Experiments in quantum optics : scalable entangled states and quantum computation with cluster states

Abstract

Quantum optics is an extremely fruitful area of research in that it allows the probing of fundamental and basic science, as well as the opportunity to investigate potential applications that exploit these exotic phenomena. Entanglement is a uniquely quantum phenomenon that is a key feature of quantum mechanics. In this thesis we investigate the scalability of entangled states of light, as well as modest demonstrations of one-way quantum computations, based on a resource that exploits entanglement. Scalability has been a bottleneck of entanglement experiments since the first demonstrations began surfacing a few decades ago. In Part II of this thesis we investigate different methods of multiplexing quantum light modes in order to achieve a scalable architecture. 2 experiments are presented in spatial multiplexing, that lead to programmable linear optics networks that can produce up to 8 entangled modes. Building on this, and shifting domains we then demonstrate temporal multiplexing, where we entangle tens of thousands of quantum modes together in a highly entangled state called a cluster state. This is several orders of magnitude larger than any entangled state created to date, in any platform (be it atomic or electronic). The highly entangled cluster state is a universal resource for quantum computing, and we investigate a particular class of computations in Part III. We demonstrate that a cluster state consisting of only 4 highly entangled quantum modes is sufficient for universal 1-mode Gaussian quantum computing. Further, we demonstrate a tuneable entanglement gate that can be retrofitted to cluster states of arbitrary size or shape. Although the experiments in Part II and Part III are conceptually quite different, they employ very similar optical and electronic equipment. These experimental techniques as well as a survey of relevant quantum optics theory is presented in Part I.