Sistemas de comunicação quânticos baseados em Qubits codificados na polarização

We are now facing a second quantum revolution, that started in the early 21st century, bringing significant technological advances to science, industry and society based on advances on quantum information. The eminent emergence of a quantum computer has boosted concerns about the security of current classical public-key cryptography systems. One important topic in the research field of quantum information is the way we distribute keys in order to allow secure communication between distant parties. QKD systems are already in a pre-commercial stage attracting companies and government heavy investment in researching for quantum information technologies. However, there still are a lot of research to be done in this field, specially regarding high rate transmission, achievable distance reach, and obviously the practical implementation cost. In this thesis, we start by experimentally implement a polarization-encoded discrete variables based quantum communication system which allowed us to identify issues that must be solved in order to make it suitable for QKD protocols practical implementation. In this way, we propose a non-intrusive heuristic method to automatically compensate polarization random drift in standard opticalfiber channels due birefringence effects, and that induces errors during qubit transmission. The compensation of polarization drifts induced by the quantum channel is fundamental to enable the deployment of polarization encoded single-photons transmission over the current optical fiber networks. Furthermore, in this thesis we also propose and validated though numerical simulations a novel polarization-based DV-QKD system that combines the use of phase-modulators for state of polarization (SOP) generation and basis switching with a polarization diversity coherent detection scheme. This enables a full implementation of DV-QKD systems using only classical hardware, which low the cost of QKD systems based on polarization encoded single-photons at the same time that increases the transmission rate. Our results open the door to very high baud-rate polarization qubits transmission in access and metro networks. We report continuous qubit transmission, even in environments subjected to high polarization drift, without consuming extra-bandwidth with a maximum Quantum Bit Error Rate (QBER) of 2%.Estamos perante a segunda revolução quântica, a qual começou no início do século 21 trazendo avanços significativos na ciência, na indústria e na sociedade baseados nos avanços da teoria da informação. A emergência eminente de um computador quântico tem despoletado preocupações relativamente à segurança dos atuais sistemas de criptografia pública clássica. Um tópico importante no campo da investigação de informação quântica diz respeito à forma de distribuição de chaves criptográficas de forma a garantir comunicações seguras entre partes distantes. Os sistemas de distribuição de chaves quânticas estão já num estágio comercial, o que tem atraído investimento de empresas e governos para a investigação nas tecnologias de informação quântica. Contudo, existe ainda muita investigação a ser feita neste campo, especialmente no que diz respeito a elevadas taxas de transmissão, distância atingida, e obviamente o custo duma implantação prática. Neste trabalho de doutoramento, começamos por implementar experimentalmente um sistema de comunicações quânticas que usa variáveis discretas com codificação na polarização, o que nos permite identificar os problemas a serem resolvidos de forma a tornar possível a implementação prática de protocolos de distribuição de chave quântica. Desta forma, propomos um método heurístico não intrusivo para compensar automaticamente a deriva aleatória de polarização em canais padrão de fibra ótica devido a efeitos de birrefringência, e que induzem erros durante a transmissão de Qubits. A compensação da deriva de polarização induzida pelo canal quântico é fundamental para permitir a implementação prática generalizada da transmissão de fotões únicos com codificação na polarização nas redes atuais de fibra ótica. Neste trabalho de doutoramento propomos ainda e validamos através de simulações numéricas um novo sistema de DV-QKD baseado na polarização que combina o uso de moduladores de fase para gerar quatro estados de polarização e mudança de base com um esquema de deteção coerente. Este sistema permite a implementação de sistemas de DV-QKD usando unicamente equipamento clássico, o que garante um custo reduzido da implementação de sistemas Quantum Key Distribution (QKD) baseados em fotões únicos codificados na polarização e ao mesmo tempo um aumento da taxa de transmissão. Os nossos resultados abrem a porta a sistemas de transmissão de qubits a débitos elevados aquando da sua implementação nas redes instaladas de fibra ótica. Reportamos transmissões continuas de qubits mesmo em ambientes sujeitos a elevada deriva da polarização, sem a necessidade de consumir largura de banda extra com uma taxa de erro quântico máxima de 2%.Programa Doutoral em Engenharia Eletrotécnic

Roadmap on structured light

Structured light refers to the generation and application of custom light fields. As the tools and technology to create and detect structured light have evolved, steadily the applications have begun to emerge. This roadmap touches on the key fields within structured light from the perspective of experts in those areas, providing insight into the current state and the challenges their respective fields face. Collectively the roadmap outlines the venerable nature of structured light research and the exciting prospects for the future that are yet to be realized.Peer ReviewedPostprint (published version

Quantum optical memory protocols in atomic ensembles

We review a series of quantum memory protocols designed to store the quantum information carried by light into atomic ensembles. In particular, we show how a simple semiclassical formalism allows to gain insight into various memory protocols and to highlight strong analogies between them. These analogies naturally lead to a classification of light storage protocols into two categories, namely photon echo and slow-light memories. We focus on the storage and retrieval dynamics as a key step to map the optical information into the atomic excitation. We finally review various criteria adapted for both continuous variables and photon-counting measurement techniques to certify the quantum nature of these memory protocols

Roadmap on structured light

Structured light refers to the generation and application of custom light fields. As the tools and technology to create and detect structured light have evolved, steadily the applications have begun to emerge. This roadmap touches on the key fields within structured light from the perspective of experts in those areas, providing insight into the current state and the challenges their respective fields face. Collectively the roadmap outlines the venerable nature of structured light research and the exciting prospects for the future that are yet to be realized

Quantum information with Gaussian states

Quantum optical Gaussian states are a type of important robust quantum states which are manipulatable by the existing technologies. So far, most of the important quantum information experiments are done with such states, including bright Gaussian light and weak Gaussian light. Extending the existing results of quantum information with discrete quantum states to the case of continuous variable quantum states is an interesting theoretical job. The quantum Gaussian states play a central role in such a case. We review the properties and applications of Gaussian states in quantum information with emphasis on the fundamental concepts, the calculation techniques and the effects of imperfections of the real-life experimental setups. Topics here include the elementary properties of Gaussian states and relevant quantum information device, entanglement-based quantum tasks such as quantum teleportation, quantum cryptography with weak and strong Gaussian states and the quantum channel capacity, mathematical theory of quantum entanglement and state estimation for Gaussian states.Comment: 170 pages. Minors of the published version are corrected and listed in the Acknowledgement part of this versio

QKD and high-speed classical data hybrid metropolitan network

Quantum Key Distribution (QKD) is currently receiving much attention as it provides a secure source of encryption keys. Discrete-Variable QKD (DV-QKD) is possible for single photon transmission in QKD to-coexist with and encode classical wavelength division multiplexed (WDM) data with appropriate system design. Nevertheless, previous QKD field trials adopted either or both of key relay via trusted nodes and transparent link via optical switching. The former requires guaranteed physical security of the relay nodes, but can expand key distribution distance arbitrarily. The latter can realize key establishment for more users with less complexity of key management over an untrusted network. To realise the adaption of the QKD system for future high speed and long distance metropolitan world exploitation at lower cost, there has to be investigations on existing fibre infrastructures. Prior to this work, previous researches over similar distances feature extremely low secure key rates. For example, the Swiss Quantum Network between three sites displayed secure bit rates of 2.5 kbps at a fibre length of 17km. Quantum Key distribution within the 25km Cambridge Quantum Network have demonstrated the highest long-term secure key rates yet demonstrated in a field trial of at least 2.5Mb/s which is the fastest and much higher than 0.8 kbps which was reached over the similar channel loss field trial up to date. Additional field trials have been performed on the UK Quantum Network using a 66km path having 16dB loss. Combined wavelength division multiplexed 2 x 100 Gb/s traffic encrypted using QKD co-existing on the same fibres has operated for several months, with a long-term key rate of 80kb/s that is also faster than any other similar long-term QKD trial systems. In addition to this advanced commercial QKD system, there have been secure key rate analysis comparisons between laboratory fibre coils and practical field trials more than field trials only conducted before.These comparisons help to identify factors that limit future QKD network scale in both quantity and quality aspects. Also, the limit for the highest secure key rate at longest fibre length QKD in the multiplexing environment is discussed and determined in this research thesis. Nevertheless, in this thesis, improvements have been made to minimise the corresponding negative effects by investigations on the dependence of temperature have been done in order to ensure system operation environment effects. It was found from the trial results that there exists a relationship between temperature and secure key rate and further study has been done to evaluate the system sensitivity to operating temperature. Although the conventional DV-QKD system, original BB84 coding scheme, was designed to exploit the quantum properties of single photon polarization states, the trial equipment operates based upon the phase coding schemes. These coding schemes are based on the properties of interferometers and the coding is implemented by changing the relative optical path lengths or phase between the internal arms of the interferometer, while in the real transmission environment, temperature or polarization variation happens unpredictably. The existing polarisation controllers operate at relative low speed align within the interferometer, which slows to operation environment such as a punch to fibre causing phase difference. Therefore, in this project, there has been an improvement in the QKD-WDM system performance by adding an external polarization controller to minimize the Raman noise and increase the secure key rate at the longest fibre length up to date. In Summary, transmitting quantum keys over a coil of fibre in the lab differs a lot from actually putting it in the ground. This work contrasts the world fastest QKD system at the longest distance in field trials with lab fibre reels and then characterises and identifies two of the key factors, temperature and polarizations, influencing performance in practical wavelength-multiplexed secure communication systems. This is a significant step towards the coexistence of the quantum and conventional data channels on the same fibre for metropolitan networks and paves a way for an information-secure communication infrastructure

Matter manipulation with extreme terahertz light: Progress in the enabling THz technology

Terahertz (THz) light has proven to be a fine tool to probe and control quasi-particles and collective excitations in solids, to drive phase transitions and associated changes in material properties, and to study rotations and vibrations in molecular systems. In contrast to visible light, which usually carries excessive photon energy for collective excitations in condensed matter systems, THz light allows for direct coupling to low-energy (meV scale) excitations of interest, The development of light sources of strong-field few-cycle THz pulses in the 2000s opened the door to controlled manipulation of reactions and processes. Such THz pulses can drive new dynamic states of matter, in which materials exhibit properties entirely different from that of the equilibrium. In this review, we first systematically analyze known studies on matter manipulation with strong-field few-cycle THz light and outline some anticipated new results. We focus on how properties of materials can be manipulated by driving the dynamics of different excitations and how molecules and particles can be controlled in useful ways by extreme THz light. Around 200 studies are examined, most of which were done during the last five years. Secondly, we discuss available and proposed sources of strong-field few-cycle THz pulses and their state-of-the-art operation parameters. Finally, we review current approaches to guiding, focusing, reshaping and diagnostics of THz pulses. (C) 2019 The Author(s). Published by Elsevier B.V

Roadmap of optical communications

© 2016 IOP Publishing Ltd. Lightwave communications is a necessity for the information age. Optical links provide enormous bandwidth, and the optical fiber is the only medium that can meet the modern society's needs for transporting massive amounts of data over long distances. Applications range from global high-capacity networks, which constitute the backbone of the internet, to the massively parallel interconnects that provide data connectivity inside datacenters and supercomputers. Optical communications is a diverse and rapidly changing field, where experts in photonics, communications, electronics, and signal processing work side by side to meet the ever-increasing demands for higher capacity, lower cost, and lower energy consumption, while adapting the system design to novel services and technologies. Due to the interdisciplinary nature of this rich research field, Journal of Optics has invited 16 researchers, each a world-leading expert in their respective subfields, to contribute a section to this invited review article, summarizing their views on state-of-the-art and future developments in optical communications

Plasmonics and its Applications

Plasmonics is a rapidly developing field that combines fundamental research and applications ranging from areas such as physics to engineering, chemistry, biology, medicine, food sciences, and the environmental sciences. Plasmonics appeared in the 1950s with the discovery of surface plasmon polaritons. Plasmonics then went through a novel propulsion in the mid-1970s, when surface-enhanced Raman scattering was discovered. Nevertheless, it is in this last decade that a very significant explosion of plasmonics and its applications has occurred. Thus, this book provides a snapshot of the current advances in these various areas of plasmonics and its applications, such as engineering, sensing, surface-enhanced fluorescence, catalysis, and photovoltaic devices