Routes towards optical quantum technology: new architectures and applications

As this century unfolds we will witness the rise of the quantum computer. A quantum computer is a device that utilizes the laws of one of the most fundamental theories of physics— quantum mechanics. By utilizing properties of quantum mechanics a quantum computer is able to perform certain information processing tasks much more effciently than an ordinary classical computer. The last century of modern society was revolutionized by some of the simplest ideas in quantum mechanics like energy quantization and quantum tunnelling, which led to technologies like the laser and the transistor. We can expect to see additional technological revolutions occur in this century since we are beginning to build technologies with some of the more complex properties of quantum mechanics such as quantum entanglement and quantum superposition. These quantum effects make more radical technologies like the quantum computer possible. This thesis is based upon the work I have done during my PhD candidature at Macquarie University. In this work we develop quantum technologies that are directed towards realising a quantum computer. Specifically, we have made many theoretical advancements in a type of quantum information processing protocol called Boson Sampling. This device effciently simulates the interaction of quantum particles called bosons, which no classical computer can effciently simulate. In this thesis we explore quantum random walks, which are the basis of how the bosons in Boson Sampling interfere with each other. We explore implementing BosonSampling using the most readily available photon source technology. We invented a completely new architecture which can implement Boson Sampling in time rather than space and has since been used to make the worlds largest Boson Sampling experiment ever performed. We look at variations to the traditional Boson Sampling architecture by considering other quantum states of light. We show a worlds first application inspired by Boson Sampling in quantum metrology where measurements may be made more accurately than with any classical method. Lastly, dealing with Boson Sampling, we look at reformulating the formalism of Boson Sampling using a quantum optics approach. In addition, but not related to BosonSampling, we show a protocol for effciently generating large-photon Fock states, which are a type of quantum state of light, that are useful for quantum computation. Also, we show a method for generating a specific quantum state of light that is useful for quantum error correction — an essential component of realising a quantum computer — by coupling together light and atoms.