JultikaUniversity of Oulu repository

Abstract

We are interested in a model for quantum computer that consists of superconducting qubits, transmons. These transmons are known to experience decoherence which causes unwanted dynamics in their quantum state. In this thesis, one form of decoherence experienced by the transmon, dephasing, is studied. The goal of this research is to differentiate the thermalization phase from the many-body localization phase in a transmon chain by studying dephasing of a single transmon. This phase transition is measured by simulating an array of transmons using the Bose–Hubbard model and calculating the dephasing rate R2 that a single transmon experiences. The many-body localization is shown to have significantly reduced dephasing rates compared to the rates observed at the thermalization phase in three different cases with varying ratio between the interaction strength and the hopping rate U/J = 2, 3.5 and 5. From these results we see that the phase transition is seen when the disorder strength is in the range of W/J = 10−15 on all three cases. An interesting future outlook could be to study the deeper features of the phase transition by examining the the effect of spin-echo pulses on the transmon dephasing.