National Repository of Grey Literature 32 records found  1 - 10nextend  jump to record: Search took 0.00 seconds. 
Quantum key distribution over optical fibre infrastructure
Klíčník, Ondřej ; Horváth, Tomáš (referee) ; Münster, Petr (advisor)
The aim of this bachelor thesis is to create a comprehensive view of the current technology of quantum key distribution (QKD) over optical fiber, in theoretical terms, a completely secure key exchange. The thesis can be divided into theoretical and practical parts. The theoretical part illuminates the reasons for the use of these systems and the fundamentals of quantum mechanics needed to understand the function of individual QKD protocols. Furthermore, the principles of operation of both the protocols and related services such as post-quantum cryptography (PQC) and quantum number generation (QRNG) are described. The last chapter is devoted to the architecture of QKD networks and describes current standards for QKD communications. In the practical part, a detailed analysis of commercially available devices is performed. Subsequently, simulation results of selected QKD protocols are presented and a custom QKD polygon is designed, built and tested.
Quantum key distribution
Klíčník, Ondřej ; Burda, Karel (referee) ; Münster, Petr (advisor)
This thesis is indirectly related to the bachelor thesis Quantum key distribution over optical fiber infrastructure. Unlike the previous paper, the focus will be mainly on the practical application of the QKD (Quantum key distribution) system Clavis3. For this reason, physical phenomena related to practically used QKD protocols are briefly explained in the theoretical part. These are mostly based on phase coding. In particular, special attention is paid to the Coherent one-way protocol (COW) implemented in Clavis3 devices. This protocol is also compared with practical implementations of the BB84 protocol. Furthermore, the principles of other advanced QKD techniques are outlined and the phenomena in the optical fiber that may affect the quantum channel are discussed. A separate chapter is also devoted to standardization and topologies of QKD networks. Last but not least, the thesis addresses the topic of attacks against practical implementations of QKD protocols. In the practical part, measurements aimed at practical deployment of Clavis3 devices in a common communication network are performed. These include the possibility of combining a quantum channel into a single fiber together with classical channels using Wavelength-division multiplexing (WDM) and the analysis of the effect of Raman noise on the maximum communication distance. At the same time, the robustness of the system against polarization changes and fiber manipulation is verified. Finally, the performance of the system using three-state and four-state versions of the COW protocol is compared and the eavesdropping simulation module is tested.
The role of fermions and bosons in magnetic resonance imaging
ADAMEC, Filip
Abstract Currently, there are only partial theories describing the role and application of fermions and bosons in magnetic resonance imaging, but there are no interdisciplinary theories. I believe, that by combining diverse disciplines (physics, mathematics and the medical nature of MR), an interdisciplinary theory may emerge, that can also be used as a material to deepen the knowledge of future or already active radiologists in radiological physics and magnetic resonance imaging. The following objectives have been set for this Bachelor thesis: Global target: Creation of an expert description of the role of fermions and bosons in magnetic resonance imaging using interdisciplinary communication. Sub-objectives: C1: Data processing - physics of elementary particles C2: Data processing - classical form of electromagnetic field theory C3: Data processing - structure of magnetic resonance imaging C4: Data processing - quantum mechanics of protons Due to the goals of this bachelor thesis, the following hypotheses were made: H1: The classical dimension of the physical nature of MR can be created by comparison of the structure of magnetic resonance and the classical form of electromagnetic field H2: The quantum dimension of the physical nature of MR can be created by comparison of the structure of magnetic resonance and the quantum mechanics of protons H3: The role of fermions and bosons in the description of the physical nature of magnetic resonance can be described by the comparative structure of magnetic resonance and the quantum dimension of the physical substance of magnetic with the standard model of elementary particles and their interactions In the introduction to the theoretical part of the work, I dealt with elementary particles, their physics and mutual interactions. Furthermore, these particles were divided into fermions and bosons, which were described in more detail. I devoted another part of the theory to the classical form of the electromagnetic field, where I discussed mainly Maxwell's equations. The next part was devoted to quantum mechanics of particles and at the end of the theoretical part I focused on the medical part of magnetic resonance. The practical part compares the knowledge gained in the theoretical part of this work. The results were divided into two sections, according to established hypotheses. The results of this bachelor thesis point to the relatively crucial role of fermions and bosons in magnetic resonance imaging, thus confirming all three established hypotheses. The results of this bachelor thesis point out the relatively crucial role of fermions (electrons, protons) and bosons (photons, Cooper pairs) based on verified and accepted hypotheses.
Studies of Lanthanide Complexes by a Combination of Spectroscopic Methods
Krupová, Monika ; Bouř, Petr (advisor) ; Kapitán, Josef (referee)
Studies of Lanthanide Complexes by a Combination of Spectroscopic Methods Monika Krupová (Department of Physical and Macromoecular Chemistry, Faculty of Science, Charles University in Prague) Since conventional structural analysis offers rather limited means for the chirality detection, a series of lanthanide tris-(β-diketonates) are investigated as effective receptors for a better chirality sensing in biomolecular substrates. These lanthanide complexes containing β-diketonate ligands are electrically neutral; they can further coordinate with various small organic molecules such as chiral alcohols, amino alcohols or amino acids in organic solvents and produce a strong chiral signal. Previously, a resonance in Raman scattering was observed in the studied systems due to the correspondence of europium electronic transition energy to the laser excitation wavelength, about a 100-fold signal enhancement if compared to non-resonant vibrational ROA was observed. This enabled shorter detection times as well as lower sample concentrations. In the current work, interaction of the Eu(FOD) complex with (R)- and (S)- enantiomer of 1-phenylethanol in n-hexane was studied using IR spectroscopy, Raman spectroscopy and Raman optical activity (ROA), UV-Vis spectroscopy and ultraviolet circular dichroism (UVCD). Only...
Two new uncertainty relations and their applications
Lužová, Martina ; Skála, Lubomír (advisor) ; Kapsa, Vojtěch (referee)
The historical development of uncertainty relation, begining with first Heisenberg's thoughts of uncertainty principle is summed up in this thesis. After proving validity of Schwarz inequality general uncertainty relation for two hermitian operators is obtained, and from this general version the validity of Heisenberg uncertainty relation is than proved. The most important part of this work is the obtention of two new uncertainty relations, which are stronger then Heisenberg or Robertson-Schrödinger uncertainty relation, and their specific form for two examples - a free particle in a state discribed by the gaussian wave packet and the linear harmonic oscillator with a wave function in the shape of gaussian packet.
Quantum key distribution over optical fibre infrastructure
Klíčník, Ondřej ; Horváth, Tomáš (referee) ; Münster, Petr (advisor)
The aim of this bachelor thesis is to create a comprehensive view of the current technology of quantum key distribution (QKD) over optical fiber, in theoretical terms, a completely secure key exchange. The thesis can be divided into theoretical and practical parts. The theoretical part illuminates the reasons for the use of these systems and the fundamentals of quantum mechanics needed to understand the function of individual QKD protocols. Furthermore, the principles of operation of both the protocols and related services such as post-quantum cryptography (PQC) and quantum number generation (QRNG) are described. The last chapter is devoted to the architecture of QKD networks and describes current standards for QKD communications. In the practical part, a detailed analysis of commercially available devices is performed. Subsequently, simulation results of selected QKD protocols are presented and a custom QKD polygon is designed, built and tested.
Excitation Energy Transfer in Photosynthetic Reaction Centres
Ptáček, Michal ; Mančal, Tomáš (advisor) ; Dostál, Jakub (referee)
The photosynthetic reaction centres have uppermost importance in photosynthesis. They represent the actual place where the energy carried by photons is turned into charge-separated states which then enable to establish the electrochemical H+ transmembrane gradient used by ATP synthases. The photosynthetic light- harvesting complexes gather the energy of light radiation and direct it in the form of electronic excitation energy into the reaction centres. The efficiency of this process is exceptionally high, close to unity, what is capturing the interest of researchers for decades. The development of experimental techniques has led to better understanding of this process down to atomic scale. Nowadays, this insight along with the theoretical basis stemming from quantum mechanics can be used to perform accurate computer simulations which can determine properties of the whole molecular aggregates independently of experiments. This thesis provides an introduction into the field of theoretical photosynthesis research, and it summarises the progress made in past two decades. The detailed theoretical approaches are being put into perspective of the reaction centres of photosynthetic purple bacterium Rhodobacter sphaeroides which is a valuable model organism. Both experimental and theoretical results of...
Quantum mechanical study of the electron hoping processes of conjugated systems.
Fatková, Kateřina ; Pospíšil, Miroslav (advisor) ; Pittner, Jiří (referee)
This thesis uses previously proposed methodology for simulations of all-trans- polyenes with conjugated systems. Dynamic properties, especially the mean lifeti- mes of the excited states, of these molecules were systematically simulated. Obta- ined data shows that the method is still too time-consuming for polyene molecules with more than 20 carbon atoms, including most carotenoids. Thus, a study of active space reduction was performed with the model tetradecaheptaene molecule with regards to excited state mean lifetimes. A new, less time-consuming method would need further simulation studies. Moreover, static spectra of the these mo- lecules were studied as well, yielding a comparison of different DFT and ab-initio approaches. 1
Electron-phonon Coupling in Finite Multi-chromophoric Systems
Herman, Daniel ; Mančal, Tomáš (advisor) ; Profant, Václav (referee)
Quantum systems in nature interact with other quantum systems, and these are examples of open quantum systems. In this work, we provide an introduction to the theory of open quantum system with a particular focus on the dynamics of molecular systems embedded in the protein environment, such as those found in photosynthetic antennas. We devote some time to the techniques of constructing equations of motion for the dynamics of a selected quantum system under the interaction with the bath, where we restrict ourselves to a finite number of degrees of freedom. We compare the exact calculation of the whole finite system with the results of approximate equations derived from an ansatz for the time evolution for the degrees of freedom of the bath part. We also reformulate the exact equations into a time non-local master equation using projection operator techniques, and we study the quality of results obtained with the modified quantum master equation. The time evolution of studied systems is also compared to the time evolution obtained by Schrödiger and Liouville-von Neumann equations. 1
Short-lived Delocalization and Absorption by Light
Vokrouhlický, David ; Mančal, Tomáš (advisor) ; Veis, Libor (referee)
Coherent exciton delocalization improves the light harvesting function of photosyn- thetic antennae by creating conditions for very fast excitation transfer in space. This thesis focuses on two different effects creating coherence - short-lived excitation by light and weak coupling between pigments that is present in the system on longer timescales. The evolution and relaxation of simple systems - the dimer and trimer - are calculated. The core of this thesis are newly developed numerical methods for distinguishing and quantifying the effect of the two types of coherence throughout evolution, which are applied to the aforementioned systems. 1

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