Recently, industrial trends strongly favor the concepts of high density, low power consumption and low cost applications of Datacom and Telecom pluggable transceiver modules. Hence, thermal management plays an important role, especially in the design of high-performance compact optical transceivers. Extensive care should be taken on wavelength drift for thermal tuning lasers using thermoelectric cooler and indeed, accurate expression is needed to describe transient characteristics of the Peltier device to achieve maximum controllability. In this study, the exact solution of governing equation is presented, considering Joule heating, heat conduction, heat flux of laser diode and thermoelectric effect in one dimension.
In classical cryptography, the bit commitment scheme is one of the most important primitives. We review the state of the art of bit commitment protocols, emphasizing its main achievements and applications. Next, we present a practical quantum bit commitment scheme, whose security relies on current technological limitations, such as the lack of long-term stable quantum memories. We demonstrate the feasibility of our practical quantum bit commitment protocol and that it can be securely implemented with nowadays technology.
We show how to generate, encode, transmit and detect single photons. By using single photons we can address two of the more challenging problems that communication engineers face nowadays: capacity and security. Indeed, by decreasing the number of photons used to encode each bit, we can efficiently explore the full capacity to carry information of optical fibers, and we can guarantee privacy at the physical layer. We present results for single and entangled photon generation. We encode information in the photons polarization and after transmission we retrieve that information. We discuss the impact of fiber birefringence on the photons polarization.
We present a calculation of the required number of bits to be received in a system of communications in order to achieve a given level of confidence. The calculation assumes a binomial distribution function for the errors. The function is numerically evaluated and the results are compared with the ones obtained from Poissonian and Gaussian approximations. The performance in terms of the signal-to-noise ratio is also studied. We conclude that for higher number of errors in detection the use of approximations allows faster and more efficient calculations, without loss of accuracy.
The rapid increase on the information sharing around the world, leads to an utmost requirement for capacity and bandwidth. However, the need for security in the transmission and storage of information is also of major importance. The use of quantum technologies provides a practical solution for secure communications systems. Quantum key distribution (QKD) was the first practical application of quantum mechanics, and nowadays it is the most developed one. In order to share secret keys between two parties can be used several methods of encoding. Due to its simplicity, the encoding into polarization is one of the most used. However, when we use optical fibers as transmission channels, the polarization suffers random rotations that may change the state of polarization (SOP) of the light initially sent to the fiber to a new one at the output. Thus, in order to enable real-time communication using this encoding method it is required the use of a dynamic control system. We describe a scheme of transmission of quantum information, which is based in the polarization encoding, and that allows to share secret keys through optical fibers without interruption. The dynamic polarization control system used in such scheme is described, both theoretically and experimentally. Their advantages and limitations for the use in quantum communications are presented and discussed.
Quantum communications can provide almost perfect security through the use of quantum laws to detect any
possible leak of information. We discuss critical issues in the implementation of quantum communication systems
over installed optical fibers. We use stimulated four-wave mixing to generate single photons inside optical fibers,
and by tuning the separation between the pump and the signal we adjust the average number of photons per pulse.
We report measurements of the source statistics and show that it goes from a thermal to Poisson distribution with
the increase of the pump power. We generate entangled photons pairs through spontaneous four-wave mixing.
We report results for different type of fibers to approach the maximum value of the Bell inequality. We model
the impact of polarization rotation, attenuation and Raman scattering and present optimum configurations to
increase the degree of entanglement. We encode information in the photons polarization and assess the use
of wavelength and time division multiplexing based control systems to compensate for the random rotation of
the polarization during transmission. We show that time division multiplexing systems provide a more robust
solution considering the values of PMD of nowadays installed fibers. We evaluate the impact on the quantum
channel of co-propagating classical channels, and present guidelines for adding quantum channels to installed
WDM optical communication systems without strongly penalizing the performance of the quantum channel. We
discuss the process of retrieving information from the photons polarization. We identify the major impairments
that limit the speed and distance of the quantum channel. Finally, we model theoretically the QBER and present
results of an experimental performance assessment of the system quality through QBER measurements.
A single-photon source based on the stimulated four-wave mixing (SFWM) process in optical fibers is presented.
At the output of the source, the state of polarization (SOP) of the photons can be adjusted in order to obtain
any linear polarization. A theoretical model to describe the average photon counts recorded in the avalanche
photodiodes (APDs) is presented. The experimental results show an accurate detection of two non-orthogonal
linear SOPs after propagation through a 60 km quantum channel, and good agreement with theory. This source,
operating in a low power regime, can be used for quantum key distribution (QKD) using polarization-encoding
in quantum communications.
In this work we develop an analysis of polarization control schemes suitable for quantum key distribution systems.
Both time division multiplexing and wavelength division multiplexing based schemes are considered. A model
for the optimization of the temporal separation between reference pulses and polarization encoded photons
is presented. The model accounts for the reference pulse shape, the single photon detector gate width, and
the respective temporal separation between them. The theoretical results are validated through experimental
measurements. These results can be used to optimize the performance of polarization control schemes and
therefore to optimize the polarization encoded quantum key distribution systems.
Quantum laws can be used to implement secure communication channels; this has been named quantum cryptography.
In quantum cryptography the security does not depend of limited computational power, but is inherent
to the laws that govern the propagation and detection of single and entangled photons. We show how single
and entangled photon-pairs can be efficiently generated using four-wave mixing in optical fibers. We analyze the
source statistics, degree of entanglement and impact of spontaneous Raman scattering. By coding information
in the photons polarization we are able to transmit quantum information over 20 km of standard single mode