<p dir="ltr">Quantum networking holds tremendous promise in transforming computation and communication. While matter-based systems excel as memory nodes, photons are ideal for long-distance transmission. Hence, a hybrid network combining both becomes essential. Moreover, developing entangled photon pair sources is critical for quantum repeaters and network implementation. The realization of these capabilities on integrated photonic circuits is vital for miniaturization and scalability. In this dissertation, we focus on two key aspects: establishing efficient photon-to-memory interfaces and generating and manipulating entangled states within integrated platforms.</p><p dir="ltr">One research direction involves developing an efficient interface between photons and matter-based memory, requiring spectral and temporal mode matching. Spectral compression is inevitable to realize low-loss interconnection between intrinsically narrowband memories and broadband photons. We proposed a novel approach using electro-optic time-varying cavities for spectral compression. Currently, we are working toward realizing this approach on the thin film lithium niobate platform.</p><p dir="ltr">In the other research focus, we encode quantum information as a coherent superposition of multiple optical frequencies; this approach is favorable due to its simplicity in generating high-dimensional entanglement and compatibility with fiber transmission. We successfully generated and reconstructed the density matrix of biphoton frequency combs from integrated silicon nitride microrings, achieving an 8x8 two-qudit dimensionality, the highest to date for frequency-bin qudits. Moreover, we employ Vernier electro-optic phase modulation methods to perform time-resolved measurements of biphoton correlation functions. Currently, we are exploring bidirectional pumping of microrings to generate indistinguishable entangled pairs in both directions, aiming to demonstrate key networking operations such as entanglement swapping and GHZ state generation in the frequency domain. We are also pursuing bidirectional pumping in a Sagnac configuration to generate simultaneous entanglement in both polarization and frequency, with the goal of deployment in a wavelength-multiplexed</p><p dir="ltr">network.</p>

Metallic nano-structures provide for new and exciting domains to investigate light-matter interactions. The coupling of these metallic nano-structures to semiconductor emitters allows for the observation of cavity QED effects including Purcell enhancement and Vacuum Rabi splitting. The focus of this dissertation will be to present an introduction and background to semiconductor optics, and metallic metamaterial systems. This will be followed by the presentation of the spectroscopy systems designed and constructed during my tenure as graduate student and the experimental data obtained with these systems. Some of the results have been published, while some of the presented material is still actively being pursued for publication. More specifically, the dissertation will cover the research at hand, experimental techniques, and results.

This thesis is concerned with the generation of non-classical quantum states of light, the photon-level manipulation of quantum states and the accurate tomography of both quantum states and quantum processes. In optics, quantum information can be encoded and processed in both discrete and continuous variables. Hybrid approaches combining for example homodyne detection with conditional state preparation and manipulation are gaining increasing prominence. The development and characterization of a time-domain balanced homodyne detector (BHD) is presented. The detector has a bandwidth of 80 MHz, a signal-to-noise ratio of 14.5 dB and an efficiency of 86% making it well-suited to pulse-to-pulse measurement of quantum optical states. The BHD is employed to perform quantum state tomography (QST) of non-classical multi-photon Fock states generated by spontaneous parametric down-conversion. A detailed investigation of the mode-matching between the local oscillator used for homodyne detection and the generated Fock states is presented. The one-, two- and three-photon Fock states are reconstructed with a combined preparation and detection efficiency exceeding 50%. Fock states have a number of applications in quantum state engineering, where non-classical ancilla states and conditional measurements enable photon-level manipulation of quantum states. Fock state filtration (FSF) is investigated - an example of a post-selected beam splitter which is a basic building block for many quantum state engineering protocols. A model is developed incorporating the effect of experimental imperfections. An experimental implementation of a Fock state filter is fully characterized by means of coherent-state quantum process tomography (QPT). The reconstructed process is found to be consistent with the model. The filter preferentially removes the single-photon component from an arbitrary input quantum state. Calibration of optical detectors in the quantum regime is discussed. Quantum detector tomography (QDT) is reviewed and contrasted with a new technique for performing QST with a calibrated detector known as the fitting of data patterns (FDP). The first experimental characterization of a BHD is performed by probing the detector with phase-averaged coherent states. The FDP method is shown to be applicable to the estimation of quantum processes, where a detector response is not assumed - thus demonstrating the versatility of the FDP approach as a new method in the quantum tomography toolbox.

In this thesis, we are concerned with continuous-variable quantum optical state engineering protocols. Such protocols are designed to repair or enhance the nonclassical features of a given state. In particular, we build a weak measurement model of Gaussian entanglement concentration of the two mode squeezed vacuum state. This model allows the simultaneous description of all possible ancilla system variations. In addition, it provides an explanation of the Gaussian-preserving property of these protocols while providing a success criterion which links all of the degrees of freedom on the ancilla. Following this, we demonstrate the wider application of weak measurements to quantum optical state engineering by showing that they allow probabilistic noiseless amplifi cation of photon number. We then establish a connection between weak measurements and entanglement concentration as a fundamental result of weak measurements on entangled probes. After this, we explore the trade-off between Gaussian and non-Gaussian operations in the preparation of non-Gaussian pure states. In particular, we suggest that an operational cost for an arbitrary non-Gaussian pure state is the largest Fock state required for its approximate preparation. We consider the extent to which this non-Gaussian operational cost can be reduced by applying unitary Gaussian operations. This method relies on the identification of a minimal core state for any target non-Gaussian pure state.

La dissertation explore différentes applications des fibres optiques en ingénierie quantique. Deux thématiques sont développées : d'une part l'utilisation des fibres optiques monomodales en silice pour l'implémentation d'algorithmes et de protocoles de communication quantiques et d'autre part l'utilisation de la non-linéarité de ces fibres pour réaliser des sources de paires de photons corrélés. L'étude est à la fois théorique et expérimentale./ The dissertation explores various uses of optical fibers for quantum engineering. Two topics are developed : first the use of single-mode silica fibers for implementing quantum algorithms and communication protocols, second the use of these fibers for generating correlated photon-pairs.

Light, which is composed of discrete quanta, or photons, is one of the most fundamental concepts in physics. Being an elementary entity, the behaviour of photons is governed by the rules of quantum mechanics. The ability to create, manipulate and measure quantum states of light is not only useful in foundational tests of quantum theory, but also in a wide range of quantum technologies – which aim to utilize non-classical properties of quantum systems to perform tasks not possible with classical resources. Only recently has it been possible to control the properties of number states of light, which have a fixed photon-number. Two-photon states are central to testing fundamental physical theories (such as locality and reality) and the implementation of quantum information technologies. The versatility of photon-pair states is en- abled by the potential entanglement properties it can posses. Thus controlling the correlations between photons is crucial to both pure and applied physics. To produce a single photon, a photon-pair state can be used. Detection of one photon indicates its twin’s existence. Many applications, such as optical quantum computation, require pure indistinguishable single photons. Heralding single pho- tons from a photon-pair will, in general, produce single photons in a mixed quantum state due to correlations within the pair. A common approach to creating photon-pairs is through the nonlinear sponta- neous four-wave mixing interaction in optical fibres. This thesis presents a theoreti- cal and experimental implementation of a scheme to tailor the spectral correlations within the pairs. Emphasis is placed on engineering the two-photon state such that they are completely uncorrelated. Spatial entanglement is naturally avoided due to the discrete nature of the optical fibre modes. Spectral correlations are eliminated by careful choice of dispersion characteristics and conditions. The purity of the photons generated by this scheme is demonstrated by means of two-photon inter- ference from independent sources. We measure a purity of (85.9 ± 1.6)% with no spectral filtering, exhibiting the usefulness of this source for quantum technologies and applications.

Systems of ultracold atoms in optical potentials have taken a place at the forefront of research into many-body atomic systems because of the clean experimental environment they exist in and the tunability of the system parameters. In this thesis we study how light scattered from these ultracold atomic gases reveals information about the state of the atomic gas and also leads to changes in that state. We begin by investigating the angular dependence of light scattered from atoms in optical lattices at finite temperature. We demonstrate how correlations in the superfluid and Mott insulator states affect the scattering pattern, and we show that temperature affects the number of photons scattered. This effect could be used to measure the temperature of the gas, however, we show that when the lattice band structure is taken into account the efficiency of this temperature measurement is reduced. We then investigate light scattering from small optical lattices where the Bose-Hubbard Hamiltonian can be solved exactly. For small lattices, scattering a photon from the atomic system significantly perturbs the atomic system. We develop a model of the evolution of the many-body state that results from the consecutive scattering and detection of photons. This model shows that light scattering pushes the system towards eigenstates of the light scattering measurement process, in some cases leading to a superposition of atomic states. In the second half of this thesis we study light scattering that depends on the internal hyperfine spin state of the atoms, in which case the scattered light can form images of the spatial atomic spin distribution. We demonstrate how scattering spatially correlated light from the atoms can result in spin state images with enhanced spatial resolution. We also show how using spatially correlated light can lead to direct measurement of the spatial correlations of the atomic spin distribution. We then apply this theory of spin-dependent light scattering to the detection of different spin states of ultracold gases in synthetic magnetic fields. We show that it is possible to distinguish between ground states in the quantum Hall regime using light scattering. Moreover, we show how noise correlation analysis of the spin state images can be used to identify the correlations between atoms and how a variant on phase-contrast imaging can reveal the relationship between the atomic spins.

Photons have proven to be an effective test-bed for the fundamental concepts and elements of quantum-enhanced technologies. As systems become increasingly complex, however, practical considerations make the traditional approach of bulk optics and free-space propagation progressively more difficult. The major obstacles are the physical space necessary to realise and operate such a complex system, its stability, and maintaining low losses. In order to address these issues, quantum optical technologies can take a cue from their classical counterparts and look towards an integrated architecture to provide miniaturisation, greatly enhanced stability, less alignment, and low loss interfaces between different system components. In this thesis the feasibility of chip-based waveguides as a platform for metrology and information processing will be explored. In Part I, the necessary criteria for a metrology system to out-perform its classical counterpart will be investigated. It will be found that loss is a major barrier to this aim and, critically, that it is unlikely to have been achieved to date by any experiment which consumes resources of a fixed photon number. The issue of loss will be addressed by developing a scalable heralded source of a class of entangled photonic states which are both robust to losses and practically feasible to prepare. A novel tomographic technique will be developed to characterize these states and it will be explicitly demonstrated how it is possible to beat some bounds on classical performance without being able to out-perform a comparable classical system. Finally, a proof of principle demonstration of a waveguide-based interferometer with an integrated phase-shifter will be undertaken. It will be shown that the device preserves quantum interference, making it suitable for use in quantum-enhanced metrology applications. In Part II, integrated optics in the context of information processing will be discussed. First, a novel characterization technique will be developed which enables the behaviour of complex circuits to be predicted. The technique is independent of loss in the device being characterized. A method of simulating these circuits will be outlined that takes advantage of the computational speed-up available from parallelisation and sparse matrix operations. A key increase in complexity for integrated photonic systems will be demonstrated by showing quantum interference of three photons from two separate sources in eight spatial modes. The resulting interference has a visibility which beats all possible classical interference visibilities for similar circuits. Finally, a fully integrated waveguide-coupled photon-number-resolving detector will be developed and demonstrated. This proof of concept demonstration will show good resolution of different photon number events. The device will be modelled and routes to high efficiency operation will be explored.

Cavity quantum electrodynamics (cavity QED) with single atoms and single photons provides a promising route toward scalable quantum information processing (QIP) and computing. A strongly coupled atom-cavity system should act as a universal quantum interface, allowing the generation and storage of quantum information. This thesis describes the realisation of an atom-cavity system used for the production and manipulation of single photons. These photons are shown to exhibit strong sub-Poissonian statistics and indistinguishability, both prerequisites for their use in realistic quantum systems. Further, the ability to control the temporal shape and internal phase of the photons, as they are generated in the cavity, is demonstrated. This high degree of control presents a novel mechanism enabling the creation of arbitrary photonic quantum bits.

Quantum information is a highly interesting and fast emerging field that involves processing information encoded into quantum systems and their subsequent use in various information tasks. The use of quantum resources such as superposition and entanglement have shown to enhance information processing capabilities beyond classical means in a number of communication, information and computation tasks. In this thesis, we have used single photons to study the advantage of d-level quantum systems (qudits) for a communication task commonly known as random access codes (RACs). A successful experimental demonstration of quantum random access codes (QRACs) with four dimensions is realized to demonstrate that the higher dimensional QRACs not only outperform the classical RACs but also provide an advantage over their quantum bit (qubit) counterparts. QRACs are also studied in regards to two specific applications: certification of true randomness and for testing the non-classicality of quantum systems. A method for increased certification of generated randomness is realized for the former and a successful experimental demonstration of a test of non-classicality with arbitrarily low detection efficiency is provided for the latter. This is followed by an implementation of a QRAC in a one-path communication network consisting of preparation, transformation and measurement devices. We have shown that the distributed QRAC provides optimal success probabilities for a number of tasks. Moreover, a novel quantum protocol for the solution to the problem of dining cryptographers and anonymous veto voting is also presented. This single photon transmission based protocol provides an efficient solution, which is experimentally demonstrated for a 3-party description. Lastly, Nitrogen-Vacancy (NV) center in diamond is studied as a potential resource for single photon emission and two methods to enhance the photon collection efficiency are successfully explored. Due to this enhancement, single photons from an NV center may also be used in similar single quantum system based communication experiments. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript.</p>