In order to harness the enormous potential of schemes in optical quantum information processing, readily scalable photonic circuits will be required. A major obstacle for this scalability is the monolithic integration of quantum light sources with the photonic circuit on a single chip. This dissertation presents the experimental demonstration of different in-plane single photon sources that allow for this integration with planar light circuits. To this end, the spontaneous recombination of excitons in single indium arsenide quantum dots was exploited to generate single photons. The emission into on-chip waveguides was achieved by the use of advanced two-dimensional photonic crystal structures. First, slow-light effects in a unidirectional photonic crystal waveguide were exploited to achieve on-demand single photon emission with a rate of up to 18.7 MHz, corresponding to a remarkable estimated internal device efficiency of up to 47%. Waveguide-coupled L3 defect cavities with record Q-factors of up to 5150 were then studied for improved Purcell enhancement of the emission, and in-plane single photon generation from such a device was demonstrated. Finally, an electrically tunable, integrable quantum light source with a total tuning range of 1.9 nm was demonstrated by exploiting the quantum-confined Stark effect in an electrical PIN diode. These results are the first demonstrations of in-plane single photon emission at optical wavelengths and mark an important cornerstone for the realisation of fully integrated quantum photonic circuits in optical quantum information science.

Due to its inherent stability and compactness, integrated optics can allow for experimental complexity not currently achievable with bulk optics. This opens up the possibility for large-scale quantum technological applications, such as quantum communication networks and quantum information processing. Quantum information processing relies on efficient sources of entangled photon pairs. Most demonstrations in integrated photonics so far have featured the on-chip manipulation of photon states using a free-space bulk-optic source of photons. This has the drawback of introducing loss due to the spatial mode mismatch between waveguide modes of the chip and modes of the produced photons. In this way, loss limits the number of photons that are simultaneously carried in the integrated optical device, and thus limits the number of qubits. One way to avoid this loss is to generate the photons in another waveguide device. This can be done through, for example, spontaneous four-wave mixing (SFWM). In this third-order nonlinear process, two pump photons spontaneously scatter off each other to create two photons of two new frequencies, satisfying momentum and energy conservation. This has been studied in birefringent optical fibers and photonic crystal fibers. In this work, we investigate the SFWM generation of photons in a waveguide coupler comprised of two touching tapered optical fibers, which we call a microcoupler. The two silica fibers are kept in contact and tapered to be 1 micron in diameter in the 10 cm long uniform interaction region. This device has three main advantages over a standard telecom 2x2 fiber coupler. 1) The small mode area enhances the photon generation rate; 2) The microcoupler supports four modes which is the minimum number required for two-photon entanglement. So in principle the device should be able to produce polarization-entangled photon pairs; 3) The strong waveguide-waveguide coupling and waveguide dispersion (due to the tapering) forces the photons to be far in wavelength from the background light around the pump. We present the 28 allowed phasematching processes for the microcoupler, as well as predict the frequencies of the generated photons. We report the first experimental observation of photon pairs produced via SFWM in a microcoupler. We also analyze the polarization state of the observed photons to figure out which phasematching processes are responsible for generating the photons. We expect to observe more photon pairs in future devices, with the ultimate goal being the generation of polarization-entangled photon pairs for integrated optics.

The development of integrated quantum photonics is integral to many areas of quantum information science, in particular linear optical quantum computing. In this context, a diversity of physical systems is being explored and thus versatility and adaptability are important prerequisites for any candidate platform. Silicon oxynitride is a promising material because its refractive index can be varied over a wide range. This dissertation describes the development of silicon oxynitride waveguides for applications in the field of integrated quantum photonics. The project consisted of three stages: design, characterisation, and application. First, the parameter space was studied through simulations. The structures were optimised to achieve low-loss devices with a small footprint at a wavelength of 900 nm. Buried channel waveguides with a cross-section of 1.6 ?m x 1.6 ?m and a core (cladding) refractive index of 1.545 (1.505) were chosen. Second, following their fabrication with plasma-enhanced chemical vapour deposition, electron beam lithography, and reactive ion etching, the waveguides were characterised. The refractive index was shown to be tunable from the silica to the silicon nitride regime. Optimised tapers significantly improved the coupling efficiency. The minimum bend radius was measured to be less than 2 mm. Propagation losses as low as 1.45 dB cm-1 were achieved. Directional couplers with coupling ratios ranging from 0 to 1 were realised. Third, building blocks for linear optical quantum computing were demonstrated. Reconfigurable quantum circuits consisting of Mach-Zehnder interferometers with near perfect visibilities were fabricated along with a four-port switch. The potential of quantum speedup was illustrated by carrying out the Deutsch-Jozsa algorithm with a fidelity of 100 % using on-demand single photons from a quantum dot. This dissertation presents the first implementation of tunable Mach-Zehnder interferometers, which act on single photons, based on silicon oxynitride waveguides. Furthermore, for the first time silicon oxynitride photonic quantum circuits were operated with on-demand single photons. Accordingly, this work has created a platform for the development of integrated quantum photonics.

In this thesis, the effect of surfactants (Bi /Sb) on GaAs(111) is explored, particularly in regards to modifying the surface morphology and growth kinetics. Both molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) techniques are discussed in this context. InAs/GaAs(111) quantum dots (QDs) have been promoted as leading candidates for efficient entangled photon sources, owing to their high degree of symmetry (c_3v). Unfortunately, GaAs(111) suffers from a defect-ridden homoepitaxial buffer layer, and the InAs/GaAs(111) material system does not natively support Stranski{Krastanov InAs QD growth. Surfactants have been identified as effective tools to alter grown surface morphologies and growth modes, potentially overcoming these obstacles, but have yet to be studied in detail in this context. For MBE, it is shown that Bi acts as a surfactant when employed in GaAs(111) homoepitaxy, and eliminates defects/hillocks, yielding atomically-smooth surfaces with step-flow growth, and RMS roughness values of 0.13 nm. The effect is more pronounced as the Bi flux increases, and Bi is suggested to be increasing adatom diffusion. A novel reflection high energy electron diffraction (RHEED)-based experiment was also designed and performed to measure the desorption activation energy (U_Des) of Bi on GaAs(111), yielding U_Des = 1.74 ± 0.38 eV. GaAs(111) homoepitaxy was also investigated using MOCVD, with GaAs(111)B exhibiting RMS roughness values of 0.09 nm. Sb is shown to provoke a morphological transition from plastically-relaxed 2D to 3D growth for InAs/GaAs(111)B, showing promise in its ability to induce QDs. Finally, simulations for GaAs-based quantum well (QW) photoluminescence were conducted, and such QWs are shown to potentially produce very sharp linewidths of 3.9 meV. These results enhance understanding of Bi surfactant behaviour on GaAs(111) and can open up its use in many technological applications, paving the way for the realization of high efficiency/viable QD entangled photon sources. / Thesis / Master of Applied Science (MASc)

Integrated quantum photonic circuits have gained increasing interest in the field of quantum information, due to their compactness, the intrinsic stability and the potential scalability. Photons are the promising candidate for quantum information processing. Among all the optical platforms, femtosecond-laser waveguide writing technique has shown the extraordinary versatility in producing different components of a complete quantum system. In the last decade, femtosecond-laser writing has greatly expanded its applications in quantum technology. The aim of this thesis is to study and optimize the fundamental optical devices for integrated quantum circuits using femtosecond-laser waveguide writing technique. We investigate relevant theory of optical waveguides, the methods to fabricate and characterize laser-written waveguides in glass. In this work, we demonstrate the femtosecond-laser writing of integrated devices including Mach-Zehnder interferometer and path-encoded CNOT quantum gate. These devices can further serve as building blocks to produce complete integrated quantum system. / Integrerade kvantfotoniska kretsar har fått ett ökande intresse inom området kvantinformation, på grund av deras kompakthet, den inneboende stabiliteten och den potentiella skalbarheten. Fotoner är den lovande kandidaten för bearbetning av kvantinformation. Bland alla optiska plattformar har femtosekund-laservågledarskrivteknik visat den extraordinära mångsidigheten i att producera olika komponenter i ett komplett kvantsystem. Under det senaste decenniet har femtosekundlaserskrivning kraftigt utökat sina tillämpningar inom kvantteknologi. Syftet med denna avhandling är att studera och optimera de grundläggande optiska enheterna för integrerade kvantkretsar med hjälp av femtosekund-laservågledarskrivteknik. Vi undersöker relevant teori om optiska vågledare, metoderna för att tillverka och karakterisera laserskrivna vågledare i glas. I detta arbete demonstrerar vi femtosekundlaserskrivning av integrerade enheter inklusive Mach-Zehnder-interferometer och vägkodad CNOT-kvantgrind. Dessa enheter kan vidare fungera som byggstenar för att producera kompletta integrerade kvantsystem.

<p>  </p> <p>Low-dimensional materials (LDMs) are substances that have at least one dimension with thicknesses in the nanometer (nm) scale. They have attracted tremendous research interests in many fields due to their unique properties that are absent in bulk materials. For instance, in quantum optics/photonics, LDMs offer unique advantages for effective light extraction and coupling with photonic/plasmonic structures; in chemistry, the large surface-to-volume ratio of LDMs enables more efficient chemical processes that are useful for numerous applications. In this thesis, several types of LDMs are studied and engineered with the goal to improve their impact in plasmonic and quantum photonic applications. Two-dimensional hexagonal boron nitride (hBN) is receiving increasing attention in quantum optics/photonics as it hosts various types of quantum emitters that are promising for quantum computing, quantum sensing, etc. In the first study, we explore and demonstrate a radiation- and lithography-free route to deterministically create single-photon emitters (SPEs) in hBN by nanoindentation with an atomic force microscopy. The method applies to hBN on flat, chip-compatible silicon-based substrates, and an SPE yield of up to 36% is achieved. This marks an important step toward the deterministic creation and integration of hBN SPEs with photonic and plasmonic devices. In the second study, the recently discovered negatively charged boron vacancy (V_B^-) spin defect in hBN is investigated. V_B^- defects are optically active with spin properties suitable for sensing at extreme scales. To resolve the low brightness issue of V_B^- defects, we couple them with an optimized nano-patch antenna structure and observe emission intensity enhancement that is nearly an order of magnitude higher than previous reports. Our achievements pave the way for the practical integration of V_B^- defects for quantum sensing. Zero-dimensional nanodiamond is another important host material for solid-state SPEs. Specifically, the negatively charged silicon vacancy (SiV) center in nanodiamonds exhibits optical properties that are suitable for quantum information technologies. In the third study, we, for the first time, demonstrate the creation of single SiV centers in nanodiamonds with an average size of ~20 nm using ion implantation. Stable single-photon emission is confirmed at room temperature, with zero-phonon line (ZPL) wavelengths in the range of 730 – 803 nm. This confirms the feasibility of single-photon emitter creation in nanodiamonds with ion implantation, and offers new opportunities to integrate diamond color centers for hybrid quantum photonic systems. Finally, we have also explored using metal-semiconductor hybrid nanoparticles for plasmon-enhanced photocatalysis. A core-shell nanoparticle structure is synthesized, with titanium nitride (TiN) and titanium dioxide (TiO₂) being the core and shell material respectively. It is observed that such core-shell nanoparticles effectively catalyze the generation of single oxygen molecules under 700-nm laser excitation. The main mechanism behind is the hot electron injection from the TiN core to the TiO₂ shell. Considering the chemical inertness and low cost of TiN, TiN@TiO₂ NPs hold great potential as plasmonic photosensitizers for photodynamic therapy and other photocatalytic applications at red-to-near-infrared (NIR) wavelengths.</p>

<p>Bose-Einstein condensates (BECs) in light-induced synthetic gauge fields and spaces can provide a highly-tunable platform for quantum simulations. Chapter 1 presents a short introduction to the concepts of BECs and our BEC machine. Chapter 2 introduces some basic ideas of how to use light-matter interactions to create synthetic gauge fields and spaces for neutral atoms. Three main research topics of the thesis are summarized below.</p> <p>Chapter 3: Recently, using bosonic quasiparticles (including their condensates) as spin carriers in spintronics has become promising for coherent spin transport over macroscopic distances. However, understanding the effects of spin-orbit (SO) coupling and many-body interactions on such a spin transport is barely explored. We study the effects of synthetic SO coupling (which can be turned on and off, not allowed in usual materials) and atomic interactions on the spin transport in an atomic BEC.</p> <p>Chapter 4: Interplay between matter and fields in physical spaces with nontrivial geometries can lead to phenomena unattainable in planar spaces. However, realizing such spaces is often impeded by experimental challenges. We synthesize real and curved synthetic dimensions into a Hall cylinder for a BEC, which develops symmetry-protected topological states absent in the planar counterpart. Our work opens the door to engineering synthetic gauge fields in spaces with a wide range of geometries and observing novel phenomena inherent to such spaces.</p> <p>Chapter 5: Rotational properties of a BEC are important to study its superfluidity. Recent studies have found that SO coupling can change a BEC's rotational and superfluid properties, but this topic is barely explored experimentally. We study rotational dynamics of a SO-coupled BEC in an effective rotating frame induced by a synthetic magnetic field. Our work may allow for studying how SO coupling modify a BEC's rotational and superfluid properties.</p> <p>Chapter 6 presents some possible future directions.</p>

Integrated quantum photonics show monolithic waveguide chips to be a promising platform for realizing the next generation of quantum optical circuits. This work proposes the implementation of quantum page Rank algorithm on a photonic waveguide lattice. Our contributions are as follows: Continuous-time quantum stochastic walk(QSW)-an alternate paradigm of quantum computing, is a hybrid quantum walk that incorporates both unitary and non-unitary effects. We propose the use of QSW which necessitates the hopping of the quantum crawler on a directed graph, for the quantum page Rank problem. We propose the implementation of quantum page Rank on a photonic waveguide lattice, where we allow the density matrix to evolve according to the Lindblad-Kossakowski master equation, the diagonal of which gives the quantum page Rank. We have also shown the use of the metric of positional Kolmogorov Complexity as an efficient tool for determining whether or not the quantum channel has been compromised. We appositionally encode multi-photon decoy pulses within the stream of single photon pulses. This positional encoding is chosen in such a way as to have low Kolmogorov complexity. The PNS attack on the multi-photon decoy pulses causes a dip in the ratio of the transmittance of the decoy pulses to the signal pulses in the conventional analysis.