High quality entangled photon sources are a key requirement for many promising quantum optical technologies. However, the production of multi-photon entangled states with good fidelity is challenging. Current sources of multi-photon entanglement require the use of post-selection, which limits their usefulness for some applications. It has been an open challenge to create a source capable of directly producing three-photon entanglement. An important step in this direction was achieved with the demonstration of photon triplets produced by a new process called cascaded downconversion, but these previous measurements were not sufficient to show whether these photons were in an entangled state and only had detection rates of five triplets per hour. In this thesis, we show the first demonstration of a direct source of three-photon entanglement. Our source is based on cascaded downconversion, and we verify that it produces genuine tripartite entanglement in two degrees of freedom: energy-time and polarization. The energy-time entanglement is similar to a three-particle generalization of an Einstein-Podolski-Rosen state; the three photons are created simultaneously, yet the sum of their energies is well defined, which is an indication of energy-time entanglement. To prove it, we use time-bandwidth inequalities which check for genuine tripartite entanglement. Our measurements show that the state violates the inequalities with what constitute, to the best of our knowledge, the strongest violation of time-bandwidth inequalities in a tripartite continuous-variable system to date. We create polarization entanglement by modifying our experimental setup so that two downconversion processes producing orthogonally polarized triplets interfere to create Greenberger-Horne-Zeilinger states. By using highly efficient superconducting nanowire single photon detectors, we improve the detected triplet rate by 2 orders of magnitude to 660 triplets per hour. We characterize the state using quantum state tomography, and find a fidelity of 86\% with the ideal state, beating the previous best value for a three-photon entangled state fidelity measured by tomography. We also use the state to perform two tests of local realism. We violate the Mermin and Svetlichny inequalities by 10 and 5 standard deviations respectively, the latter being the strongest violation to date. Finally, we show that, unlike previous sources of tree-photon entanglement, our source can be used as a source of heralded Bell pairs. We demonstrate this by measuring a CHSH inequality with the heralded Bell pairs, and by reconstructing their state using quantum state tomography.

La nanophotonique non-linéaire offre une opportunité unique pour ouvrir de nouvelles voies vers des applications dans les détecteurs, les ordinateurs et la cryptographie quantique. Cependant, la faiblesse intrinsèque de la réponse non-linéaire des milieux de taille inférieure au micromètres limite fortement l’efficacité des sources optique à cette échelle. Combiner l'exaltation du champ électromagnétique dans les métaux (appelée résonance plasmonique) et l'efficacité non-linéaire de nanocristaux non-centosymétriques apparait extrêmement souhaitable et constitue le cœur de ce projet. Dans ce cadre, le travail présenté dans cette thèse consiste en une approche numérique quantitative des processus linéaires et non-linéaires (génération de second harmonique et de paires de photons) mis en jeu dans les nanostructures hybrides afin de pouvoir « accorder » les résonances plasmoniques et optimiser le couplage lumière-matière. L’étude menée prédit une exaltation par plusieurs ordres de grandeur des processus non linéaires modélisés au sein de particules composites. / Nonlinear nanophotonics offers a unique opportunity to open new path toward a wide range of pratical applications in sensors, quantum computers, cryptography devices. The main challenge is to enhance nonlinear response of nanosized particles in order to integrate them in optical components. On this purpose, we want to combine the electromagnetic field enhancement in metals (due to a phenomenon called plasmonic resonances) with non linear efficiency of non centrosymmetric nanocrystals.In this thesis, we present a numerical approach for simulating linear and non linear optical processes (second harmonic generation and spontaneous numerical down conversion) in hybrid nanostructures in order to “ tune “ plasmonic resonances and optimize light/matter coupling. The study predicts an enhancement by several orders of magnitude of the non linear phenomena modeled in composite nano particles.

Nonlinear optics has had extensive application into a vast array of scientific fields. One such nonlinear process, two-photon absorption (TPA), has had a wildly successful adoption into the field of biological imaging and microscopy. As far and as fast as this field is progressing, limitations stemming from the use of ultrafast lasers are starting to appear. In this work, an alternative nonclassical light source will be motivated for the application of low photon-flux two-photon microscopy. The origin and properties of the chosen nonclassical source, spontaneous parametric downconversion (SPDC), will be discussed along with the spatial and spectral properties modelled. Nonlinear processes such as TPA and sum frequency generation (SFG) will be viewed as "ultrafast coincidence measurements" of two photons arriving at a molecule within the time window of excitation. These ultrafast coincidence measurements will be viewed in an alternative manner: in terms of the second-order coherence from a light source. This degree of second-order coherence can be subdivided into two categories arising from different combinations of correlations within and between entangled photon pairs. Of interest, the energy-time correlations within the photon pair allow for enhancements in ultrafast coincidence rates over coherent light sources. The makings of an experimental setup to demonstrate enhanced rates from ultrafast two-photon coincidences taking place in SFG in a nonlinear crystal will be discussed.

This thesis presents studies on III-V semiconductor waveguides with particular emphasis on second-order optical nonlinearity. The nonlinear processes that were investigated in this thesis are the Second Harmonic Generation (SHG) and the Spontaneous Parametric Down-Conversion (SPDC). The optical waveguides are made of InGaP and the waveguide design includes tapered parts for in- and out-coupling of guided light. Simulation of light propagation and modal solutions were done using Lumerical MODE, FDTD, and COMSOL Multiphysics software. The in- and outcoupling for the design of tapered waveguide that utilize the bulk non-linearity is 65 % when the waveguide is 145 nm thick and 2.60 μm wide having PMMA as top cladding. The SHG conversion efficiency for this configuration when the waveguide length is 2 μm long, is found 31 %/W. Three cases of the utilization of the surface non-linearity are proposed too. Preliminary steps toward the fabrication of the waveguide structures are also reported. The particular mesa-isolated substrates are fabricated having a side wall with a negative angle profile that result to a significant undercut. InGaP waveguides were transferred to the target substrates successfully and the process that was used can enable heterogeneous integration of InGaP and SOI platform. / Denna avhandling presenterar studier av III-V-halvledarvågledare med särskild tonvikt på andra ordningens optisk olinjäritet. De olinjära processer som undersöktes i denna avhandling är SHG och SPDC. De optiska vågledarna är gjorda av InGaP och vågledardesignen inkluderar avsmalnande delar för in- och utkoppling av styrt ljus. Simulering av ljusutbredning och modala lösningar gjordes med Lumerical MODE, FDTD och COMSOL Multiphysics mjukvara. In- och utkopplingen för konstruktionen av avsmalnande vågledare som utnyttjar bulkolinjäriteten är 65 % när vågledaren är 145 nm tjock och 2,60 μm bred med PMMA som toppbeklädnad. SHGkonverteringseffektiviteten för denna konfiguration när vågledarlängden är 2 μm lång, är 31 %/W. Tre fall av utnyttjande av ytolinjäriteten föreslås också. Preliminära steg mot tillverkningen av vågledarstrukturerna rapporteras också. De speciella mesa-isolerade substraten är tillverkade med en sidovägg med en negativ vinkelprofil som resulterar i en betydande underskärning. InGaP-vågledare överfördes till målsubstraten framgångsrikt och processen som användes kan möjliggöra heterogen integration av InGaP och SOI-plattformen.

Myanmar, an isolated country in Southeast Asia, held general elections for the People¡¦s Assembly in 2010, the first in twenty years and the second in fifty years. The military junta that has ruled the country for decades has been facing strong criticism from the international community. This research has systematically examined the historical development of the electoral laws and the overall electoral system of the Union of Myanmar (since October 2010 officially known as the Republic of the Union of Myanmar) during the period of 18 June 1989 up until the latest elections on 7 November 2011. Why did the military bother organizing elections? The latest elections were meant to give the impression that they would create a legitimate government. In reality, they were designed to preserve military rule under a façade of democracy. The junta learned a valuable lesson from the last free elections in 1990, which ended in a fiasco for the pro-junta parties, and did not leave it to the people's will in 2010. In order to effectively extend military rule, in 2008 the junta used a flawed referendum to approve a supremacy constitution, according to which the military automatically receives 25% of the seats in parliament. This constitution helped the junta impose several severe limitations on parties willing to participate in the 2010 elections, thus ensuring that the military-backed political parties would win most of the seats at stake. This thesis attempts to show the powerlessness of the masses against a system based on the creation of self-serving laws used by the ruling junta to control the society.

When using quantum key distribution (QKD), one of the trade-offs for security is that the generation rate of a secret key is typically very low. Recent works have shown that using a weak coherent source allows for higher secret key generation rates compared to an entangled photon source, when a channel with low loss is considered. In most cases, the system that is being studied is over a fiber-optic communication channel. Here a theoretical QKD system using the BB92 protocol and entangled photons over a free-space maritime channel with multiple spatial modes is presented. The entangled photons are generated from a spontaneous parametric down conversion (SPDC) source of type II. To employ multiple spatial modes, the transmit apparatus will contain multiple SPDC sources, all driven by the pump lasers assumed to have the same intensity. The receive apparatuses will contain avalanche photo diodes (APD), modeled based on the NuCrypt CPDS-1000 detector, and located at the focal point of the receive aperture lens. The transmitter is assumed to be located at Alice and Bob will be located 30 km away, implying no channel crosstalk will be introduced in the measurements at Alices side due to turbulence. To help mitigate the effects of atmospheric turbulence, adaptive optics will be considered at the transmitter and the receiver. An eavesdropper, Eve, is located 15 km from Alice and has no control over the devices at Alice or Bob. Eve is performing the intercept resend attack and listening to the communication over the public channel. Additionally, it is assumed that Eve can correct any aberrations caused by the atmospheric turbulence to determine which source the photon was transmitted from. One, four and nine spatial modes are considered with and without applying adaptive optics and compared to one another.

No description available.

The miniaturization of optical systems has been a longstanding interest for physicists. By facilitating the design of smaller optical systems, we can improve their versatility and cost-effectiveness. This aim applies to macroscopic imaging systems, technologies that implicitly image, and micrometer-scale optics. Parallel to this, quantum optical devices have also seen rapid developments. Notably, the need for new quantum communications and quantum imaging devices has recently risen. The thesis outlines advancements in both of these areas and, in many ways, bridges gaps between them. It discusses the development of optics that compress free space, the design of holographic optical elements, and the generation of entangled photon states in thin-film sources. First, we describe an optic designed to miniaturize free space, termed the spaceplate. Spaceplates achieve the propagation of light for a distance greater than their thickness.Therefore, they compress optical space, reducing the required distance between optical elements in an imaging system. In this thesis, we describe a spaceplate based on conventional optics in a 4-f arrangement, mimicking the transfer function of free space in a thinner system - we term this device a three-lens spaceplate. It is broadband, polarization-independent, and achieves meter-scale space compression. We experimentally measure compression ratios up to 15.6, replacing up to 4.4 meters of free space, three orders of magnitude greater than previous spaceplates. We demonstrate that three-lens spaceplates reduce the length of a full-color imaging system, albeit with reductions in resolution and contrast. We also present theoretical limits on the numerical aperture and the compression ratio. Our design presents a simple, accessible, cost-effective method for optically compressing large volumes of space. Second, we discuss the design of holographic optical elements. Holograms are extraordinarily versatile optics. They have many applications, including interferometry, spectrometry, data storage, optical filtration, and sensing. We can design various optical elements such as filters, lenses, beam splitters, and solar concentrators by tailoring the phase response of a hologram. In this thesis, we describe the nature and function of holograms, and we experimentally characterize holography in lithium niobate and photopolymers. Using this characterization, we assess the limitations of different holographic analysis methods. Further, we describe novel holographic optical elements like the holographic spaceplate - a holographic optic element whose phase response mimics free space. Lastly, this thesis outlines the production of entangled photon pairs, or biphotons, via non-phase-matched spontaneous parametric down-conversion in micrometer- and nanometer-scale devices. By producing biphotons in micrometer-scale crystals rather than in bulk, as is done traditionally, we are allowed to ignore phase matching. These devices produce broadband emission in both angle and frequency not seen in phase-matched bulk sources. We measure entangled biphotons produced via spontaneous parametric down-conversion in gallium arsenide (111) and lithium niobate - both undoped and iron-doped. Lastly, we outline and present initial experiments towards a holographic spontaneous parametric down-conversion optic that combines photon production and mode sorting - an optic of cascaded miniaturization.

L'optique non-linéaire étudie des phénomènes capables de modifier la fréquence de la lumière incidente en s'appuyant sur la symétrie intrinsèque de certains matériaux. Le défi actuel de la miniaturisation des composants va de paire avec une perte d'efficacité à l'échelle sub-micrométrique. Pour résoudre ce problème, l'idée explorée au cours de cette thèse consiste à utiliser un phénomène d'oscillation collective des électrons libres d'une nanostructure en métal, appelé résonance plasmon de surface localisé. Cet effet est associé à une exaltation du champ au voisinage immédiat d'une structure plasmonique, une propriété adaptée pour augmenter l'efficacité non-linéaire d'un matériau placé non loin. Les objectifs principaux de ma thèse consistaient à fabriquer ces objets hybrides, à développer une plate-forme expérimentale polyvalente capable de réaliser différents types d'observation à l'échelle de la particule unique, puis à analyser leur génération de second harmonique (SHG). Ces travaux ont abouti à l'obtention de structures hybrides non-linéaires efficaces, dont l'intensité SHG atteint jusqu'à 100 fois celle d'une antenne plasmonique isolée et jusqu'à plus de 1000 fois celle d'un nanocristal non-linéaire unique, confirmant l'intérêt de ces structures. Nous avons aussi tenté d'observer de la fluorescence paramétrique (SPDC) dans une nanostructure individuelle, une prouesse encore inachevée dans le monde; si nos études n'ont pas davantage abouti, elles esquissent des pistes d'amélioration pour y parvenir, et un modèle numérique innovant développé dans l'équipe annonce un rendement compatible avec des observations. Enfin, une source de photons intriqués a été développée dans le cadre d'une collaboration sur l'intelligence artificielle dans des systèmes physiques et constitue une perspective envisageable d'application pour les travaux précédents. Ces résultats ouvrent potentiellement la voie à l'amélioration de l'éfficacité et de la fiabilité des algorithmes IA actuels. / Nonlinear optics study phenomena able to modify the frequency of incoming light by using intrinsic symmetry properties of some materials. The current challenge of component miniaturization goes with an efficiency drop at the sub-micrometer scale. To solve this issue, the idea we have explored during my PhD consists in using a collective oscillation phenomenon from free electrons in a metal structure called localized surface plasmon resonance. This effect is indeed linked to an enhancement of the electromagnetic field near a plasmonic structure, a property well suited to increase the nonlinear efficiency of a material placed beside. The main objectives of my PhD consisted in fabricating these hybrid objects, developing a versatile experimental platform able to make different kinds of observations at the single particle level, and finally analyzing their second harmonic generation (SHG). This work has managed to produce efficient nonlinear hybrid structures, whose SHG intensity is up to 100 times that of an isolated plasmonic antenna and up to 1000 times that of a single nonlinear nanocrystal, confirming the potential of this type of structures. We have also tried to detect spontaneous parametric down conversion (SPDC) in a single nanostructure, a never-achieved feat that has yet to be done; although our study wasn't successful, it gives hints to improve experiments, even more since a cutting edge numerical model developed in our team has predicted intensities compatible with observations. Finally, an entangled photon source has been developed in the framework of a collaboration on artificial intelligence in physical systems and is a reachable perspective for potential applications of our work. These results pave the way to improving efficiency and liability of current AI algorithms.

This portfolio of work contributes to the remit of the National Physical Laboratory (NPL) to develop the underpinning expertise and tools for validating nascent and future optical quantum technologies based on the discrete and quantum properties of photons. This requirement overlaps with the requirement to provide validation for devices operating in the photon-counting regime. A common theme running through the portfolio is photon pairs generated through spontaneous parametric downconversion (SPDC). A Hong-Ou-Mandel (HOM) interferometer sourced with visible wavelength photon pairs from an SPDC process in beta-barium borate (BBO) was designed, built and characterised. The visibility of the HOM interference is dependent on the indistinguishability of the interfering photons, but is also influenced by imperfections of the interferometer; therefore an investigation was carried out to quantify the effects of the interferometer imperfections on the measured visibility so that the true photon indistinguishability could be measured with a quantified uncertainty. A bright source of correlated pair photons in the telecoms band based upon a pump enhanced SPDC process in periodically-poled potassium titanyl phosphate (PPKTP) was designed, built and characterised. From the characterisation measurements the source brightness was estimated to be 6.2×10⁴ pairs/ s/ mw pump. The photon pairs were further characterised through their incorporation as a source in a HOM interference experiment. The developed correlated photon pair source was at the heart of a novel scheme for the generation of polarisation entangled photon pairs, for which the design, build and characterisation work is presented. The source was demonstrated to produce two of the four maximally entangled Bell states with quantum interference visibilities of around 0.95. The generated states were also shown to break a form of Bell's inequality by around six standard deviations. The polarisation entangled photon pair source was originally built at the University of St Andrews and was later transferred to the NPL where it will extend NPL's capabilities to this key spectral region. Finally a study was carried out to investigate the possibility of a wavelength tuneable device for the absolute measurement of single photon detector quantum efficiencies based upon an established SPDC technique.