The rapidly developing interdisciplinary field of quantum information, which merges quantum and information science, studies non-classical aspects of quantum systems. These studies are motivated by the promise that the non-classicality can be used to solve tasks more efficiently than classical methods would allow. In many quantum informational studies, non-classical behaviour is attributed to the notion of entanglement. In this thesis we use photons to experimentally investigate fundamental questions such as: What happens to the entanglement in a system when it is affected by noise? In our study of noisy entanglement we pursue the challenging task of creating bound entanglement. Bound entangled states are created through an irreversible process that requires entanglement. Once in the bound regime, entanglement cannot be distilled out through local operations assisted by classical communication. We show that it is possible to experimentally produce four-photon bound entangled states and that a violation of a Bell inequality can be achieved. Moreover, we demonstrate an entanglement-unlocking protocol by relaxing the condition of local operations. We also explore the non-classical nature of quantum mechanics in several single-photon experiments. In these experiments, we show the violation of various inequalities that were derived under the assumption of non-contextuality. Using qutrits we construct and demonstrate the simplest possible test that offers a discrepancy between classical and quantum theory. Furthermore, we perform an experiment in the spirit of the Kochen-Specker theorem to illustrate the state-independence of this theorem. Here, we investigate whether or not measurement outcomes exhibit fully contextual correlations. That is, no part of the correlations can be attributed to the non-contextual theory. Our results show that only a small part of the experimental generated correlations are amenable to a non-contextual interpretation. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 5: Submitted. Paper 6: Submitted.</p>

Spontaneous parametric down conversion (SPDC) – incoherent light scattering – is one of the main entangled photons source applied in quantum optics experiments. The tradition to pump SPDC by laser radiation was established from the very first SPDC experiments in 1968. The aim of this thesis was experimentally to investigate the ability to generate an SPDC pumping by both temporal and spatially incoherent radiation - a high-power blue LED. Weak SPDC signals were registered with high sensitivity CCD cameras, photons coincidences were detected with photon counters. The theoretical simulations were performed in parallel with experiments. Therefore, mathematical simulation code was written in order to estimate the SPDC power distribution and simulate photon coincidence experiment changing the properties of pump beam and detection system. Experimental results reveal that incoherent light sources can be good alternative for the laser systems in order to generate average quality biphoton fields especially in those experiments in which low biphoton field coherency would be advantage. The main advantages of the incoherent sources over laser systems are low cost, simple production technology and the huge commercial variety of different wavelength sources. / Parametrinė fluorescencija (PF) – nekoherentinė šviesos sklaida – yra vienas pagrindinių susietųjų fotonų šaltinių taikomų kvantinės optikos eksperimentuose. Nuo pat pirmųjų PF eksperimentinių tyrimų 1968 metais įsigalėjo tradicija šį reiškinį žadinti išimtinai lazerine spinduliuote. Šios disertacijos tikslas – eksperimentiškai ištirti galimybę generuoti PF tiek laikiškai, tiek ir erdviškai nekoherentine spinduliuote – didelės galios šviesos diodu. Atliekant tyrimus didelio jautrio CCD kamera buvo registruojami silpni PF signalai, pavienių fotonų skaitliukais buvo registruojami fotonų sutapimai,. Lygiagrečiai eksperimentiniams tyrimams buvo atliekami teoriniai skaičiavimai. Šiuo tikslu buvo parašytas matematinio modeliavimo programinis kodas, skirtas įvertinti PF erdvinį galios pasiskirstymą bei modeliuoti fotonų sutapimų eksperimentą, keičiant kaupinimo pluošto ir detekcijos sistemos savybes. Šio darbo rezultatai atskleidžia, kad nekoherentiniai šaltiniai gali būti puiki alternatyva lazerinėms sistemoms siekiant žadinti vidutinės kokybės dvyninius laukus, ypatingai tokiose tyrimų srityse, kuriose mažas dvyninio lauko koherentiškumas būtų didžiulis privalumas. Pagrindiniai nekoherentinių šaltinių pranašumai prieš lazerines sistemas: maža kaina, paprasta gamybos technologija ir didžiulė komercinė skirtingo bangos ilgio šaltinių įvairovė.

Parametrinė fluorescencija (PF) – nekoherentinė šviesos sklaida – yra vienas pagrindinių susietųjų fotonų šaltinių taikomų kvantinės optikos eksperimentuose. Nuo pat pirmųjų PF eksperimentinių tyrimų 1968 metais įsigalėjo tradicija šį reiškinį žadinti išimtinai lazerine spinduliuote. Šios disertacijos tikslas – eksperimentiškai ištirti galimybę generuoti PF tiek laikiškai, tiek ir erdviškai nekoherentine spinduliuote – didelės galios šviesos diodu. Atliekant tyrimus didelio jautrio CCD kamera buvo registruojami silpni PF signalai, pavienių fotonų skaitliukais buvo registruojami fotonų sutapimai,. Lygiagrečiai eksperimentiniams tyrimams buvo atliekami teoriniai skaičiavimai. Šiuo tikslu buvo parašytas matematinio modeliavimo programinis kodas, skirtas įvertinti PF erdvinį galios pasiskirstymą bei modeliuoti fotonų sutapimų eksperimentą, keičiant kaupinimo pluošto ir detekcijos sistemos savybes. Šio darbo rezultatai atskleidžia, kad nekoherentiniai šaltiniai gali būti puiki alternatyva lazerinėms sistemoms siekiant žadinti vidutinės kokybės dvyninius laukus, ypatingai tokiose tyrimų srityse, kuriose mažas dvyninio lauko koherentiškumas būtų didžiulis privalumas. Pagrindiniai nekoherentinių šaltinių pranašumai prieš lazerines sistemas: maža kaina, paprasta gamybos technologija ir didžiulė komercinė skirtingo bangos ilgio šaltinių įvairovė. / Spontaneous parametric down conversion (SPDC) – incoherent light scattering – is one of the main entangled photons source applied in quantum optics experiments. The tradition to pump SPDC by laser radiation was established from the very first SPDC experiments in 1968. The aim of this thesis was experimentally to investigate the ability to generate an SPDC pumping by both temporal and spatially incoherent radiation - a high-power blue LED. Weak SPDC signals were registered with high sensitivity CCD cameras, photons coincidences were detected with photon counters. The theoretical simulations were performed in parallel with experiments. Therefore, mathematical simulation code was written in order to estimate the SPDC power distribution and simulate photon coincidence experiment changing the properties of pump beam and detection system. Experimental results reveal that incoherent light sources can be good alternative for the laser systems in order to generate average quality biphoton fields especially in those experiments in which low biphoton field coherency would be advantage. The main advantages of the incoherent sources over laser systems are low cost, simple production technology and the huge commercial variety of different wavelength sources.

One of the current hot topics in physics is quantum information, which, broadly speaking, is concerned with exploring the information-processing and storing tasks that can be performed in quantum mechanical systems. Besides driving forward our experimental control and understanding of quantum systems, the field is also in the early stages of developing revolutionary new technology of far reaching implication. As part of these endeavors, this thesis presents some results in experimental quantum information. Specifically, we develop several new tools for performing quantum information processing in optical quantum systems, and use them to explore a number of applications and novel physical phenomena. A central theme, and one of the most sought after applications of quantum information, is the pursuit of a programmable quantum computer. This thesis is divided into 3 parts. In Part I we develop some new optical quantum logic gates, which are tools for manipulating quantum information and the fundamental building blocks of a quantum computer. We also develop a new technique for simplifying the construction of quantum logic circuits, by exploiting multi-level quantum systems, that has the potential for application in any physical encoding of quantum information. In Part II we use these tools to perform some of the first demonstrations of quantum algorithms. Each of these could, in principle, efficiently solve an important problem that is thought to be fundamentally intractable using conventional `classical' techniques. Firstly we implement a simplified version of the quantum algorithm for factoring numbers, and demonstrate the core processes, coherent control, and resultant entangled states required for a full-scale implementation. Secondly we implement an algorithm for calculating the energy of many-body quantum systems. Specifically, we calculate the energy spectrum of the Hydrogen molecule, in a minimal basis. Finally we demonstrate an algorithm for a novel model of quantum computing that uses mixed states. Here we perform the first characterisation of intrinsically non-classical correlations between fully separable quantum systems, captured by the 'discord'---a measure of quantum correlations in mixed states that goes beyond entanglement. Part III presents a technique that extends experimental control over biphotons---the novel quantum information carriers formed by the polarisation of two photons in the same spatial and temporal mode. We also generate and explore new forms of entanglement: producing the first instance of qubit-qutrit entanglement, by entangling the polarisation of a photon and a biphoton, and developing a technique that enables full control over the level of `W-class' of multi-partite entanglement between the polarisation of three photons.

In this thesis I consider the general problem of how to make the best possible phase measurements using feedback. Both the optimum input state and optimum feedback are considered for both single-mode dyne measurements and two-mode interferometric measurements. I derive the optimum input states under general dyne measurements when the mean photon number is fixed, both for general states and squeezed states. I propose a new feedback scheme that introduces far less phase uncertainty than mark II feedback, and is very close to the theoretical limit. I also derive results for the phase variance when there is a time delay in the feedback loop, showing that there is a lower limit to the introduced phase variance, and this is approached quite accurately under some conditions. I derive the optimum input states for interferometry, showing that the phase uncertainty scales as 1/N for all the common measures of uncertainty. This is contrasted with the |j0> state, which does not scale as 1/N for all measures of phase uncertainty. I introduce an adaptive feedback scheme that is very close to optimum, and can give scaling very close to 1/N for the uncertainty. Lastly I consider the case of continuous measurements, for both the dyne and interferometric cases.

Les fontaines atomiques, basées sur une transition micro-onde du Césium ou du Rubidium, constituent l'état de l'art des horloges atomiques, avec une exactitude relative avoisinant 10^{-16}. Il apparaît cependant clairement aujourd'hui qu'il sera difficile de dépasser significativement ce niveau de performance avec un dispositif de ce type.<br /><br />L'utilisation d'une transition optique, toutes choses étant égales par ailleurs, ouvre la perspective d'une amélioration de 4 ou 5 ordres de grandeur de la stabilité et de l'incertitude relative sur la plupart des effets systématiques. Les effets liés au mouvement des atomes peuvent être, quant à eux, contrôlés d'une façon totalement différente, en les piégeant dans un réseau optique pour éviter la phase de vol ballistique caractéristique des fontaines. Le point clef de cette approche réside dans le fait que les paramètres de ce piège peuvent être ajustés de façon à s'affranchir du déplacement lumineux si l'on sélectionne une transition d'horloge faiblement permise J=0 -> J=0.<br /><br />A cet égard, l'atome de strontium est l'un des candidats les plus prometteurs, la transition ^1S_0 -> ^3P_0 présente une largeur naturelle de 1 mHz, et plusieurs autres transitions facilement accessibles peuvent être utilisées en vue d'un refroidissement laser efficace des atomes jusqu'à une température de 10 µK. Ce manuscrit de thèse d'une part démontre la faisabilité expérimentale d'une horloge à réseau optique basée sur l'atome de strontium, et d'autre part expose une évaluation préliminaire de l'exactitude relative avec l'isotope fermionique ^{87}Sr, à un niveau de quelques 10^{-15}.

À la surface d’un métal, la lumière visible peut se coupler avec les électrons libres pour engendrer une quasi-particule particulièrement intéressante, le plasmon-polariton de surface. Cet objet a pour propriété d’être évanescent dans les directions perpendiculaires à la surface, ce qui en fait un support idéal pour transporter l’information lumineuse à deux dimensions, et sur des échelles sub-longueur d’onde. S’il est excité par une source quantique, il conserve cet aspect quantique du signal, même si des millions d’électrons sont impliqués dans sa propagation.Dans ce manuscrit, je présente les résultats expérimentaux et théoriques obtenus en plasmonique de surface durant mon doctorat. En associant l’utilisation de centres colorés azote-lacune (NV) dans les nanodiamants, qui sont des émetteurs de photons uniques, et d’un microscope optique en champ proche (SNOM), j’ai pu étudier de nombreuses propriétés du centre NV et des plasmons de surface dans les domaines classique et quantique.Notamment, j’ai réalisé une étude complète de la photo-dynamique interne du centre NV, dans différents régimes d’excitation. De plus, j’ai étudié le mode d’imagerie des plasmons de surface qu’est la microscopie à fuite radiative, en mettant en lumière certaines aberrations optiques pouvant survenir dans des conditions de désaccord d’indices optiques. J’ai ensuite effectué des mesures de corrélations spatio-temporelles de plasmons de surface excités par des centres NV, grâce à un système expérimental spécifique que j’ai mis en œuvre.Enfin, je décris dans ce manuscrit les toutes premières études de l’interaction des plasmons avec différentes cavités elliptiques et paraboliques gravées dans le métal, qui ont mené notamment à des mesures de densité locales d’états (LDOS) plasmonique. / On a metal surface, visible light can couple with surface free electrons to form a very interesting quasi-particle, the surface plasmon-polariton. The main property of this object is to be evanescent in the directions perpendicular to the surface. This feature makes the plasmon ideally suited to transport electromagnetic information in two dimensions and on a sub-wavelength scale. If it is excited by a quantum source, it retains this quantum aspect of the signal, even if millions of electrons are involved in its propagation.In this manuscript, I present the experimental and theoretical results obtained during my PhD in surface plasmonics. By combining the use of nitrogen vacancy (NV) color centers in nanodiamonds, which are single photon emitters, and of a scanning near field optical microscope (SNOM), I was able to study numerous properties of the NV center and surface plasmons, both in the classical and quantum regimes.In particular, I have performed a complete study of the internal photo-dynamics of the NV center in different excitation regimes. Moreover, I have studied the leakage radiation microscopy, a dedicated imaging mode in plasmonics , by highlighting some optical aberrations that can arise in conditions of optical index mismatch. Furthermore, I have ran spatio-temporal correlation measurements on surface plasmons excited by NV centers with a specific experimental system I implemented.Finally, I describe in the manuscript the very first studies of the interaction between plasmons and different elliptical and parabolic cavities milled in the metal. This has led to the measurements of the plasmonic local density of states.

Ces dernières années, de considérables efforts expérimentaux ont été dévoués au développement d’outils de nanoélectronique quantique, dans le but d’atteindre un niveau de contrôle sur le transport électronique quantique à l’échelle de l’électron unique. Ces avancées ont poussé à un changement de paradigme dans le domaine du transport électronique cohérent et donné naissance à l’optique quantique électronique, domaine dans lequel cette thèse s’inscrit. Cette thèse est consacrée à deux problématiques. Tout d’abord, elle s’intéresse au problème des interactions Coulombiennes entre électrons, qui donnent lieu à un phénomène de décohérence qu’il est nécessaire de caractériser et de prédire au mieux afin de le contrôler. En utilisant une approche analytique et numérique, il a été possible de prédire l’effet de ces interactions sur un système expérimentalement accessible, prédiction qui a ensuite été confirmée par l’expérience. Dans la foulée de ce résultat, cette thèse présente des possibilités de contrôle de ces interactions, et propose un moyen de les mettre en œuvre qui devrait pouvoir être testé dans une expérience. Je me suis également confronté à la problématique de la caractérisation d’états quantiques complexes. En particulier, suite à la démonstration expérimentale d’un protocole de tomographie pour des états mono-électroniques, je me suis tourné vers l’extension de ce protocole à des états plus complexes, pouvant exhiber des propriétés de cohérence à deux électrons, voire plus. Ces états étant également sensibles aux interactions de Coulomb, une extension au cas multi-électronique des outils utilisés pour traiter ces interactions est proposée dans cette thèse. / Over the last few years, extensive experimental efforts have been devoted to thedevelopment of quantum nanoelectronics tools aiming at controlling electronic trans-port down to the single electron level. These advances led to a paradigm shift inthe domain of coherent electronic transport, giving birth to electron quantum optics,which is the domain of this work.This manuscript is devoted to two problems. The first of these is the one ofCoulomb interactions between electrons, which lead to a decoherence phenomenonthat must be characterized and predicted in order to be controlled. Using an analyt-ical and numerical approach, it became possible to predict the effect of interactionson an experimentally relevant system, a prediction that was then confirmed in the ex-periment. After this result, this manuscript displays some ideas aiming at controllinginteractions and proposes some ways to test them experimentally.In this work, I also took on the problem of characterizing complex quantum states.In particular, following the experimental demonstration of a tomography protocol forfirst order coherences, I tried to extend this protocol to more complex states thatcould exhibit two-electron coherences, or more. These states being also sensitive to Coulomb interactions, an extension of the tools used to treat interactions to thismulti-electronic state is also presented in this work.

Quantum correlations play a fundamental role in quantum information science. The variety of their manifestations has become increasingly apparent following the development of novel light sources, protocols and photodetectors. One broad classification identifies two instances of non-classical correlations: particle and mode entanglement. These categories mirror two coexisting descriptions of quantum systems in terms of discrete and continuous variables of the electromagnetic field. The past decades have generated a number of promising results based on schemes which encompass elements from both frameworks, rather than viewing the two descriptions as mutually exclusive. In this context, it is possible to conceive and realise experiments where either the quantum resource or the detection system is 'hybrid'. Optical weak-field homodyne detectors bring together phase sensitivity and photon counting; as such, they represent a detection scheme which works across continuous and discrete variables of the radiation field. In this thesis we present a two-mode weak-field homodyne detection layout with added photon-number resolution and apply it to the study of a split single-photon state and a squeezed vacuum state. As a first test of the capabilities of this system, we investigate the reconstruction of relevant features of a given quantum resource - such as its photon statistics - with our detection scheme. Further, we experimentally demonstrate the observation of an instance of non-classical optical coherence which combines the continuous- and discrete-variable descriptions explicitly. The ability to probe phenomena at the interface of wave and particle regimes opens the way to novel, improved schemes for quantum information processing. From a more fundamental perspective, such hybrid approaches may shed light on the very roots of quantum enhancement.

Orientador: Prof. Dr. Roberto Menezes Serra / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2016. / Termodinâmica Quântica é uma área emergente da Física que está associada a aplicação dos princípios termodinâmicos a sistemas que são geralmente pequenos, estão longe do equilíbrio térmico e nos quais efeitos quânticos são relevantes. Este novo campo de pesquisa deve contribuir substancialmente para o entendimento dos limites das novas tecnologias quânticas que envolvem processamento de informação, comunicação, metrologia, entre outras aplicações. No limite de pequenos sistemas longe do equilíbrio, flutuações de energia adquirem grande importância e quantidades termodinâmicas como trabalho, calor e produção de entropia tornam-se variáveis estocásticas regidas por teoremas de flutuação, sendo que o acesso às distribuições de probabilidade dessas flutuações é essencial para caracterizar completamente a termodinâmica de um processo fora do equilíbrio. Em um outro desenvolvimento, a possibilidade de aquisição e uso de informação sobre estados microscópicos foi incorporada na descrição teórica dos teoremas de flutuação, dando origem a contundentes conexões entre Termodinâmica e Teoria de Informação. Essas conexões possibilitam, entre outras coisas, obter uma expressão quantitativa em termos de variáveis microscópicas para o conceito filosófico da seta do tempo em um sistema quântico e entender, de um ponto de vista bastante fundamental, de onde emerge a assimetria entre passado e futuro, possibilitando também que se projete um sistema de feedback (retroalimentação) que pode operar como um demônio de Maxwell genuíno. Embora haja uma intensa atividade teórica na área, experimentos em Termodinâmica Quântica tem demorado a aparecer devido à dificuldade de acesso a flutuações de energia durante a evolução de sistemas quânticos fora do equilíbrio. Aplicações da técnica de Ressonância Magnética Nuclear (RMN) que desenvolvemos nesta tese são capazes de contornar essa dificuldade. A partir dessas técnicas, reportamos a realização dos primeiros experimentos em Termodinâmica Quântica, incluindo a comprovação experimental dos teoremas de flutuação no regime quântico, a observação da emergência da seta do tempo (caracterizada pela produção de entropia) em um sistema quântico fechado não autônomo, a construção de uma máquina térmica quântica empregando spins nucleares e a implementação de um demônio de Maxwell em regime quântico. O estudo da Termodinâmica Quântica ainda está em sua infância e, por ser tão novo, ainda há divergências (em parte da comunidade) sobre alguns conceitos. Esperamos que essa tese contribua experimentalmente e teoricamente para avançar a noção de que as definições de grandezas termodinâmicas devem ser consistentes com os teoremas de flutuação, que continuam válidos no regime quântico e de poucas partículas. Em nossa opinião esses teoremas junto com as relações entre informação e energia formam o arcabouço teórico fundamental para o avanço da área. / Quantum Thermodynamics is an emerging topic in Physics, associated with applying the principles of Thermodynamics to systems that are usually small, far from thermal equilibrium and for which quantum effects are relevant. This new research field should give substantial contributions to understanding the limits and limitations of new quantum technologies involving information processing, communication, metrology, among others. In the limit of small systems far from equilibrium, energy fluctuations become very important and thermodynamical quantities such as work, heat and entropy production are treated as stochastic variables obeying fluctuation theorems. The possibility to access the probability distribution of fluctuations is essential to fully characterise the thermodynamics of an out-of-equilibrium process. In a related approach, the possibility to acquire and use information has been included in the theoretical framework of fluctuation theorems, paving the way for powerful connections between Thermodynamics and Information Theory. Such connections enable us, among other things, to derive a quantitative expression in terms of microscopic variables for the philosophical concept known as the arrow of time in a quantum system, and to understand in a fundamental level the origin of the asymmetry between past and future. These ideas can be used to design a feedback mechanism able to act as a genuine Maxwell demon. Even though there is intense theoretical activity, experiments in Quantum Thermodynamics have been taking a long time to appear, due to the difficulty in measuring energy fluctuations during an out-of-equilibrium quantum system evolution. However, techniques from Nuclear Magnetic Resonance (NMR) developed for this thesis are able to circumvent that problem. Using them, we report the first experiments in Quantum Thermodynamics, including the verification of the fluctuation theorems, the observation of the emergence of the arrow of time (quantified by entropy production) in a closed non-autonomous quantum system, the implementation of a quantum thermal machine using nuclear spins and the realization of a Maxwell demon in the quantum domain. The field of Quantum Thermodynamics is still in its infancy, and there are divergences (in part of the community) about some of its concepts. We hope that this thesis can contribute experimentally and theoretically to advance the notion that the definitions of thermodynamical quantities must be consistent with the fluctuation theorems, which remain valid in the the quantum, low-particle-number regime. It is our opinion that these theorems, together with relations between information and energy, form the fundamental theoretical framework that will help advance the field.