Implementation of a Universal Fine-Structure Splitting Eraser for Quantum Dots

Abstract

Entangled photons are an important resource for different applications in quantum computation, quantum information, and quantum communication. Sources based on semiconductor quantum dots have successfully obtained on-demand, highly entangled photon pairs with a high repetition rate. However, the fidelity of the photon pair to a Bell state is reduced due to fine structure splitting (FSS) in the exciton state of the biexciton-exciton cascade. The FSS causes the exciton to precess during its lifetime resulting in a time-dependent entangled state which, when larger than the linewidth of a photon ($\approx$1 $\mu$eV), can reveal ‘which-path’ information for the exciton and biexciton cascade. A detector with a small timing jitter can resolve the dynamics of the entangled photon states. Real-world applications make it desirable to remove the FSS. Current techniques to remove the FSS include applying a strain, electric, or magnetic field. These techniques require post-processing of QD sources and are difficult to implement in conjunction with nanostructures such as nanowires or micropillar cavities. Here, we present an all-optical method of removing the FSS, which requires no post-processing of the source and works for any quantum dot sources. We propose frequency shifting single photons by emulating a fast-rotating half-waveplate using an electro-optical lithium niobate waveguide. In this thesis we show this method is capable of frequency shifting photons with high efficiency (88.4\%). We also examine how the efficiency is affected by the frequency that we are shifting by, and how the the set-up performs when operating over an extended period of time. We also examine several electro-optical devices to see if their behaviour is consistent and predictable.

In this thesis we also examine the building blocks for implementing a quantum circuit using entangled photons on a silicon nitride chip. These chips can be used instead of traditional free-space optics which take up a significant footprint, make it challenging to switch between experiments, and face losses because of coupling inefficiencies and attentuation. Silicon nitride is a low-loss waveguide material that, unlike the more traditional silicon, allows for the transmission of near-infrared photons. However, the newer material doesn't have significant libraries of pre-built components that can be used to create larger circuits. We examine the process of simulating, building, and integrating two of these components into a open-source library. This is the start of a process of integrating quantum dot entangled sources with building scalable quantum systems.