# Ordered spin states and quantum coherence in low-dimensional structures : quantum dots and nanowires

Kornich, Viktoriia. Ordered spin states and quantum coherence in low-dimensional structures : quantum dots and nanowires. 2016, Doctoral Thesis, University of Basel, Faculty of Science.

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Official URL: http://edoc.unibas.ch/diss/DissB_11688

We also studied Si/SiGe quantum dots as a potential candidate for a qubit. Apart from the absence of hyperfine interaction and bulk spin-orbit interaction in the isotopically purified $^{28}$Si, its electron-phonon interaction is different from GaAs that also leads to longer qubit lifetimes. We study $S$-$T_-$ qubit near the anticrossing of the basis states. This particular region is interesting due to possibilities in operating the qubit. We show that the type of singlet plays a crucial role, i.e. whether it is a singlet with each dot singly occupied or a singlet with only one dot doubly occupied. Depending on the type of singlet the qubit lifetimes change by several orders of magnitude. We also study the influence of a micromagnet, usually used in experiments to operate the qubit, on the relaxation time and decoherence time and present the regime where its effect is negligible. We suggest how to test experimentally our theory of one-phonon and two-phonon processes separately. We also show how the relaxation and decoherence time depend on different system parameters for $S$-$T_0$ qubits.