Quantum optics has successfully been used to test fundamental principles in quantum physics and to demonstrate the potential of quantum-enhanced technologies. Linear quantum optics, in which large quantum states of light are produced by optical interference of smaller quantum states, has proved to be particularly fruitful. Further progress will rely both on developing improved experimental tools to manipulate and measure quantum light, and on expanding and refining the range of applications of these technologies. This thesis presents our contributions towards both of these endeavours. We first focus on some of the components necessary for linear quantum optics, starting with the requirement for reconfigurable interference between several optical modes. We propose a novel design for interferometers that satisfy this requirement, which is based on a new mathematical decomposition of unitary matrices used to describe optical interference. We show that our design is more efficient than previously known designs. We also experimentally demonstrate a modular approach to building these devices, which is based on the assembly of multiple UV-written integrated photonic chips. These chips are characterised, and three of them are assembled into a structure shown to enable a wide range of optical transformations. We then study methods of photon detection, showing how photon detectors can be calibrated and discussing the operation of superconducting photon number resolving transition edge sensors. Next, we study two applications of linear optics. We examine the applicability of a proposal for simulating molecular spectroscopy using quantum optics in the presence of experimental imperfections. Our findings are illustrated with a proof of principle experiment in which we simulate part of the vibronic spectrum of the tropolone molecule. Finally, we study a class of optical devices, known as optical Ising machines, that has been shown to find solutions to difficult combinatorial problems. Describing the optical pulses in these devices in phase space as Gaussian quasi-probability distributions that evolve stochastically, we analyse the computational mechanism of these machines and show in theory that they can be simplified without affecting their performance.
Quantum optics has successfully been used to test fundamental principles in quantum physics and to demonstrate the potential of quantum-enhanced technologies. Linear quantum optics, in which large quantum states of light are produced by optical interference of smaller quantum states, has proved to be particularly fruitful. Further progress will rely both on developing improved experimental tools to manipulate and measure quantum light, and on expanding and refining the range of applications of these technologies. This thesis presents our contributions towards both of these endeavours. We first focus on some of the components necessary for linear quantum optics, starting with the requirement for reconfigurable interference between several optical modes. We propose a novel design for interferometers that satisfy this requirement, which is based on a new mathematical decomposition of unitary matrices used to describe optical interference. We show that our design is more efficient than previously known designs. We also experimentally demonstrate a modular approach to building these devices, which is based on the assembly of multiple UV-written integrated photonic chips. These chips are characterised, and three of them are assembled into a structure shown to enable a wide range of optical transformations. We then study methods of photon detection, showing how photon detectors can be calibrated and discussing the operation of superconducting photon number resolving transition edge sensors. Next, we study two applications of linear optics. We examine the applicability of a proposal for simulating molecular spectroscopy using quantum optics in the presence of experimental imperfections. Our findings are illustrated with a proof of principle experiment in which we simulate part of the vibronic spectrum of the tropolone molecule. Finally, we study a class of optical devices, known as optical Ising machines, that has been shown to find solutions to difficult combinatorial problems. Describing the optical pulses in these devices in phase space as Gaussian quasi-probability distributions that evolve stochastically, we analyse the computational mechanism of these machines and show in theory that they can be simplified without affecting their performance.
This dissertation explores the use of single photons in integrated photonic circuits as a platform for quantum computation and quantum simulation. In the noisy intermediate-scale quantum (NISQ) era, photonic implementations of boson sampling provide one of the most promising routes to demonstrating quantum advantage, while also enabling insight-driven experiments that probe many-body interference and computational complexity. The first part of the thesis is devoted to the hardware development and characterization of programmable silicon nitride photonic processors. We demonstrate large-scale multi-photon interference in a 20-mode universal processor, establishing the potential of integrated photonics for scaling up boson sampling experiments. In parallel, we introduce a validation protocol based on detector binning, implemented on a 12-mode predecessor of the device. Together, these studies advance the photonic platform itself, focusing on scalability, versatility, and benchmarking, rather than direct applications. The second part turns to quantum simulation. We first present a photonic simulation of a spin-foam amplitude from loop quantum gravity, illustrating how multi-photon interference can provide a testbed for problems in fundamental physics. We then implement a particle-number-difference Maxwell demon using conditional operations on photon subsets in a multi-mode interferometer. This experiment probes the link between thermodynamics and information theory, showing how a photonic platform with single-photon resolution and quantum interference can be used to explore information-driven control of physical systems. The final part addresses quantum computation, where we demonstrate a boson-sampling-accelerated Monte Carlo integrator. This hybrid quantum–classical algorithm represents one of the few proposals to extend boson sampling toward practical applications while plausibly retaining its computational hardness. We validate the method in a proof-of-principle experiment reproducing perturbative corrections in a model inspired by Efimov physics. Altogether, the results presented in this thesis position integrated photonics as both a scalable hardware platform for multi-photon experiments and a versatile tool for advancing quantum simulation and computation.
Measurements on states of light underpins the entire fields of classical and quantum optics. Without measurement we are unable to gain any information about the light we are studying or about the processes it has undergone. The property of the density matrix of light prior to measurement that we want to measure also relies heavily on the measurement technique we employ. We can use a single-photon detector to measure the particle nature of light--the photon--the discrete quanta of the electromagnetic field. We can conversely measure the wave nature of the light field by making phase-sensitive measurements. In this thesis, we shall explore some advanced implementations and applications of these measurement schemes. First, we shall focus on a specific type of single-photon detector: the Superconducting-Nanowire Single-Photon Detector. We will first consider the integration of these detectors on optical waveguides, and discuss their optical characterisation. Then we will consider a practical application of these detectors by investigating a method to perform broadband spectroscopy with a single detector. We shall then progress to phase-sensitive measurements, and consider the intermediate regime between measuring the wave-like and particle-like nature of light. First we will perform a fundamental investigation of the transition between both measurement regimes. We will see how varying the phase reference itself allows us to tune between both regimes. Following this we will discuss an application of this intermediate regime, by considering an experimental setup to perform state tomography with a weak phase reference. The final consideration of this thesis will be a theoretical benchmarking scheme for a practical application of quantum optics. The manipulation of certain quantum states of light followed by a photon-number measurement can have practical applications in a variety of fields, and therefore validating the results is of extreme importance. On this final topic, we will discuss an experimentally friendly technique to benchmark the output of such measurements by considering smaller subsystems, and the practical requirements of this technique.
In 2009, physicists at the National Institute of Standards and Technology in Colorado, Boulder developed what could arguable be called the first rudimentary quantum computer [1]. The past decade has seen unprecedented improvements in quantum information science culminating in the demonstration of quantum supremacy --- that quantum computers can solve problems that are impractical to be solved on the best supercomputers [2]. This remarkable progress necessitates the development of techniques to characterize the quantum devices that are being developed. In my thesis, I will focus on such devices that manipulate and detect light. In Chapter 1, I will introduce the reader to the historical underpinnings of the study of light. After surveying the history of light since time immemorial, I will delve into the developments in the study of light in the 19th and the early 20th century. I will then present a brief introduction to quantum mechanics. I end this chapter by demonstrating how light is quantized. In Chapter 2, I will introduce the various tools necessary to delve into this dissertation. After defining quadrature operators and Wigner functions, I will introduce various states of light used in this thesis. I will then discuss how the evolution, as well as detection of quantum optical states, are modelled. I will then present the formalism of Gaussian quantum information that simplifies the manipulation of certain states of light. Finally, I will briefly talk about quantum metrology and show its advantage in phase estimation. In Chapter 3, I will introduce a method to characterize linear and quadratically nonlinear optical systems. After motivating the need for this work, I will introduce the modifications necessary to the standard Mach-Zehnder interferometer in order to characterize optical systems with it. I will then show how to characterize linear optical systems with coherent probes. I will also show that this scheme is shot-noise limited. I will then show that by using single photons in addition, we can characterize quadratically nonlinear optical systems. Finally, I will show that no advantage is gained in sensitivity by using squeezed light along with coherent light as probes. In Chapter 4, I will introduce a method to characterize photodetectors. This is done by developing an experimental method to find the Wigner functions of the POVM set corresponding to a photodetector. After motivating the necessity of this work as well as describing the proposed experimental setup, I will show how this scheme can
It is believed that the exotic properties of quantum systems can be harnessed to perform certain computational tasks more efficiently than classical theories allow. The production, manipulation and detection of single photons constitutes a potential platform for performing such non-classical information processing. The development of integrated quantum photonics has provided a miniaturised, monolithic architecture that is promising for the realisation of near-term analog quantum devices as well as full-scale universal quantum computers. In this thesis we investigate the viability of these photonic quantum computational approaches from an experimental and theoretical perspective. We implement the first universally reconfigurable linear optical network; a key capability for the rapid prototyping of photonic quantum protocols. We propose and demonstrate the use of these devices as a new platform for the programmable quantum simulation of molecular vibrational dynamics. We then tackle an important outstanding problem in linear optical quantum computing; quantifying how partial-distinguishability amongst photons aff ects logical error rates. Finally, we propose a series of schemes aimed at counteracting these distinguishability errors in order to achieve practical quantum technologies with imperfect photonic components.
Nonlinear photonics studies situations in which light passing through a material can be used to create, reshape, or convert other light in ways that are not possible in ordinary optical systems. In recent years, this has become especially important for quantum photonics, where carefully prepared states of light are used as resources for emerging technologies such as secure communication, ultra-precise measurement, and new forms of information processing. A major challenge, however, is that these effects are delicate, difficult to control, and highly sensitive to the imperfections that arise when trying to build practical devices on a chip. This thesis examines how such nonlinear quantum-optical processes can be both engineered more deliberately and modelled more realistically, with the broader goal of helping move quantum photonic technology from proof-of-principle laboratory demonstrations toward robust and scalable implementation. To do this, the thesis first places nonlinear quantum photonics in a broader technological context and explains the main physical ideas needed to understand how quantum states of light are generated, transformed, and measured. It then develops this picture through a set of concrete examples. These include a method for generating quantum light on a chip while avoiding one of the field’s standard fabrication steps, a proposed chip-scale device that could help connect otherwise incompatible quantum systems by changing the colour and spectral width of light, and a modelling framework that links quantum theory directly to realistic fabricated waveguides, including common imperfections such as loss, non-uniformity, and writing errors. The thesis also reviews a wider class of design methods in which the nonlinear properties of a material are deliberately patterned in order to tailor the light produced. Taken together, the central message is that progress in quantum photonics will depend not only on discovering new effects, but on learning how to design, fabricate, and model these devices in ways that make them practical outside idealized laboratory conditions.
Technologies based on quantum mechanics promise to revolutionize the collection, processing and communication of information. However, due to the fragility of quantum coherence, complex quantum states can only exist in highly isolated and stable environments. One suitable environment is that of a quantum photonic chip. Quantum integrated photonics seeks to generate, process and detect complex quantum states inside a photonic chip. This thesis presents theory and experimental verification of novel approaches for the integration of various functionalities into quantum photonic chips in a scalable way. As such, this thesis encompasses a broad area of physics including quantum optics and nonlinear photonics. The results presented in this thesis have applications in the areas of quantum enhanced measurement, communication and information processing. In particular we develop the theory and experimentally demonstrate flexible on-chip sources of spatially entangled photons, the state of which can be reconfigured alloptically. We show how such techniques could enable the realization of simple cluster state quantum computing algorithms using spatially encoded two-photon states. Furthermore, we suggest new and practical approaches for the efficient characterization of mass produced nonlinear quantum photonic chips. Finally we develop and experimentally demonstrate a scalable method for the full quantum state tomography of multi-photon states on-chip. Importantly this technique only requires a linearly increasing number of single photon detectors relative to the number of photons in the state being characterized, and is also highly compatible with on-chip single photon detectors.