In the age of information explosion, there is huge amount of information generated every second. Some of the information generated, for example news, is supposed to be shared by public and anyone in the world can get a copy of it. However, sometimes, information is only supposed to be maintain private or only shared by a given group of people. In the latter case, information protection becomes very important. There are various ways to protect information. One of the technical ways is cryptography, which is an area of interest for mathematicians, computer scientists and physicists. As a new area in cryptography, physical layer security has been paid great attention recently. Quantum key distribution is a hot research topic for physical layer security in the two decades. This thesis focuses on two quantum key distribution protocols that can potentially increase the key generation rate and lower the cost. On protocol is based on amplified spontaneous emission as signal source and the other one is based on discretely signaled continuous variable quantum communication. The security analysis and experimental implementation issues for both protocols are discussed.

Quantum networks lie at the heart of distributed quantum computing and secure quantum communication - research areas that have seen a strong increase of interest over the last decade. Their basic architecture consist of stationary nodes composed of quantum processors which are linked via photonic channels. The key requirement, and at the same time the most demanding challenge, is the efficient distribution of entanglement between distant nodes. The two ground states of single spins confined in self-assembled InGaAs quantum dots provide an effective two-level system for the implementation of quantum bits. Moreover, they offer strong transition dipole moments with outstanding photonic properties allowing for the realisation of close to ideal, high-bandwidth spin-photon interfaces. These properties are combined with the benefits of working in the solid state, such as scalability and integrability of devices, to form a promising candidate for the implementation of fast entanglement distribution. In this dissertation we provide the first implementation of a unit cell of a quantum network based on single electron spins in InGaAs. We use a probabilistic scheme based on spin-photon entanglement and the erasure of which path information to project the two distant spins into a maximally entangled Bell state. The successful generation of entanglement is verified through a reconstruction of the final two-spin state and we achieve an average fidelity of $61.6\pm2.3\%$ at a record-high generation rate of $5.8\,\mathrm{kHz}$. One of the main constraints to the achieved fidelity is the limited coherence of the electron spin. We show that it can be extended by three orders of magnitude through decoupling techniques and develop a new measurement technique, allowing us to investigate the origins of the decoherence which has previously been obscured by nuclear feedback processes. Our results evidence that further extension of coherence is ultimately limited by intrinsic mechanisms closely related to local strain due to the growth method of self-assembled quantum dots. After establishing the intrinsic limits to the electron coherence we investigate the coherence properties of the single hole spin as an alternative two-level system with the potential for higher coherence times. We show that the hole spin coherence is indeed superior to the one of the electron and realise the first successful dynamic decoupling scheme implemented in these systems. We find that the decoherence at low external magnetic fields is still governed by coupling to the nuclear spins whereas it is dominated by electrical noise for fields exceeding a few Tesla. This noise source is extrinsic to the quantum dots and a better understanding offers the potential for further improvement of the coherence time. The findings of this work present a complete study of the coherence of the charge carriers in self-assembled quantum dots and provide the knowledge needed to improve the implementation of a quantum-dot based quantum network. In particular, the combination of spin-spin entanglement and the hole coherence times enable further research towards multidimensional photonic cluster states.

Les pères fondateurs de la mécanique quantique exploraient les implications de leur théorie avec des "expériences de pensée". Les améliorations continuelles en matière de manipulation de systèmes quantiques individuels ont ouvert la voie à des recherches théoriques et expérimentales. C'est la base de l'information quantique: quand un contenu informationnel est associé à des transformations et mesures sur des systèmes quantiques, cela offre un nouveau paradigme à la théorie de l'information. Une des promesses de l'information quantique est la réalisation d'un internet quantique: des liaisons quantiques permettraient de partager des états quantiques entre les noeuds du réseau. Le contexte de notre travail est l'optique quantique expérimentale dans des fibres optiques aux longueurs d'onde des télécommunications, avec comme perspective des applications en communication quantique. Nous démontrons une nouvelle méthode pour manipuler des photons intriqués en énergie-temps, en utilisant des composants fibrés et optoélectroniques standard. Les photons produits par paires par une source de conversion paramétrique sont envoyés dans des modulateurs de phase électro-optiques indépendants, qui agissent comme des diviseurs de faisceau en fréquence. Nous utilisons ensuite des filtres fréquentiels et des détecteurs de photons uniques pour discriminer les fréquences des photons. Nos résultats expérimentaux incluent l'obtention d'interférences à deux photons robustes, à haute visibilité et à haute dimension, qui permettent la violation d'inégalités de Bell. Cela montre qu'une telle "intrication en bins fréquentiels" est une plate-forme intéressante pour la communication Quantique. / The founding fathers of quantum mechanics explored the implications of their theory with "gedanken experiments". Continuous improvement of the experimental manipulation of individual quantum systems has opened the way to exciting research, both on blackboards and in laboratories. lt is the basis for quantum information processing : when an information content is associated with transformations and measurements of quantum systems, it offers a new paradigm, full of potentialities, to information theory. One of the promises of quantum information is the realization of a quantum internet: quantum communication links would allow to share quantum states between the nodes of the network.Our work lies in the context of experimental quantum optics in optical fibers at telecommunication wavelengths, in view of quantum communication applications. We demonstrate a new method for manipulating photons entangled in their energy-time degree of freedom, by using standard fiber-optic and optoelectronic components. The photon pairs produced by a parametric down-conversion source are sent through independent electro­optic phase modulators, which act as high-dimensional frequency beam splitters. We then use frequency filters and single-photon detectors to discriminate the frequencies of the photons. Our experimental results include robust, high-visibility and high-dimensional two­photon interference patterns allowing Bell inequality violations. This shows that such a "frequency -bin entanglement" provides an interesting platform for quantum communication.

Thesis (MTech (Information Technology))--Cape Peninsula University of Technology, 2019 / The availability of high-speed/high-volume Data Link Layer (Layer 2) transmission networks fuelled by the implementation of mission critical and performance-intensive technologies, such as Cloud and Data Centre services transmitting sensitive data over the wide area network (WAN) has shifted the attention of hackers, eavesdroppers, cyber-criminals and other malicious attackers to the exploitation of these data transmission technologies. It is argued that security on the current classical technologies that store, transmit and manipulate information on the OSI Layer 2 have historically not been adequately addressed when it comes to secure communication and exchange of information. Quantum teleportation (QT) stemming from quantum communication a branch of quantum information science (QIS) has emerged as a technology that promise unconditional security and providing new ways to design and develop frameworks that operate based on the laws of quantum physics. It is argued that it has a potential to address the data transmission security GAP for OSI layer 2 technologies. This research study aims to propose a framework for the implementation of secure quantum teleportation infrastructures in South Africa. There is currently a lack of generic models and methods to guide the implementation of QT infrastructures that will enable secure transmission of information. A design science research (DSR) was undertaken in order to develop a secure quantum teleportation artefact called (SecureQT-Framework). SecureQT-Framework is a generic model and method that guides the selection and implementation of QT infrastructures motivated by multi-disciplinary domains such as QIS, Quantum Physics, Computer Science as well as information and communication technology (ICT). The DSR process employed a primary DSR cycle with four DSR sub-cycles which involved the awareness and suggestion phase guided by a systematic literature review (SLR), development and evaluation phase guided by Software Defined Network’s OpenFlow, Mininet, Mininet-Wifi and computer simulations for QT using SQUANCH framework. We investigated, examined and collected credible QT techniques and its variant protocols to develop and simulate secure transmission of information over the WAN, We studied their features and challenges. We concluded the study by describing the QT techniques, protocols and implementations that has potential to bridge the security GAP for OSI Layer 2 technologies over the WAN. The results gained were used in the construction of a framework for the implementation of a secure quantum teleportation infrastructure in South Africa. The framework describes the main factors that need to be taken into consideration when implementing quantum teleportation infrastructures.

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We present a theoretical study that investigates the applicability of a graph theoretical approach to realize various quantum experiments. Crucially, we may represent quantum optical experiments involving tabletop optical elements in terms of highly interpretable, coloured, weighted multi-graphs. We introduce the formalism behind this approach; then through the digital discovery framework PyTheus, we uncover over 100 different quantum experiments which realizes complex, novel quantum states. Towards enhancing our interpretation of the AI-based framework's solutions, we also leverage eXplainable-AI (XAI) techniques from computer vision to investigate what a trained neural network learns about quantum experiments. Crucially, we find that we are able to conceptualize the learned strategies which the neural network applies to optimize for a target quantum property, and discover how the network conceives of its solution. We conclude by presenting an experimental proposal which yields realizable solutions that, for the first time, solves high-dimensional variants of a quantum retrodiction puzzle known as the Mean King's Problem. We, therefore, present a case study which investigates the potential for new scientific discoveries through a joint collaboration between human and artificial intelligence.

Quantum communication protocols require maximally entangled state of pair of qubits (spin-1/2 states in this context) to be shared between sender and the receiver. The entangled qubits lose entanglement because of random magnetic field disturbances. The dynamics in the form of joint density matrix of random pure entangled state provide the steady (joint) state and the associated timescales (time taken by the pair to reach the steady state) providing a scope in future to quantify the effective utilization of quantum communication protocols. / The dynamics of an identical pair of entangled spin-1/2 particles, both subjected to the identical, independent, correlated random magnetic fields is studied. The dynamics of the pure joint state of the pair is derived using stochastic calculus. In case of identical fields, an ensemble of such pure states are combined using the modified spin joint density matrix and the joint relaxation time is obtained for the pair of spin-1/2 particles. These dynamics can be interpreted as special kind of correlations involving the spatial components of the Bloch polarization vectors of the constituent entangled spin-1/2 particles. In case of independent random magnetic fields, the dynamics are obtained by considering a pure joint state of entangled spin-1/2 particles. The disentanglement time defined as the time taken for the particles to become disentangled, is obtained. In case of correlated random magnetic fields, the dynamics of a maximally entangled pair of spin-1/2 particles are derived in terms of the joint density matrix of the entangled pair from which the steady state density matrix and the associated timescale for it to be reached are obtained. The asymptotic density matrix in this case represents a state of (partial) disentanglement. In other words, there is a persistent entanglement in case of correlated field disturbances. / Thesis / Doctor of Philosophy (PhD) / Maximally entangled pair of quantum bits (in the form of spin-1/2 states) lose entanglement either partially or completely depending upon the nature of random magnetic field disturbances around them (correlated/independent/identical fields). The dynamics of entangled states (in the form of density matrix of a random pure state) in the presence of random magnetic fields are obtained using the ideas of stochastic calculus to understand the steady state of the pair and the associated timescales to be reached.

This thesis probes the usefulness of non-classical correlations within imperfect continuous variable decoherent quantum systems. Although a consistent function and practical usefulness of these correlations is largely unknown, it is important to examine their characteristics in more realistic dissipative systems, to gain further insight into any possible advantageous behaviour. A bipartite separable discordant state under the action of controlled loss on one subsystem was considered. Under these conditions the Gaussian quantum discord not only proved to be robust against loss, but actually improves as loss is intensified. Harmful imperfections which reduce the achievable level of discord can be counteracted by this controlled loss. Through a purification an explanation of this effect was sought by considering system-environment correlations, and found that a flow of system-environment correlations increases the quantumness of the state. Entanglement recovery possibilities were discussed and revealed the importance of hidden quantum correlations along bi-partitions across the discordant state and a classically prepared "demodulating" system, acting in such a way as to partially cancel the entanglement preventing noise. Entanglement distribution by separable states was studied by a similar framework, in an attempt to explain the emergence of quantum entanglement by a specific flow of correlations in the globally pure system. Discord appears to play a less fundamental role compared to the qubit version of the protocol. The strengthening of non-classical correlations can be attributed to a flow of classical and quantum correlations. This work proves that discord can be created in unique ways and, in select circumstances, can act to counteract harmful imperfections in the apparatus. Due to this advantageous behaviour discord indeed may ultimately aid in more applicable "real world" applications, which are by definition decoherent.

Nesta tese abordamos o problema de transferência e manipulação de informação quântica em sistemas dissipativos. Inicialmente apresentamos uma técnica para construir, dentro de redes bosônicas dissipativas, canais livres de decoerência (CLD): um grupo de modos normais de osciladores com taxas de amortecimento efetivas nulas. Verificamos que os estados protegidos dentro do CLD definem subespaços livres de decoerência (SLD) quando mapeados de volta para a base dos osciladores naturais da rede. Portanto, a nossa técnica para obter canais protegidos formados por modos normais é uma forma alternativa para construir SLD, que oferece vantagens em relação ao método convencional. Nosso protocolo permite o cálculo de todos os estados da rede protegidos de uma só vez, assim como leva naturalmente ao conceito de subespaço quase livre de decoerência (SQLD), dentro do qual um estado de superposição é quase completamente protegido. O conceito de SQLD, é mais fraco do que a dos SLD, pode proporcionar um mecanismo mais manejável para controlar decoerência. Em seguida desenvolvemos um protocolo para transferência quase perfeita de estados de poláriton de um sistema emissor para um receptor, separados espacialmente, ambos acoplados por um canal de transmissão não ideal que é modelado por uma rede de cavidades dissipativas. Esse protocolo consiste no acoplamento dispersivo entre o estado de poláriton preparado no emissor com os modos normais da rede que forma o canal, o que possibilita que o estado tunele para o receptor. Após a obtenção de um Hamiltoniano efetivo para o acoplamento entre o emissor e receptor, calculamos a fidelidade para a transferência de alguns estados de poláriton, por exemplo, estados tipo gato de Schrödinger. Mostramos que as taxas de decaimento da fidelidade são proporcionais a cooperatividade, parâmetro esse que avalia a relação entre a taxa de dissipação e o acoplamento efetivo. Analisamos a dependência da fidelidade e do tempo de transferência em relação à topologia da rede. Por fim, propomos o mecanismo de tunelamento não local para transferência de estados bosônicos e fermiônicos com alta fidelidade. Demonstramos que a incoerência decorrente das não idealidades quânticas do canal é quase totalmente contornada pelo mecanismo de tunelamento que possibilita um processo de transferência de alta fidelidade. Aplicamos esse mecanismo para transferência e processamento de informações entre múltiplos circuitos quântico (CQs) não ideais. Um conjunto de saídas é simultaneamente acoplado ao conjunto correspondente de entradas de outro QC espacialmente separado do primeiro, através de um único canal quântico não ideal. Mostramos que além da transferência de estados, podemos realizar operações logicas entre qubits distantes e gerar uma pletora de estados quânticos emaranhados. / In this thesis we address the problem of transfer and manipulation of quantum information in dissipative systems. First we present a technique to build, within a dissipative bosonic network, decoherence-free channels (DFCs): a group of normal-mode oscillators with null effective damping rates. We verify that the states protected within the DFC define the wellknown decoherence-free subspaces (DFSs) when mapped back into the natural network oscillators. Therefore, our technique to build protected normal-mode channels turns out to be an alternative way to build DFSs, which offers advantages over the conventional method. It enables the computation of all the network-protected states at once, as well as leading naturally to the concept of the decoherence quasi-free subspace (DQFS), inside which a superposition state is quasi-completely protected against decoherence. The concept of the DQFS, weaker than that of the DFS, may provide a more manageable mechanism to control decoherence. Finally, as an application of the DQFSs, we show how to build them for quasi-perfect state transfer in networks of coupled quantum dissipative oscillators. Then we present a scheme for quasi perfect transfer of polariton states from a sender to a spatially separated receiver, both composed of high-quality cavities filled by atomic samples. The sender and the receiver are connected by a nonideal transmission channel the data bus modelled by a network of lossy empty cavities. In particular, we analyze the influence of a large class of data-bus topologies on the fidelity and transfer time of the polariton state. Moreover, we also assume dispersive couplings between the polariton fields and the data-bus normal modes in order to achieve a tunneling-like state transfer. Such a tunneling-transfer mechanism, by which the excitation energy of the polariton effectively does not populate the data-bus cavities, is capable of attenuating appreciably the dissipative effects of the data-bus cavities. After deriving a Hamiltonian for the effective coupling between the sender and the receiver, we show that the decay rate of the fidelity is proportional to a cooperativity parameter that weigh the cost of the dissipation rate against the benefit of the effective coupling strength. The increase of the fidelity of the transfer process can be achieved at the expense of longer transfer times. We also show that the dependence of both the fidelity and the transfer time on the network topology for distinct regimes of parameters. It follows that the data-bus topology can be explored to control the time of the state-transfer process. Finally we propose the nonlocal tunneling mechanism for high-fidelity state transfer between distant parties. We apply this mechanism for highfidelity information transfer and processing between remote multi-branch nonideal quantum circuits (QCs). We show that in addition to the transfer of states, we can perform logic operations between distant qubits and generate a plethora of entangled quantum states.

Nesta tese abordamos o problema de transferência e manipulação de informação quântica em sistemas dissipativos. Inicialmente apresentamos uma técnica para construir, dentro de redes bosônicas dissipativas, canais livres de decoerência (CLD): um grupo de modos normais de osciladores com taxas de amortecimento efetivas nulas. Verificamos que os estados protegidos dentro do CLD definem subespaços livres de decoerência (SLD) quando mapeados de volta para a base dos osciladores naturais da rede. Portanto, a nossa técnica para obter canais protegidos formados por modos normais é uma forma alternativa para construir SLD, que oferece vantagens em relação ao método convencional. Nosso protocolo permite o cálculo de todos os estados da rede protegidos de uma só vez, assim como leva naturalmente ao conceito de subespaço quase livre de decoerência (SQLD), dentro do qual um estado de superposição é quase completamente protegido. O conceito de SQLD, é mais fraco do que a dos SLD, pode proporcionar um mecanismo mais manejável para controlar decoerência. Em seguida desenvolvemos um protocolo para transferência quase perfeita de estados de poláriton de um sistema emissor para um receptor, separados espacialmente, ambos acoplados por um canal de transmissão não ideal que é modelado por uma rede de cavidades dissipativas. Esse protocolo consiste no acoplamento dispersivo entre o estado de poláriton preparado no emissor com os modos normais da rede que forma o canal, o que possibilita que o estado tunele para o receptor. Após a obtenção de um Hamiltoniano efetivo para o acoplamento entre o emissor e receptor, calculamos a fidelidade para a transferência de alguns estados de poláriton, por exemplo, estados tipo gato de Schrödinger. Mostramos que as taxas de decaimento da fidelidade são proporcionais a cooperatividade, parâmetro esse que avalia a relação entre a taxa de dissipação e o acoplamento efetivo. Analisamos a dependência da fidelidade e do tempo de transferência em relação à topologia da rede. Por fim, propomos o mecanismo de tunelamento não local para transferência de estados bosônicos e fermiônicos com alta fidelidade. Demonstramos que a incoerência decorrente das não idealidades quânticas do canal é quase totalmente contornada pelo mecanismo de tunelamento que possibilita um processo de transferência de alta fidelidade. Aplicamos esse mecanismo para transferência e processamento de informações entre múltiplos circuitos quântico (CQs) não ideais. Um conjunto de saídas é simultaneamente acoplado ao conjunto correspondente de entradas de outro QC espacialmente separado do primeiro, através de um único canal quântico não ideal. Mostramos que além da transferência de estados, podemos realizar operações logicas entre qubits distantes e gerar uma pletora de estados quânticos emaranhados. / In this thesis we address the problem of transfer and manipulation of quantum information in dissipative systems. First we present a technique to build, within a dissipative bosonic network, decoherence-free channels (DFCs): a group of normal-mode oscillators with null effective damping rates. We verify that the states protected within the DFC define the wellknown decoherence-free subspaces (DFSs) when mapped back into the natural network oscillators. Therefore, our technique to build protected normal-mode channels turns out to be an alternative way to build DFSs, which offers advantages over the conventional method. It enables the computation of all the network-protected states at once, as well as leading naturally to the concept of the decoherence quasi-free subspace (DQFS), inside which a superposition state is quasi-completely protected against decoherence. The concept of the DQFS, weaker than that of the DFS, may provide a more manageable mechanism to control decoherence. Finally, as an application of the DQFSs, we show how to build them for quasi-perfect state transfer in networks of coupled quantum dissipative oscillators. Then we present a scheme for quasi perfect transfer of polariton states from a sender to a spatially separated receiver, both composed of high-quality cavities filled by atomic samples. The sender and the receiver are connected by a nonideal transmission channel the data bus modelled by a network of lossy empty cavities. In particular, we analyze the influence of a large class of data-bus topologies on the fidelity and transfer time of the polariton state. Moreover, we also assume dispersive couplings between the polariton fields and the data-bus normal modes in order to achieve a tunneling-like state transfer. Such a tunneling-transfer mechanism, by which the excitation energy of the polariton effectively does not populate the data-bus cavities, is capable of attenuating appreciably the dissipative effects of the data-bus cavities. After deriving a Hamiltonian for the effective coupling between the sender and the receiver, we show that the decay rate of the fidelity is proportional to a cooperativity parameter that weigh the cost of the dissipation rate against the benefit of the effective coupling strength. The increase of the fidelity of the transfer process can be achieved at the expense of longer transfer times. We also show that the dependence of both the fidelity and the transfer time on the network topology for distinct regimes of parameters. It follows that the data-bus topology can be explored to control the time of the state-transfer process. Finally we propose the nonlocal tunneling mechanism for high-fidelity state transfer between distant parties. We apply this mechanism for highfidelity information transfer and processing between remote multi-branch nonideal quantum circuits (QCs). We show that in addition to the transfer of states, we can perform logic operations between distant qubits and generate a plethora of entangled quantum states.