Hybrid devices for protected quantum information processing

Abstract

Quantum information is very fragile, and it is clear that we need to develop better qubits and employ error correction, in order to make use of the full potential that quantum computation has. In this thesis we explore known error correction codes and promising platforms for topological quantum computation. Furthermore, we look at new ways of engineering states which are useful for error correction using hybrid devices, and investigate properties of superconductor-semiconductor hybrid devices which have gained much attention due to their many interesting properties, such as possibly hosting topological superconductivity. In the first part of the thesis we explore the engineering of quantum states by the use of hybrid devices. First, we look at a qubit coupled to a microwave cavity. By driving the qubit through level crossings, we show that it is possible to create Schrödinger-cat states. After this, we look at a spin-qubit coupled to an anisotropic ferromagnet, which we show is a physical realization of the quantum Rabi model. Furthermore, we show that by expanding the hybrid device to include 3 qubits, we are able to drive all three qubits simultaneously, creating a GHZ state, in a way that is robust against qubit asymmetries. In the second part of the thesis, we look at hybrid devices made from superconductors and semiconductors. We start by introducing concepts that are necessary to describe these systems, before looking at one of the possible applications of these systems, namely in topologically protected quantum computation. The detailed spin dynamics of these devices can, however, heavily depend on features such as the microscopic details of the device or strain. The two last chapters investigates the spin-dynamics of 1D and 2D hybrid devices. We first look at a nanowire superconductor-normal-superconductor (SNS) junction, with spin-orbit coupling and an external magnetic field, where we derive an analytical expression for the critical current of the junction. Lastly, we look at a 2D hole gas SNS junction, with spin-orbit coupling and an external magnetic field, were we also here derive (semi)-analytic expressions for the critical current in limiting cases.