79 research outputs found
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
Transmission of optical communication signals through ring core fiber using perfect vortex beams
Orbital angular momentum can be used to implement high capacity data
transmission systems that can be applied for classical and quantum
communications. Here we experimentally study the generation and transmission
properties of the so-called perfect vortex beams and the Laguerre-Gaussian
beams in ring-core optical fibers. Our results show that when using a single
preparation stage, the perfect vortex beams present less ring-radius variation
that allows coupling of higher optical power into a ring core fiber. These
results lead to lower power requirements to establish fiber-based
communications links using orbital angular momentum and set the stage for
future implementations of high-dimensional quantum communication over space
division multiplexing fibers.Comment: 13 pages, 6 figure
The Security of Practical Quantum Key Distribution
Quantum key distribution (QKD) is the first quantum information task to reach
the level of mature technology, already fit for commercialization. It aims at
the creation of a secret key between authorized partners connected by a quantum
channel and a classical authenticated channel. The security of the key can in
principle be guaranteed without putting any restriction on the eavesdropper's
power.
The first two sections provide a concise up-to-date review of QKD, biased
toward the practical side. The rest of the paper presents the essential
theoretical tools that have been developed to assess the security of the main
experimental platforms (discrete variables, continuous variables and
distributed-phase-reference protocols).Comment: Identical to the published version, up to cosmetic editorial change
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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
Theory and Practice of Cryptography and Network Security Protocols and Technologies
In an age of explosive worldwide growth of electronic data storage and communications, effective protection of information has become a critical requirement. When used in coordination with other tools for ensuring information security, cryptography in all of its applications, including data confidentiality, data integrity, and user authentication, is a most powerful tool for protecting information. This book presents a collection of research work in the field of cryptography. It discusses some of the critical challenges that are being faced by the current computing world and also describes some mechanisms to defend against these challenges. It is a valuable source of knowledge for researchers, engineers, graduate and doctoral students working in the field of cryptography. It will also be useful for faculty members of graduate schools and universities
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