum algorithms : Innovative initialization techniques and extensions to qudit systems,” Institute of Science and Technology Austria, 2023."},"day":"30","year":"2023","user_id":"ba8df636-2132-11f1-aed0-ed93e2281fdd","ddc":["530"],"date_published":"2023-11-30T00:00:00Z","OA_place":"publisher","degree_awarded":"PhD","doi":"10.15479/at:ista:14622","oa":1,"author":[{"orcid":"0000-0001-5400-8508","first_name":"Stefan","id":"dd622248-f6e0-11ea-865d-ce382a1c81a5","last_name":"Sack","full_name":"Sack, Stefan"}],"project":[{"name":"IMB PhD Nomination Fellowship - Stefan Sack","_id":"bd660c93-d553-11ed-ba76-fb0fb6f49c0d"},{"name":"Non-Ergodic Quantum Matter: Universality, Dynamics and Control","grant_number":"850899","call_identifier":"H2020","_id":"23841C26-32DE-11EA-91FC-C7463DDC885E"}],"month":"11","department":[{"_id":"GradSch"},{"_id":"MaSe"}],"tmp":{"short":"CC BY-NC-SA (4.0)","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)"},"corr_author":"1","article_processing_charge":"No","publisher":"Institute of Science and Technology Austria","related_material":{"record":[{"id":"13125","status":"public","relation":"part_of_dissertation"},{"relation":"part_of_dissertation","status":"public","id":"11471"},{"id":"9760","status":"public","relation":"part_of_dissertation"}]},"supervisor":[{"last_name":"Serbyn","full_name":"Serbyn, Maksym","first_name":"Maksym","id":"47809E7E-F248-11E8-B48F-1D18A9856A87","orcid":"0000-0002-2399-5827"}],"publication_status":"published","file":[{"embargo":"2024-11-30","checksum":"068fd3570506ec42b2faa390de784bc4","file_name":"PhD_Thesis.pdf","relation":"main_file","file_id":"14635","creator":"ssack","date_updated":"2024-11-30T23:30:03Z","access_level":"open_access","content_type":"application/pdf","file_size":11947523,"date_created":"2023-11-30T15:53:10Z"},{"file_id":"14636","relation":"source_file","date_updated":"2024-11-30T23:30:03Z","creator":"ssack","file_name":"PhD Thesis (1).zip","checksum":"0fa3bc0d108aed0ac59d2c6beef2220a","embargo_to":"open_access","file_size":18422964,"date_created":"2023-11-30T15:54:11Z","content_type":"application/zip","access_level":"closed"}],"publication_identifier":{"issn":["2663-337X"]},"date_created":"2023-11-28T10:58:13Z","alternative_title":["ISTA Thesis"],"title":"Improving variational quantum algorithms : Innovative initialization techniques and extensions to qudit systems","oa_version":"Published Version"},{"date_updated":"2026-04-27T22:30:23Z","publication":"Physical Review A","isi":1,"file_date_updated":"2023-06-13T07:28:36Z","issue":"6","ec_funded":1,"scopus_import":"1","status":"public","has_accepted_license":"1","citation":{"ista":"Sack S, Medina Ramos RA, Kueng R, Serbyn M. 2023. Recursive greedy initialization of the quantum approximate optimization algorithm with guaranteed improvement. Physical Review A. 107(6), 062404.","ama":"Sack S, Medina Ramos RA, Kueng R, Serbyn M. Recursive greedy initialization of the quantum approximate optimization algorithm with guaranteed improvement. <i>Physical Review A</i>. 2023;107(6). doi:<a href=\"https://doi.org/10.1103/physreva.107.062404\">10.1103/physreva.107.062404</a>","chicago":"Sack, Stefan, Raimel A Medina Ramos, Richard Kueng, and Maksym Serbyn. “Recursive Greedy Initialization of the Quantum Approximate Optimization Algorithm with Guaranteed Improvement.” <i>Physical Review A</i>. American Physical Society, 2023. <a href=\"https://doi.org/10.1103/physreva.107.062404\">https://doi.org/10.1103/physreva.107.062404</a>.","ieee":"S. Sack, R. A. Medina Ramos, R. Kueng, and M. Serbyn, “Recursive greedy initialization of the quantum approximate optimization algorithm with guaranteed improvement,” <i>Physical Review A</i>, vol. 107, no. 6. American Physical Society, 2023.","short":"S. Sack, R.A. Medina Ramos, R. Kueng, M. Serbyn, Physical Review A 107 (2023).","mla":"Sack, Stefan, et al. “Recursive Greedy Initialization of the Quantum Approximate Optimization Algorithm with Guaranteed Improvement.” <i>Physical Review A</i>, vol. 107, no. 6, 062404, American Physical Society, 2023, doi:<a href=\"https://doi.org/10.1103/physreva.107.062404\">10.1103/physreva.107.062404</a>.","apa":"Sack, S., Medina Ramos, R. A., Kueng, R., &#38; Serbyn, M. (2023). Recursive greedy initialization of the quantum approximate optimization algorithm with guaranteed improvement. <i>Physical Review A</i>. American Physical Society. <a href=\"https://doi.org/10.1103/physreva.107.062404\">https://doi.org/10.1103/physreva.107.062404</a>"},"day":"02","abstract":[{"lang":"eng","text":"The quantum approximate optimization algorithm (QAOA) is a variational quantum algorithm, where a quantum computer implements a variational ansatz consisting of p layers of alternating unitary operators and a classical computer is used to optimize the variational parameters. For a random initialization, the optimization typically leads to local minima with poor performance, motivating the search for initialization strategies of QAOA variational parameters. Although numerous heuristic initializations exist, an analytical understanding and performance guarantees for large p remain evasive.We introduce a greedy initialization of QAOA which guarantees improving performance with an increasing number of layers. Our main result is an analytic construction of 2p + 1 transition states—saddle points with a unique negative curvature direction—for QAOA with p + 1 layers that use the local minimum of QAOA with p layers. Transition states connect to new local minima, which are guaranteed to lower the energy compared to the minimum found for p layers. We use the GREEDY procedure to navigate the exponentially increasing with p number of local minima resulting from the recursive application of our analytic construction. The performance of the GREEDY procedure matches available initialization strategies while providing a guarantee for the minimal energy to decrease with an increasing number of layers p. "}],"_id":"13125","language":[{"iso":"eng"}],"external_id":{"arxiv":["2209.01159"],"isi":["001016927100012"]},"type":"journal_article","quality_controlled":"1","date_published":"2023-06-02T00:00:00Z","intvolume":"       107","article_number":"062404","year":"2023","ddc":["530"],"user_id":"4359f0d1-fa6c-11eb-b949-802e58b17ae8","volume":107,"author":[{"last_name":"Sack","full_name":"Sack, Stefan","orcid":"0000-0001-5400-8508","id":"dd622248-f6e0-11ea-865d-ce382a1c81a5","first_name":"Stefan"},{"first_name":"Raimel A","id":"CE680B90-D85A-11E9-B684-C920E6697425","orcid":"0000-0002-5383-2869","full_name":"Medina Ramos, Raimel A","last_name":"Medina Ramos"},{"last_name":"Kueng","full_name":"Kueng, Richard","first_name":"Richard"},{"orcid":"0000-0002-2399-5827","first_name":"Maksym","id":"47809E7E-F248-11E8-B48F-1D18A9856A87","full_name":"Serbyn, Maksym","last_name":"Serbyn"}],"article_type":"original","project":[{"call_identifier":"H2020","_id":"23841C26-32DE-11EA-91FC-C7463DDC885E","grant_number":"850899","name":"Non-Ergodic Quantum Matter: Universality, Dynamics and Control"}],"department":[{"_id":"MaSe"}],"month":"06","acknowledgement":"We thank V. Verteletskyi for a joint collaboration on numerical studies of the QAOA during his internship at ISTA that inspired analytic results on TS reported in this work. We acknowledge A. A. Mele and M. Brooks for discussions and D. Egger, P. Love, and D. Wierichs for valuable feedback on the manuscript. S.H.S., R.A.M., and M.S. acknowledge support by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 850899). R.K. is supported by the SFB BeyondC (Grant No. F7107-N38) and the project QuantumReady (FFG 896217). ","oa":1,"doi":"10.1103/physreva.107.062404","related_material":{"record":[{"relation":"dissertation_contains","id":"17208","status":"public"},{"relation":"dissertation_contains","status":"public","id":"14622"}]},"corr_author":"1","tmp":{"short":"CC BY (4.0)","image":"/images/cc_by.png","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"article_processing_charge":"No","publisher":"American Physical Society","file":[{"file_name":"2023_PhysRevA_Sack.pdf","checksum":"0d71423888eeccaa60d8f41197f26306","relation":"main_file","date_updated":"2023-06-13T07:28:36Z","file_id":"13131","creator":"dernst","access_level":"open_access","file_size":2524611,"date_created":"2023-06-13T07:28:36Z","content_type":"application/pdf","success":1}],"publication_status":"published","title":"Recursive greedy initialization of the quantum approximate optimization algorithm with guaranteed improvement","date_created":"2023-06-07T06:57:32Z","oa_version":"Published Version","publication_identifier":{"issn":["2469-9926"],"eissn":["2469-9934"]},"arxiv":1},{"oa_version":"Published Version","date_created":"2023-01-08T23:00:53Z","title":"Entanglement-based observables for quantum impurities","publication_identifier":{"issn":["2643-1564"]},"file":[{"file_size":2941167,"date_created":"2023-01-20T12:03:31Z","success":1,"content_type":"application/pdf","access_level":"open_access","date_updated":"2023-01-20T12:03:31Z","creator":"dernst","relation":"main_file","file_id":"12328","file_name":"2022_PhysicalReviewResearch_Stocker.pdf","checksum":"556820cf6e4af77c8476e5b8f4114d1a"}],"publication_status":"published","publisher":"American Physical Society","tmp":{"short":"CC BY (4.0)","image":"/images/cc_by.png","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"article_processing_charge":"No","article_type":"original","department":[{"_id":"MaSe"}],"month":"12","author":[{"full_name":"Stocker, Lidia","last_name":"Stocker","first_name":"Lidia"},{"id":"dd622248-f6e0-11ea-865d-ce382a1c81a5","first_name":"Stefan","full_name":"Sack, Stefan","last_name":"Sack"},{"last_name":"Ferguson","full_name":"Ferguson, Michael S.","first_name":"Michael S."},{"full_name":"Zilberberg, Oded","last_name":"Zilberberg","first_name":"Oded"}],"volume":4,"acknowledgement":"We thank G. Blatter, T. Ihn, K. Ensslin, M. Goldstein, C. Carisch, and J. del Pino for illuminating discussions and acknowledge financial support from the Swiss National Science Foundation (SNSF) through Project No. 190078, and from the Deutsche Forschungsgemeinschaft (DFG) - Project No. 449653034. Our numerical implementations are based on the ITensors JULIA library [64].","doi":"10.1103/PhysRevResearch.4.043177","oa":1,"date_published":"2022-12-01T00:00:00Z","year":"2022","ddc":["530"],"user_id":"2DF688A6-F248-11E8-B48F-1D18A9856A87","intvolume":"         4","article_number":"043177","has_accepted_license":"1","citation":{"ista":"Stocker L, Sack S, Ferguson MS, Zilberberg O. 2022. Entanglement-based observables for quantum impurities. Physical Review Research. 4(4), 043177.","ama":"Stocker L, Sack S, Ferguson MS, Zilberberg O. Entanglement-based observables for quantum impurities. <i>Physical Review Research</i>. 2022;4(4). doi:<a href=\"https://doi.org/10.1103/PhysRevResearch.4.043177\">10.1103/PhysRevResearch.4.043177</a>","chicago":"Stocker, Lidia, Stefan Sack, Michael S. Ferguson, and Oded Zilberberg. “Entanglement-Based Observables for Quantum Impurities.” <i>Physical Review Research</i>. American Physical Society, 2022. <a href=\"https://doi.org/10.1103/PhysRevResearch.4.043177\">https://doi.org/10.1103/PhysRevResearch.4.043177</a>.","short":"L. Stocker, S. Sack, M.S. Ferguson, O. Zilberberg, Physical Review Research 4 (2022).","ieee":"L. Stocker, S. Sack, M. S. Ferguson, and O. Zilberberg, “Entanglement-based observables for quantum impurities,” <i>Physical Review Research</i>, vol. 4, no. 4. American Physical Society, 2022.","mla":"Stocker, Lidia, et al. “Entanglement-Based Observables for Quantum Impurities.” <i>Physical Review Research</i>, vol. 4, no. 4, 043177, American Physical Society, 2022, doi:<a href=\"https://doi.org/10.1103/PhysRevResearch.4.043177\">10.1103/PhysRevResearch.4.043177</a>.","apa":"Stocker, L., Sack, S., Ferguson, M. S., &#38; Zilberberg, O. (2022). Entanglement-based observables for quantum impurities. <i>Physical Review Research</i>. American Physical Society. <a href=\"https://doi.org/10.1103/PhysRevResearch.4.043177\">https://doi.org/10.1103/PhysRevResearch.4.043177</a>"},"day":"01","type":"journal_article","quality_controlled":"1","_id":"12111","language":[{"iso":"eng"}],"abstract":[{"lang":"eng","text":"Quantum impurities exhibit fascinating many-body phenomena when the small interacting impurity changes the physics of a large noninteracting environment. The characterisation of such strongly correlated nonperturbative effects is particularly challenging due to the infinite size of the environment, and the inability of local correlators to capture the buildup of long-ranged entanglement in the system. Here, we harness an entanglement-based observable—the purity of the impurity—as a witness for the formation of strong correlations. We showcase the utility of our scheme by exactly solving the open Kondo box model in the small box limit, and thus describe all-electronic dot-cavity devices. Specifically, we conclusively characterize the metal-to-insulator phase transition in the system and identify how the (conducting) dot-lead Kondo singlet is quenched by an (insulating) intraimpurity singlet formation. Furthermore, we propose an experimentally feasible tomography protocol for the measurement of the purity, which motivates the observation of impurity physics through their entanglement build up."}],"scopus_import":"1","issue":"4","status":"public","publication":"Physical Review Research","date_updated":"2023-02-13T09:08:28Z","file_date_updated":"2023-01-20T12:03:31Z"},{"ddc":["530"],"user_id":"4359f0d1-fa6c-11eb-b949-802e58b17ae8","year":"2022","article_number":"020365","intvolume":"         3","date_published":"2022-06-29T00:00:00Z","quality_controlled":"1","type":"journal_article","external_id":{"arxiv":["2201.08194"],"isi":["000822564300001"]},"language":[{"iso":"eng"}],"_id":"11471","abstract":[{"text":"Variational quantum algorithms are promising algorithms for achieving quantum advantage on nearterm devices. The quantum hardware is used to implement a variational wave function and measure observables, whereas the classical computer is used to store and update the variational parameters. The optimization landscape of expressive variational ansätze is however dominated by large regions in parameter space, known as barren plateaus, with vanishing gradients, which prevents efficient optimization. In this work we propose a general algorithm to avoid barren plateaus in the initialization and throughout the optimization. To this end we define a notion of weak barren plateaus (WBPs) based on the entropies of local reduced density matrices. The presence of WBPs can be efficiently quantified using recently introduced shadow tomography of the quantum state with a classical computer. We demonstrate that avoidance of WBPs suffices to ensure sizable gradients in the initialization. In addition, we demonstrate that decreasing the gradient step size, guided by the entropies allows WBPs to be avoided during the optimization process. This paves the way for efficient barren plateau-free optimization on near-term devices. ","lang":"eng"}],"day":"29","citation":{"chicago":"Sack, Stefan, Raimel A Medina Ramos, Alexios Michailidis, Richard Kueng, and Maksym Serbyn. “Avoiding Barren Plateaus Using Classical Shadows.” <i>PRX Quantum</i>. American Physical Society, 2022. <a href=\"https://doi.org/10.1103/prxquantum.3.020365\">https://doi.org/10.1103/prxquantum.3.020365</a>.","ista":"Sack S, Medina Ramos RA, Michailidis A, Kueng R, Serbyn M. 2022. Avoiding barren plateaus using classical shadows. PRX Quantum. 3(2), 020365.","ama":"Sack S, Medina Ramos RA, Michailidis A, Kueng R, Serbyn M. Avoiding barren plateaus using classical shadows. <i>PRX Quantum</i>. 2022;3(2). doi:<a href=\"https://doi.org/10.1103/prxquantum.3.020365\">10.1103/prxquantum.3.020365</a>","apa":"Sack, S., Medina Ramos, R. A., Michailidis, A., Kueng, R., &#38; Serbyn, M. (2022). Avoiding barren plateaus using classical shadows. <i>PRX Quantum</i>. American Physical Society. <a href=\"https://doi.org/10.1103/prxquantum.3.020365\">https://doi.org/10.1103/prxquantum.3.020365</a>","ieee":"S. Sack, R. A. Medina Ramos, A. Michailidis, R. Kueng, and M. Serbyn, “Avoiding barren plateaus using classical shadows,” <i>PRX Quantum</i>, vol. 3, no. 2. American Physical Society, 2022.","short":"S. Sack, R.A. Medina Ramos, A. Michailidis, R. Kueng, M. Serbyn, PRX Quantum 3 (2022).","mla":"Sack, Stefan, et al. “Avoiding Barren Plateaus Using Classical Shadows.” <i>PRX Quantum</i>, vol. 3, no. 2, 020365, American Physical Society, 2022, doi:<a href=\"https://doi.org/10.1103/prxquantum.3.020365\">10.1103/prxquantum.3.020365</a>."},"has_accepted_license":"1","status":"public","scopus_import":"1","ec_funded":1,"issue":"2","file_date_updated":"2022-06-30T07:14:48Z","isi":1,"publication":"PRX Quantum","date_updated":"2026-04-27T22:30:23Z","keyword":["General Medicine"],"arxiv":1,"publication_identifier":{"issn":["2691-3399"]},"oa_version":"Published Version","title":"Avoiding barren plateaus using classical shadows","date_created":"2022-06-29T20:21:32Z","publication_status":"published","file":[{"file_name":"2022_PRXQuantum_Sack.pdf","checksum":"a7706b28d24a0e32a55ea04b82a2df43","date_updated":"2022-06-30T07:14:48Z","file_id":"11472","creator":"dernst","relation":"main_file","access_level":"open_access","file_size":4231591,"date_created":"2022-06-30T07:14:48Z","success":1,"content_type":"application/pdf"}],"publisher":"American Physical Society","article_processing_charge":"No","corr_author":"1","tmp":{"short":"CC BY (4.0)","image":"/images/cc_by.png","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"related_material":{"record":[{"relation":"dissertation_contains","status":"public","id":"17208"},{"id":"14622","status":"public","relation":"dissertation_contains"}]},"oa":1,"doi":"10.1103/prxquantum.3.020365","acknowledgement":"We thank Marco Cerezo, Zoe Holmes, and Nicholas Hunter-Jones for fruitful discussion and valuable feedback. We also acknowledge Adam Smith, Johannes Jakob Meyer, and Victor V. Albert for comments on the paper. The simulations were performed in the Julia programming\r\nlanguage [65] using the Yao module [66]. S.H.S., R.A.M., A.A.M. and M.S. acknowledge support by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 850899).","month":"06","department":[{"_id":"MaSe"}],"article_type":"original","project":[{"grant_number":"850899","_id":"23841C26-32DE-11EA-91FC-C7463DDC885E","call_identifier":"H2020","name":"Non-Ergodic Quantum Matter: Universality, Dynamics and Control"}],"volume":3,"author":[{"orcid":"0000-0001-5400-8508","id":"dd622248-f6e0-11ea-865d-ce382a1c81a5","first_name":"Stefan","full_name":"Sack, Stefan","last_name":"Sack"},{"id":"CE680B90-D85A-11E9-B684-C920E6697425","first_name":"Raimel A","orcid":"0000-0002-5383-2869","full_name":"Medina Ramos, Raimel A","last_name":"Medina Ramos"},{"first_name":"Alexios","id":"36EBAD38-F248-11E8-B48F-1D18A9856A87","orcid":"0000-0002-8443-1064","full_name":"Michailidis, Alexios","last_name":"Michailidis"},{"full_name":"Kueng, Richard","last_name":"Kueng","first_name":"Richard"},{"orcid":"0000-0002-2399-5827","first_name":"Maksym","id":"47809E7E-F248-11E8-B48F-1D18A9856A87","full_name":"Serbyn, Maksym","last_name":"Serbyn"}]},{"arxiv":1,"publication_identifier":{"eissn":["2521-327X"]},"oa_version":"Published Version","date_created":"2021-08-01T22:01:21Z","title":"Quantum annealing initialization of the quantum approximate optimization algorithm","publication_status":"published","file":[{"checksum":"9706c2bb8e748e9b5b138381995a7f6f","file_name":"2021_Quantum_Sack.pdf","relation":"main_file","date_updated":"2021-08-06T06:44:31Z","file_id":"9774","creator":"cchlebak","access_level":"open_access","content_type":"application/pdf","file_size":2312482,"date_created":"2021-08-06T06:44:31Z"}],"publisher":"Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften","article_processing_charge":"Yes","corr_author":"1","tmp":{"short":"CC BY (4.0)","image":"/images/cc_by.png","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"related_material":{"record":[{"status":"public","id":"14622","relation":"dissertation_contains"}]},"doi":"10.22331/Q-2021-07-01-491","oa":1,"acknowledgement":"We would like to thank D. Abanin and R. Medina for fruitful discussions and A. Smith and I. Kim for valuable feedback on the manuscript. We acknowledge support by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 850899).","department":[{"_id":"GradSch"},{"_id":"MaSe"}],"month":"07","project":[{"_id":"23841C26-32DE-11EA-91FC-C7463DDC885E","call_identifier":"H2020","grant_number":"850899","name":"Non-Ergodic Quantum Matter: Universality, Dynamics and Control"}],"article_type":"original","author":[{"orcid":"0000-0001-5400-8508","first_name":"Stefan","id":"dd622248-f6e0-11ea-865d-ce382a1c81a5","full_name":"Sack, Stefan","last_name":"Sack"},{"last_name":"Serbyn","full_name":"Serbyn, Maksym","orcid":"0000-0002-2399-5827","id":"47809E7E-F248-11E8-B48F-1D18A9856A87","first_name":"Maksym"}],"volume":5,"user_id":"2DF688A6-F248-11E8-B48F-1D18A9856A87","ddc":["530"],"year":"2021","article_number":"491","intvolume":"         5","date_published":"2021-07-01T00:00:00Z","quality_controlled":"1","type":"journal_article","external_id":{"arxiv":["2101.05742"],"isi":["000669830600001"]},"abstract":[{"lang":"eng","text":"The quantum approximate optimization algorithm (QAOA) is a prospective near-term quantum algorithm due to its modest circuit depth and promising benchmarks. However, an external parameter optimization required in the QAOA could become a performance bottleneck. This motivates studies of the optimization landscape and search for heuristic ways of parameter initialization. In this work we visualize the optimization landscape of the QAOA applied to the MaxCut problem on random graphs, demonstrating that random initialization of the QAOA is prone to converging to local minima with suboptimal performance. We introduce the initialization of QAOA parameters based on the Trotterized quantum annealing (TQA) protocol, parameterized by the Trotter time step. We find that the TQA initialization allows to circumvent\r\nthe issue of false minima for a broad range of time steps, yielding the same performance as the best result out of an exponentially scaling number of random initializations. Moreover, we demonstrate that the optimal value of the time step coincides with the point of proliferation of Trotter errors in quantum annealing. Our results suggest practical ways of initializing QAOA protocols on near-term quantum devices and reveal new connections between QAOA and quantum annealing."}],"_id":"9760","language":[{"iso":"eng"}],"day":"01","citation":{"chicago":"Sack, Stefan, and Maksym Serbyn. “Quantum Annealing Initialization of the Quantum Approximate Optimization Algorithm.” <i>Quantum</i>. Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften, 2021. <a href=\"https://doi.org/10.22331/Q-2021-07-01-491\">https://doi.org/10.22331/Q-2021-07-01-491</a>.","ista":"Sack S, Serbyn M. 2021. Quantum annealing initialization of the quantum approximate optimization algorithm. Quantum. 5, 491.","ama":"Sack S, Serbyn M. Quantum annealing initialization of the quantum approximate optimization algorithm. <i>Quantum</i>. 2021;5. doi:<a href=\"https://doi.org/10.22331/Q-2021-07-01-491\">10.22331/Q-2021-07-01-491</a>","apa":"Sack, S., &#38; Serbyn, M. (2021). Quantum annealing initialization of the quantum approximate optimization algorithm. <i>Quantum</i>. Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften. <a href=\"https://doi.org/10.22331/Q-2021-07-01-491\">https://doi.org/10.22331/Q-2021-07-01-491</a>","short":"S. Sack, M. Serbyn, Quantum 5 (2021).","ieee":"S. Sack and M. Serbyn, “Quantum annealing initialization of the quantum approximate optimization algorithm,” <i>Quantum</i>, vol. 5. Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften, 2021.","mla":"Sack, Stefan, and Maksym Serbyn. “Quantum Annealing Initialization of the Quantum Approximate Optimization Algorithm.” <i>Quantum</i>, vol. 5, 491, Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften, 2021, doi:<a href=\"https://doi.org/10.22331/Q-2021-07-01-491\">10.22331/Q-2021-07-01-491</a>."},"has_accepted_license":"1","status":"public","scopus_import":"1","ec_funded":1,"file_date_updated":"2021-08-06T06:44:31Z","isi":1,"publication":"Quantum","date_updated":"2026-04-27T22:30:23Z"}]