Bosonic many-body localization and collective phenomena in arrays of transmon devices

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Linnanmaa, lecture hall L5

Topic of the dissertation

Bosonic many-body localization and collective phenomena in arrays of transmon devices

Doctoral candidate

Master of science Tuure Orell

Faculty and unit

University of Oulu Graduate School, Faculty of Science, Nano and molecular systems research unit

Subject of study

Physics

Opponent

Assistant Professor Ana Asenjo-Garcia , Columbia University

Custos

Docent Matti Silveri, University of Oulu

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Bosonic many-body localization and collective phenomena in arrays of transmon devices

Quantum computer is a machine that can solve specific problems much faster than a classical one by exploiting quantum mechanical phenomena. Computations in a classical computer are performed with bits, that can be in a state 0 or 1. Quantum computer instead uses qubits, that can be in the states 0 and 1 simultaneously. Most promising candidates for the qubits are superconducting transmon circuits.

In order to perform quantum computations, the qubits must be isoleted from their environment, because coupling with the environment destroys their quantum nature. Errors caused by the environment are currently one of the biggest challenges of the quantum computation. Moreover, it is difficult to manufacture identical superconducting circuits. Due to this, the qubit systems contain disorder, which hinders their applicability. In addition to quantum computation, these systems can also be applied in quantum simulations, where complicated quantum systems are modeled experimentally on a simpler one. In this work different many body phenomena that could be simulated on currently achievable devices were studied numerically and theoretically.

In the first part of this work, the interaction of the qubits with an electromagnetic field inside a waveguide was studied. This interaction causes collective phenomena that can be seen a rapidly decaying bright states, and very slowly decaying dark states. These dark states formed by multiple qubits could be used in quantum computation as single logical qubits, since they are much better isolated from the environment than single qubits. This part of the work was done in collaboration with an experimental group, whose measurements agreed with theoretical results to high accuracy.

In the second part the effects of disorder were studied. Due to the manufacturing process, there exists deviation in the frequencies of the qubits. Interacting systems with sufficiently strong disorder undergo a phase transition to many-body localized phase. In this phase the system does not obey the laws of thermodynamics. This phase transition was studied numerically, and it was discovered that in superconducting circuits it occurs with experimentally obtainable parameters. These devices are thus well suited for the study of disordered systems, and localization phenomena have readily been observed also experimentally.

Essential part of the study was to take into account also the higher excitation states of the superconducting devices. It was observed that these states have a large impact on the behavior of these systems.
Last updated: 22.4.2022