Theoretical Framework of Exchange Coupled Tripartite Spin Systems with Magnetic Anisotropy and Predictions of Spin and Electronic Transport Properties for Their Use in Quantum Architectures

Abstract

There has been significant interest in spin systems involving two or more coupled spins as a single logical qubit, particularly for scalable quantum computing architectures. Recent realizations include the so-called singlet-triplet qubits and coupled magnetic molecules. An important class of coupled-spin systems, the three-spin paradigm for spin greater than 1/2, has not yet been fully realized in scalable qubit architectures. In this thesis, I develop the theoretical framework to investigate a class of tripartite spin models for realistic systems. First, I model a spin 1/2 particle (e.g., an electron) and two spin 1 particles (in a dimer arrangement) coupled with an exchange interaction. I find that if the two spin particles possess zero-field magnetic anisotropy, there exists resonance conditions that enable read, manipulate, and write operations on the representative qubit using the electron. Next, I generalize this result for any spin S, and describe how the resonance conditions change based on the type of exchange coupling, magnetic anisotropy, and magnitude of applied magnetic fields. The rest of the thesis is dedicated to utilizing the tools described in the framework to uncover the properties of potential scalable quantum architectures. To guide the correspondence between experiment and model Hamiltonians of effective tripartite spin systems connected to leads, I investigate the transport properties of a three-terminal quantum dot coupled to a magnetic molecular dimer using the generalized master equation. I then model both steady state and transient phenomena using equilibrium and non-equilibrium Green\u27s functions (NEGF), and comment on the applicability of a newly-developed NEGF-derived quantum master equation. Finally, I characterize two examples of novel quantum systems: the spin qubit candidate h-BN VB- and the thin film FeBipy spin-crossover molecule