Beyond hard-core bosons in transmon arrays: ground state phases and dynamics

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

University of Oulu, auditorium L6

Topic of the dissertation

Beyond hard-core bosons in transmon arrays: ground state phases and dynamics

Doctoral candidate

Master of Science Olli Mansikkamäki

Faculty and unit

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

Subject of study



Professor André Eckardt, Technical University of Berlin


Docent Matti Silveri, University of Oulu

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Beyond the second energy level in transmon arrays

Transmons are currently the most common basic building block of quantum computers. While they are usually used as a qubit, that is, only the two lowest energy levels are utilised, there are possible uses for the higher levels as well. Beyond the two-level approximation, the behaviour of transmon arrays can be well approximated with the Bose-Hubbard model with attractive interactions. Here we discuss analytical and numerical studies of the model, focusing on the higher excited levels of transmons.

We begun by studying the ground states of coupled transmons including the effect of disorder, an unavoidable feature of manufactured devices, and found three distinct ground state phases. Next, we studied the dynamics of the model. Using the wide-gapped band structure of the Bose-Hubbard spectrum, we provide an approximate description of the dynamics within each band. The dynamics effectively occur within a single band, resulting in various forms of collective behaviour, such as bosons on a single site moving as a single quasiparticle and effective longer-range interactions between multiple quasiparticles. Finally, to account for the imperfect isolation of the transmons from their environment, we include its effect into our model of the dynamics. An understanding of the dynamics of higher levels of the transmons may be used in designing novel implementations of multi-qubit gates, or multi-level qudit based quantum information protocols.
Last updated: 11.4.2023