CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior / O presente trabalho à divido em duas partes. Na primeira, a utilizaÃÃo do interferÃmetro de Sagnac em informaÃÃo quÃntica à analisada atravÃs da aplicaÃÃo do mesmo em trÃs problemas: MediÃÃo livre de interaÃÃo, distribuiÃÃo quÃntica de chaves e compartilhamento de segredo. Para a mediÃÃo livre de interaÃÃo, dois sistemas usando o Sagnac foram propostos. Considerando detectores ideais e ausÃncia de perdas, um deles apresenta probabilidade de sucesso de 25% por fÃton consumido, enquanto que o segundo apresenta probabilidade de determinar corretamente a presenÃa do objeto prÃxima a 100% por fÃton consumido. Para a distribuiÃÃo quÃntica de chaves foi proposta uma configuraÃÃo diferente das existentes, sendo a principal diferenÃa a ausÃncia de retorno do pulso enviado, como ocorre nos sistemas com Sagnac anteriormente propostos. Isto evita ataques do tipo Cavalo de TrÃia. Por fim, foi proposta uma configuraÃÃo Ãptica que permite que um segredo compartilhado por cinco pessoas, localmente distantes, seja usado apenas quando todos os cinco concordarem. A segunda parte da dissertaÃÃo apresenta um estudo analÃtico e numÃrico da variaÃÃo do entrelaÃamento de estados bipartites de qubits quando da propagaÃÃo dos mesmos em canais quÃnticos ruidosos. Em particular, foi encontrada uma fÃrmula exata que relaciona os entrelaÃamentos dos estados na entrada e saÃda do canal, quando o estado na entrada à puro e o canal à modelado pela interaÃÃo do estado bipartite (sinal) com um qubit (estado do canal) atravÃs de uma operaÃÃo unitÃria canÃnica. / This work is divided into two parts. In the first one, the use of the Sagnac interferometer in quantum information is analyzed applying it in three problems: interaction-free measurement, quantum key distribution, and secret sharing. For the interaction-free measurement two systems using Sagnac interferometer were proposed. Considering ideal detectors and loss less devices, one of them has a success probability of 25% for each photon used while the other presents the probability of getting success to detect the presence of the object close to 100% for each photon used. For quantum key distribution, it was proposed a different setup, where the main difference is that the pulse sent by the transmitter does not come back to him/her as happen with the systems based on Sagnac proposed before. This avoids the Trojan horse attack. Finally, it was proposed an optical configuration where it is possible to share a secret among five users, locally distant, that could be used only when all five persons agreed. The second part of this dissertation presents an analytical and numerical study of the entanglement variation of bipartite states of qubits during propagation in a quantum noisy channel. Particularly, it was found an exactly formula which relates the entanglement of states in the input and output of the channel, when the input state is pure and the channel is modeled by a canonical unitary operation.

<p>Quantum key growing, often called quantum cryptography or quantum key distribution, is a method using some properties of quantum mechanics to create a secret shared cryptography key even if an eavesdropper has access to unlimited computational power. A vital but often neglected part of the method is unconditionally secure message authentication. This thesis examines the security aspects of authentication in quantum key growing. Important concepts are formalized as Python program source code, a comparison between quantum key growing and a classical system using trusted couriers is included, and the chain rule of entropy is generalized to any Rényi entropy. Finally and most importantly, a security flaw is identified which makes the probability to eavesdrop on the system undetected approach unity as the system is in use for a long time, and a solution to this problem is provided.</p>

Quantum key growing, often called quantum cryptography or quantum key distribution, is a method using some properties of quantum mechanics to create a secret shared cryptography key even if an eavesdropper has access to unlimited computational power. A vital but often neglected part of the method is unconditionally secure message authentication. This thesis examines the security aspects of authentication in quantum key growing. Important concepts are formalized as Python program source code, a comparison between quantum key growing and a classical system using trusted couriers is included, and the chain rule of entropy is generalized to any Rényi entropy. Finally and most importantly, a security flaw is identified which makes the probability to eavesdrop on the system undetected approach unity as the system is in use for a long time, and a solution to this problem is provided. / ICG QC

Coding is an essential technology for efficient fiber-optic communications and secure quantum communications. In particular, low-density parity-check (LDPC) coding is favoured due to its strong error correction capability and high-throughput implementation feasibility. In fiber-optic communications, it has been realized that advanced high-order modulation formats and soft-decision forward error correction (FEC) such as LDPC codes are the key technologies for the next-generation high-speed optical communications. Therefore, energy-efficient LDPC coding in combination with advanced modulation formats is an important topic that needs to be studied for fiber-optic communications. In secure quantum communications, large-alphabet quantum key distribution (QKD) is becoming attractive recently due to its potential in improving the efficiency of key exchange. To recover the carried information bits, efficient information reconciliation is desirable, for which the use of LDPC coding is essential. In this dissertation, we first explore different efficient LDPC coding schemes for optical transmission of polarization-division multiplexed quadrature-amplitude modulation (QAM) signals. We show that high energy efficiency can be achieved without incurring extra overhead and complexity. We then study the transmission performance of LDPC-coded turbo equalization for QAM signals in a realistic fiber link as well as that of pragmatic turbo equalizers. Further, leveraging the polarization freedom of light, we expand the signal constellation into a four-dimensional (4D) space and evaluate the performance of LDPC-coded 4D signals in terms of transmission reach. Lastly, we study the security of a proposed weak-coherent-state large-alphabet QKD protocol and investigate the information reconciliation efficiency based on LDPC coding.

<p>Strong coupling between light and matter has been demonstrated both in classical</p><p>cavity quantum electrodynamics (QED) systems and in more recent circuit-QED</p><p>experiments. This enables the generation of strong nonlinear photon-photon interactions</p><p>at the single-photon level, which is of great interest for the observation</p><p>of quantum nonlinear optical phenomena, the control of light quanta in quantum</p><p>information protocols such as quantum networking, as well as the study of</p><p>strongly correlated quantum many-body systems using light. Recently, strong</p><p>coupling has also been realized in a variety of one-dimensional (1D) waveguide-</p><p>QED experimental systems, which in turn makes them promising candidates for</p><p>quantum information processing. Compared to cavity-QED systems, there are</p><p>two new features in waveguide-QED: the existence of a continuum of states and</p><p>the restricted 1D phase space, which together bring in new physical effects, such</p><p>as the bound-state effects. This thesis consists of two parts: 1) understanding the</p><p>fundamental interaction between local quantum objects, such as two-level systems</p><p>and four-level systems, and photons confined in the waveguide; 2) exploring</p><p>its implications in quantum information processing, in particular photonic</p><p>quantum computation and quantum key distribution.</p><p>First, we demonstrate that by coupling a two-level system (TLS) or three/fourlevel</p><p>system to a 1D continuum, strongly-correlated photons can be generated</p><p>inside the waveguide. Photon-photon bound states, which decay exponentially as a function of the relative coordinates of photons, appear in multiphoton scattering</p><p>processes. As a result, photon bunching and antibunching can be observed</p><p>in the photon-photon correlation function, and nonclassical light source can be</p><p>generated on demand. In the case of an N-type four-level system, we show</p><p>that the effective photon-photon interaction mediated by the four-level system,</p><p>gives rise to a variety of nonlinear optical phenomena, including photon blockade,</p><p>photon-induced tunneling, and creation of single-photon states and photon</p><p>pairs with a high degree of spectral entanglement, all in the absence of a cavity.</p><p>However, to enable greater quantum networking potential using waveguide-</p><p>QED, it is important to study systems having more than just one TLS/qubit.</p><p>We develop a numerical Green function method to study cooperative effects in</p><p>a system of two qubits coupled to a 1D waveguide. Quantum beats emerge in</p><p>photon-photon correlations, and persist to much longer time scales because of</p><p>non-Markovian processes. In addition, this system can be used to generate a</p><p>high-degree of long-distance entanglement when one of the two qubits is driven</p><p>by an on-resonance laser, further paving the way toward waveguide-QED-based</p><p>quantum networks.</p><p>Furthermore, based on our study of light-matter interactions in waveguide-</p><p>QED, we investigate its implications in quantum information processing. First,</p><p>we study quantum key distribution using the sub-Possonian single photon source</p><p>obtained by scattering a coherent state off a two-level system. The rate for key</p><p>generation is found to be twice as large as for other sources. Second, we propose</p><p>a new scheme for scalable quantum computation using flying qubits--propagating</p><p>photons in a one-dimensional waveguide--interacting with matter qubits. Photonphoton</p><p>interactions are mediated by the coupling to a three- or four-level system,</p><p>based on which photon-photon -phase gates (Controlled-NOT) can be implemented for universal quantum computation. We show that high gate fidelity is</p><p>possible given recent dramatic experimental progress in superconducting circuits</p><p>and photonic-crystal waveguides. The proposed system can be an important</p><p>building block for future on-chip quantum networks.</p> / Dissertation

[pt] A criptografia quântica foi proposta como uma solução para o problema da distribuição de chaves criptográficas com segurança total garantida pelos princípios da mecânica quântica. Através dessa técnica é possível saber se um espião tentou interceptar a transmissão, o que é impossível utilizando técnicas de transmissão clássicas. Nesse trabalho foi feito um breve resumo da teoria de criptografia quântica, de suas técnicas de transmissão e dos problemas tecnológicos enfrentados. Foi analisada em detalhes a técnica de transmissão de qubits utilizando codificação de freqüência e feita uma comparação dos diferentes esquemas de modulação frente aos protocolos BB84 e B92. Foi demonstrado que os dois esquemas de modulação existentes (AM-AM e PM-PM) são na realidade equivalentes e foi proposto um novo esquema, o AM-PM o único que suporta o protocolo BB84 clássico. Medidas foram realizadas classicamente nos formatos AM-AM e AM-PM. / [en] Quantum cryptography has been proposed as a solution to the cryptographic key distribution problem with absolute security guaranteed by the principles of quantum mechanics. Through this scheme it is possible to find out whether a spy tried to eavesdrop on the transmission, which was impossible to discover using classical transmission techniques. In this work a brief review of quantum cryptography theory, transmission techniques and technological problems involved were performed. It was analyzed in detail the transmission technique employing frequency coding, and a comparison was made between the different modulation schemes and the BB84 and B92 protocols. It was demonstrated that the two existing modulation formats (AM-AM and PM-PM) are in fact equivalent and a new format (AM-PM) was proposed, the only one able to accommodate classical BB84. Classical measurements were performed on the AM-AM and AMPM formats.

Die Quantenschlüsselverteilung (QKD), die erste anwendbare Quantentechnologie, verspricht informationstheoretisch sichere Kommunikation. In der vorliegenden Arbeit wurde das Zeit-Frequenz (TF)-QKD-Protokoll untersucht, das Zeit und Frequenz, nämlich Puls-Positionsmodulation (PPM) im Zeitbereich und Frequenzumtastung (FSK) im Frequenzbereich als die beiden komplementären Basen verwendet. Seine Sicherheit beruht den Quanteneigenschaften von Licht und auf der Zeit-Frequenz-Unschärferelation. TF-QKD kann mit größtenteils Standard-Telekommunikationstechnologie im 1550-nm-Band implementiert werden. Die PPM-Basis kann mit Modulatoren und die FSK-Basis mit Hilfe der Wellenlängenmultiplex-Technologie realisiert werden. Das TF-QKD-Protokoll ist in der Lage, ein beliebig großes Alphabet bereitzustellen, was mehr als 1 bit/Photon ermöglicht. Darüber hinaus ist es robust gegenüber athmosphärischen Störungen und somit für die Übertragung über den Freiraumkanal geeignet. In der vorliegenden Arbeit wird das TF-QKD-Protokoll theoretisch bewertet, mit Standardkomponenten für 1 bit/Photon implementiert und die Freiraumübertragung mit optischem Tracking über eine 388 m Teststrecke wird bei Tageslicht demonstriert. Unter Verwendung der vorhandenen Komponenten konnte eine sichere Schlüsselrate von 364 kbit/s back-to-back und 9 kbit/s über den Freiraumkanal demonstriert werden. / Quantum key distribution (QKD), the first applicable quantum technology, promises information theoretically secure communication. In the presented work the time-frequency (TF)-QKD protocol was examined, which uses time and frequency, namely pulse position modulation (PPM) in the time domain and frequency shift keying (FSK) in the frequency domain as the two complementary bases. Its security relies on the quantum properties of light and the time-frequency uncertainty relation. TF-QKD can be implemented mostly with standard telecom-technology in the 1550 nm band. The PPM basis can be implemented with modulators and the FSK basis with help of wavelength-division multiplexing technology. The TF-QKD protocol is capable of providing an arbitrarily large alphabet enabling more than 1 bit/photon. Moreover, it is robust in the atmosphere making it suitable for transmission over the free-space channel. In the present work the TF-QKD protocol is assessed theoretically, implemented with off-the-shelf components for 1 bit/photon and free-space transmission with optical tracking over a 388 m testbed is demonstrated in daylight. Using components at hand, secret key rates of 364 kbit/s back-to-back and 9 kbit/s over the free-space channel could be demonstrated.

<p>La distribution quantique de clés est une technique cryptographique permettant l'échange de clés secrètes dont la confidentialité est garantie par les lois de la mécanique quantique. Le comportement particulier des particules élémentaires est exploité. En effet, en mécanique quantique, toute mesure sur l'état d'une particule modifie irrémédiablement cet état. En jouant sur cette propriété, deux parties, souvent appelées Alice et Bob, peuvent encoder une clé secrète dans des porteurs quantiques tels que des photons uniques. Toute tentative d'espionnage demande à l'espion, Eve, une mesure de l'état du photon qui transmet un bit de clé et donc se traduit par une perturbation de l'état. Alice et Bob peuvent alors se rendre compte de la présence d'Eve par un nombre inhabituel d'erreurs de transmission.</p> <p>L'information échangée par la distribution quantique n'est pas directement utilisable mais doit être d'abord traitée. Les erreurs de transmissions, qu'elles soient dues à un espion ou simplement à du bruit dans le canal de communication, doivent être corrigées grâce à une technique appelée réconciliation. Ensuite, la connaissance partielle d'un espion qui n'aurait perturbé qu'une partie des porteurs doit être supprimée de la clé finale grâce à une technique dite d'amplification de confidentialité.</p> <p>Cette thèse s'inscrit dans le contexte de la distribution quantique de clé où les porteurs sont des états continus de la lumière. En particulier, une partie importante de ce travail est consacrée au traitement de l'information continue échangée par un protocole particulier de distribution quantique de clés, où les porteurs sont des états cohérents de la lumière. La nature continue de cette information implique des aménagements particuliers des techniques de réconciliation, qui ont surtout été développées pour traiter l'information binaire. Nous proposons une technique dite de réconciliation en tranches qui permet de traiter efficacement l'information continue. L'ensemble des techniques développées a été utilisé en collaboration avec l'Institut d'Optique à Orsay, France, pour produire la première expérience de distribution quantique de clés au moyen d'états cohérents de la lumière modulés continuement.</p> <p>D'autres aspects importants sont également traités dans cette thèse, tels que la mise en perspective de la distribution quantique de clés dans un contexte cryptographique, la spécification d'un protocole complet, la création de nouvelles techniques d'amplification de confidentialité plus rapides à mettre en œuvre ou l'étude théorique et pratique d'algorithmes alternatifs de réconciliation.</p> <p>Enfin, nous étudions la sécurité du protocole à états cohérents en établissant son équivalence à un protocole de purification d'intrication. Sans entrer dans les détails, cette équivalence, formelle, permet de valider la robustesse du protocole contre tout type d'espionnage, même le plus compliqué possible, permis par les lois de la mécanique quantique. En particulier, nous généralisons l'algorithme de réconciliation en tranches pour le transformer en un protocole de purification et nous établissons ainsi un protocole de distribution quantique sûr contre toute stratégie d'espionnage.</p> <p>Quantum key distribution is a cryptographic technique, which allows to exchange secret keys whose confidentiality is guaranteed by the laws of quantum mechanics. The strange behavior of elementary particles is exploited. In quantum mechnics, any measurement of the state of a particle irreversibly modifies this state. By taking advantage of this property, two parties, often called Alice and bob, can encode a secret key into quatum information carriers such as single photons. Any attempt at eavesdropping requires the spy, Eve, to measure the state of the photon and thus to perturb this state. Alice and Bob can then be aware of Eve's presence by a unusually high number of transmission errors.</p> <p>The information exchanged by quantum key distribution is not directly usable but must first be processed. Transmission errors, whether they are caused by an eavesdropper or simply by noise in the transmission channel, must be corrected with a technique called reconciliation. Then, the partial knowledge of an eavesdropper, who would perturb only a fraction of the carriers, must be wiped out from the final key thanks to a technique called privacy amplification.</p> <p>The context of this thesis is the quantum key distribution with continuous states of light as carriers. An important part of this work deals with the processing of continuous information exchanged by a particular protocol, where the carriers are coherent states of light. The continuous nature of information in this case implies peculiar changes to the reconciliation techniques, which have mostly been developed to process binary information. We propose a technique called sliced error correction, which allows to efficiently process continuous information. The set of the developed techniques was used in collaboration with the Institut d'Optique, Orsay, France, to set up the first experiment of quantum key distribution with continuously-modulated coherent states of light.</p> <p>Other important aspects are also treated in this thesis, such as placing quantum key distribution in the context of a cryptosystem, the specification of a complete protocol, the creation of new techniques for faster privacy amplification or the theoretical and practical study of alternate reconciliation algorithms.</p> <p>Finally, we study the security of the coherent state protocol by analyzing its equivalence with an entanglement purification protocol. Without going into the details, this formal equivalence allows to validate the robustness of the protocol against any kind of eavesdropping, even the most intricate one allowed by the laws of quantum mechanics. In particular, we generalize the sliced error correction algorithm so as to transform it into a purification protocol and we thus establish a quantum key distribution protocol secure against any eavesdropping strategy.</p>

We are currently experiencing a rapid development of quantum information, a new branch of science, being an interdisciplinary of quantum physics, information theory, telecommunications, computer science, and many others. This new science branch was born in the middle of the eighties, developed rapidly during the nineties, and in the current decade has brought a technological breakthrough in creating secure quantum key distribution (QKD), quantum secret sharing, and exciting promises in diverse technological fields. Recent QKD experiments have achieved high rate QKD at 200 km distance in optical fiber. Significant QKD results have also been achieved in free-space. Due to the rapid broadband access deployment in many industrialized countries and the standing increasing transmission security treats, the natural development awaiting quantum communications, being a part of quantum information, is its migration into commercial switched telecom networks. Such a migration concerns both multiuser quantum key distribution and multiparty quantum secret sharing that have been the main goal of my PhD studies. They are also the main concern of the thesis. Our research efforts in multiuser QKD has led to a development of the five-user setup for transmissions over switched fiber networks in a star and in a tree configuration. We have achieved longer secure quantum information distances and implemented more nodes than other multi-user QKD experiments. The measurements have shown feasibility of multiuser QKD over switched fiber networks, using standard fiber telecom components. Since circular architecture networks are important parts of both intranets and the Internet, Sagnac QKD has also been a subject of our research efforts. The published experiments in this area have been very few and results were not encouraging, mainly due to the single mode fiber (SMF) birefringence. Our research has led to a development of a computer controlled birefringence compensation in Sagnac that open the door to both classical and quantum Sagnac applications. On the quantum secret sharing side, we have achieved the first quantum secret sharing experiment over telecom fiber in a five-party implementation using the "plug &amp; play" setup and in a four-party implementation using Sagnac configuration. The setup measurements have shown feasibility and scalability of multiparty quantum communication over commercial telecom fiber networks.

Quantum key distribution (QKD) protocols are intended to allow cryptographic keys to be generated and distributed in way that is provably secure based on inherent limitations, such as the no-cloning principle, imposed by quantum mechanics. This unique advantage compared with classical cryptography comes with an added difficulty: key bits in QKD protocols are encoded in analogue quantum states and their preparation is consequently subject to the usual imprecisions inevitable in any real world experiment. The negative impact of such imprecisions is illustrated for the BB84 QKD protocol. Following this, the main part of this thesis is concerned with the incorporation of such imprecisions in security proofs of the BB84 and two semi-device-independent protocols against the class of collective attacks. On a technical level, by contrast with the vast majority of security proofs developed since the turn of the century, in which recasting the protocol into an equivalent entanglement-based form features heavily in the analysis, the main results obtained here are approached directly from the prepare-and-measure perspective and in particular the connection with the no-cloning theorem and an early security proof by Fuchs et al. against the class of individual attacks is emphasised.<p><p>This thesis also summarises, as an appendix, a separate project which introduces and defines a hierarchy of polytopes intermediate between the local and no-signalling polytopes from the field of Bell nonlocality. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished