Electronic, Magnetic, and Optical Properties of Carbon-based Molecules for Qubit Application
Author
Andersen, Kasper Porsborg
Term
4. term (FYS10)
Education
Publication year
2026
Submitted on
2026-05-28
Pages
81
Abstract
Carbon-based nanographenes—tiny, precisely made fragments of graphene—are promising platforms for molecular qubits because their weak spin–orbit coupling helps keep electron spins stable. This thesis uses a specific nanographene called Clar’s goblet to examine its electronic, magnetic, and optical properties and evaluate its suitability as a qubit. The electronic structure is computed with a mean-field Hubbard model and closely matches results from density functional theory (DFT). From these calculations, total hyperfine tensors (parameters that describe interactions between electron and nuclear spins) are obtained for both the neutral molecule and its radical cation, using generic sp2-hydrocarbon fit parameters. These parameters reproduce measured continuous-wave electron paramagnetic resonance (CW-EPR) spectra for both charge states. To assess qubit performance, pulse-EPR experiments (Hahn echo and inversion recovery) are simulated using the phenomenological Bloch equations and the Lindblad master equation, yielding decay rates γz = 0.30 MHz and γ± = 0.00265 MHz. Together, these results provide a practical framework for simulating the time evolution of a qubit in this structure, which can be extended to other planar carbon-based molecules.
Kulstofbaserede nanografener—små, præcist fremstillede fragmenter af grafen—er lovende platforme for molekylære kvantebits, fordi deres svage spin–banekobling hjælper med at stabilisere elektronspins. Denne afhandling bruger en specifik nanografen kaldet Clar’s goblet til at undersøge dens elektroniske, magnetiske og optiske egenskaber og vurdere dens egnethed som kvantebit. Den elektroniske struktur beregnes med en middelfelts Hubbard-model og stemmer tæt overens med resultater fra tæthedsfunktionalteori (DFT). På baggrund af disse beregninger bestemmes de samlede hyperfine tensorer (parametre, der beskriver vekselvirkninger mellem elektron- og kernespin) for både den neutrale form af molekylet og dets radikalkation ved brug af generiske sp2-kulbrinteparametre. Disse parametre genskaber målte kontinuerlig-bølge elektronparamagnetiske resonans (CW-EPR) spektre for begge ladningstilstande. For at vurdere anvendeligheden som kvantebit simuleres puls-EPR-sekvenser (Hahn echo og inversion recovery) med de fænomenologiske Bloch-ligninger og Lindblad master-ligningen, hvilket giver henfaldsraterne γz = 0.30 MHz og γ± = 0.00265 MHz. Tilsammen giver dette et praktisk rammeværk til at simulere tidsevolutionen af en kvantebit i denne struktur, som kan udvides til andre plane kulstofbaserede strukturer.
[This apstract has been rewritten with the help of AI based on the project's original abstract]
Keywords