Universität zu Köln

Berke, Christoph (2023). Transmon-based quantum computers from a many-body perspective. PhD thesis, Universität zu Köln.

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Abstract

The quest for quantum computers is in full swing. Over the past decade, the frontiers of quantum computing have broadened from exploring few-qubit devices to developing viable multi-qubit processors. One of the protagonists of the present era is the superconducting transmon qubit. By harmoniously combining applied engineering with fundamental research in computer science and physics, transmon-based quantum processors have matured to a remarkable level. Their applications include the study of topological and nonequilibrium states of matter, and it is argued that they have already ushered us into the era of quantum advantage. Nevertheless, building a quantum computer that can solve problems of practical relevance remains a massive challenge. As the field progresses with unbridled panache, the question of whether we have a comprehensive picture of the potential dangers lurking in the wings acquires increasing urgency. In particular, it needs to be thoroughly clarified whether, with viable quantum computers of O(50) qubits at hand, new and hitherto unconsidered obstacles associated with the multi-qubit nature can emerge. For example, the high accuracy of quantum gates in small-scale devices is hard to obtain in larger processors. On the hardware side, the unique requirements posed by large quantum computers have already spawned new approaches to qubit design, control, and readout. This thesis introduces a novel, less applied perspective on multi-qubit processors. Specifically, we fuse the field of quantum engineering and many-body physics by applying concepts from the theories of localization and quantum chaos to multi-transmon arrays. From a many-body per- spective, transmon architectures are synthetic systems of interacting and disordered nonlinear quantum oscillators. While a certain amount of coupling between the transmons is indispensable for performing elementary gate operations, a delicate balancing with disorder—site-to-site varia- tions in the qubit frequencies—is required to prevent locally injected information from dispersing in extended many-body states. Transmon research has established different modalities to cope with this dilemma between inefficiency (slow gates due to small coupling or large disorder) and information loss (large couplings or too small disorder). We analyze small instances of transmon quantum computers in exact diagonalization studies, using contemporary quantum processors as blueprints. Scrutinizing the spectrum, many-body wave functions, and qubit-qubit correlations for experimentally relevant parameter regimes reveals that some of the prevalent transmon de- sign schemes operate close to a region of uncontrollable chaotic fluctuations. Furthermore, we establish a close link between the advent of chaos in the classical limit and the emergence of quantum chaotic signatures. Our concepts complement the traditional few-qubit picture that is commonly exploited to optimize device configurations on small scales. Destabilizing mecha- nisms beyond this local scale can be detected from our fresh perspective. This suggests that techniques developed in the field of many-body localization should become an integral part of future transmon processor engineering.

Item Type:	Thesis (PhD thesis)
Translated title:	Title Language Transmon-Quantencomputer aus der Vielteilchenperspektive German
Creators:	Creators Email ORCID ORCID Put Code Berke, Christoph cberke1@uni-koeln.de UNSPECIFIED UNSPECIFIED
Contributors:	Contribution Name Email Author UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-719973
Date:	2023
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute for Theoretical Physics
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Quantum Computer English Quantum Chaos English Transmons English
Date of oral exam:	15 February 2023
Referee:	Name Academic Title Trebst, Simon Prof. Dr. Rosch, Achim Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/71997