Spin-photon and spin-spin interactions with quantum dots and wells for quantum information processing

Quantum information technology holds much promise for the future, but efforts over the past two decades to implement quantum computers and quantum repeaters have been hampered by the extraordinarily difficult challenges one faces when attempting to engineer fault-tolerant, controllable large-scale quantum systems. In this thesis, I present several theoretical and experimental results that aim to advance the state-of-the-art in the design and realization of physical implementations of quantum bits that have the potential to scale to the large numbers of physical qubits that are required to implement most useful quantum information processing devices. In general, solid-state qubits offer the advantage over atomic or ionic systems that they have the potential to be scaled more easily than most atomic or ionic qubit implementations. However, atomic and ionic qubits generally tend to offer much higher operation fidelities due to their greater isolation from the environment. Our work on the generation and tomography of an entangled qubit pair consisting of an electron spin qubit in a quantum dot, and a photonic qubit emitted by the quantum dot, which is described in this thesis, shows that semiconductor spin-photon interfaces can produce states with surprisingly high fidelities, rivaling the fidelities measured in many atomic and ionic implementations. Two major outstanding challenges in the optical quantum dot spin qubit community are the implementation of high-fidelity, fast, single-shot qubit readout, and the implementation of a scalable two-qubit gate that can operate on nearest-neighbour qubits in a regular array. I present several pieces of work that are related to these two challenges. Specifically, I present two different theoretical proposals for implementing single-shot spin readout, and I present experimental work that demonstrates the underlying physical mechanism in the latter proposal. This mechanism is based on the coupling of a spin in a quantum well to a gas of exciton-polaritons, formed in a quantum well situated close to the quantum dot. Despite more than a decade having passed since the first proposal calling for the use of exchange coupling of spin qubits to delocalized excitons in order to implement two-qubit operations, there has been scant experimental progress in this direction. I present our demonstration that the class of device envisaged in the polariton-based proposals can be implemented, and that the exchange interaction between a quantum dot electron spin and an exciton-polariton's electron spin results in a measurable energy shift. This is a first step towards the realization of the polariton-exchange-interaction-based proposals, and provides some hope that these proposals may yet yield fruit.\\ One of the key features of recent proposals (for example, see references for developing a quantum computer or a quantum repeater using optically-controlled semiconductor spin qubits is the use of a regular array of site-controlled spins. However, despite the significant progress in implementing spin initialization, control, readout, and a spin-photon interface using randomly-positioned charged quantum dots, there has been relatively little work done on demonstrating all these key operations in site-controlled quantum dots. In the final vignette of this thesis, I present our work on attempting to optically pump a spin in a quantum dot that is inside a site-controlled nanowire. Recent efforts to improve the optical quality of site-controlled quantum dots realized using a variety of growth and fabrication techniques have seen considerable success, so the demonstration of spin qubit operations in site-controlled quantum dots is a line of research that will likely be possible in the near future.

Keywords: Electrical Engineering.
Advisor: Yamamoto Yoshihisa 
Mabuchi Hideo 
Vuckovic Jelena