Dissertations / Theses on the topic 'Quantum optimization'
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Arvidsson, Elisabeth. "Optimization algorithms for Quantum Annealing." Thesis, KTH, Fysik, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-279447.
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Pye, Cory C. "Applications of optimization to quantum chemistry." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/nq23109.pdf.
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Gosset, David (David Nicholas). "Case studies in quantum adiabatic optimization." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/68872.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 139-143).
Quantum adiabatic optimization is a quantum algorithm for solving classical optimization problems (E. Farhi, J. Goldstone, S. Gutmann, and M. Sipser. Quantum computation by adiabatic evolution, 2000. arXiv:quant-ph/0001106). The solution to an optimization problem is encoded in the ground state of a "problem Hamiltonian" Hp which acts on the Hilbert space of n spin 1/2 particles and is diagonal in the Pauli z basis. To produce this ground state, one first initializes the quantum system in the ground state of a different Hamiltonian and then adiabatically changes the Hamiltonian into Hp. Farhi et al suggest the interpolating Hamiltonian [mathematical formula] ... where the parameter s is slowly changed as a function of time between 0 and 1. The running time of this algorithm is related to the minimum spectral gap of H(s) for s E (0, 11. We study such transverse field spin Hamiltonians using both analytic and numerical techniques. Our approach is example-based, that is, we study some specific choices for the problem Hamiltonian Hp which illustrate the breadth of phenomena which can occur. We present I A random ensemble of 3SAT instances which this algorithm does not solve efficiently. For these instances H(s) has a small eigenvalue gap at a value s* which approaches 1 as n - oc. II Theorems concerning the interpolating Hamiltonian when Hp is "scrambled" by conjugating with a random permutation matrix. III Results pertaining to phase transitions that occur as a function of the transverse field. IV A new quantum monte carlo method which can be used to compute ground state properties of such quantum systems. We discuss the implications of our results for the performance of quantum adiabatic optimization algorithms.
by David Gosset.
Ph.D.
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Nuñez, Lobato Carlos. "Optimization of MOVPE-grown Quantum Dots for long distance quantum communication." Thesis, KTH, Tillämpad fysik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-258829.
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Stojnic, Mihailo Hassibi Babak Hassibi Babak. "Optimization algorithms in wireless and quantum communications /." Diss., Pasadena, Calif. : Caltech, 2008. http://resolver.caltech.edu/CaltechETD:etd-12032007-113628.
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Bai, Jing. "Optimization of Optical Nonlinearities in Quantum Cascade Lasers." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/19797.
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Nonlinearities in quantum cascade lasers (QCL¡¯s) have wide applications in wavelength tunability and ultra-short pulse generation. In this thesis, optical nonlinearities in InGaAs/AlInAs-based mid-infrared (MIR) QCL¡¯s with quadruple resonant levels are investigated. Design optimization for the second-harmonic generation (SHG) of the device is presented. Performance characteristics associated with the third-order nonlinearities are also analyzed.
The design optimization for SHG efficiency is obtained utilizing techniques from supersymmetric quantum mechanics (SUSYQM) with both material-dependent effective mass and band nonparabolicity. Current flow and power output of the structure are analyzed by self-consistently solving rate equations for the carriers and photons. Nonunity pumping efficiency from one period of the QCL to the next is taken into account by including all relevant electron-electron (e-e) and longitudinal (LO) phonon scattering mechanisms between the injector/collector and active regions. Two-photon absorption processes are analyzed for the resonant cascading triple levels designed for enhancing SHG. Both sequential and simultaneous two-photon absorption processes are included in the rate-equation model. The current output characteristics for both the original and optimized structures are analyzed and compared. Stronger resonant tunneling in the optimized structure is manifested by enhanced negative differential resistance. Current-dependent linear optical output power is derived based on the steady-state photon populations in the active region. The second-harmonic (SH) power is derived from the Maxwell equations with the phase mismatch included. Due to stronger coupling between lasing levels, the optimized structure has both higher linear and nonlinear output powers. Phase mismatch effects are significant for both structures leading to a substantial reduction of the linear-to-nonlinear conversion efficiency. The optimized structure can be fabricated through digitally grading the submonolayer alloys by molecular beam epitaxy (MBE).
In addition to the second-order nonlinearity, performance characteristics brought by the third-order nonlinearities are also discussed, which include third-harmonic generation (THG) and intensity dependent (Kerr) refractive index. Linear to third-harmonic (TH) conversion efficiency is evaluated based on the phase-mismatched condition. The enhanced self-mode-locking (SML) effect over a typical three-level laser is predicted, which will stimulate further investigations of pulse duration shortening by structures with multiple harmonic levels.
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Alanis, Dimitrios. "Quantum-assisted multi-objective optimization of heterogeneous networks." Thesis, University of Southampton, 2017. https://eprints.soton.ac.uk/419588/.
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Some of the Heterogeneous Network (HetNet) components may act autonomously for the sake of achieving the best possible performance. The attainable routing performance depends on a delicate balance of diverse and often conflicting Quality-of-Service (QoS)requirements. Finding the optimal solution typically becomes an NP-hard problem, as the network size increases in terms of the number of nodes. Moreover, the employment of user defined utility functions for the aggregation of the different objective functions often leads to suboptimal solutions. On the other hand, Pareto Optimality is capable of amalgamating the different design objectives by relying on an element of elitism. Although there is a plethora of bio-inspired algorithms that attempt to address the associated multi-component optimization problem, they often fail to generate all the routes constituting the Optimal Pareto Front (OPF). As a remedy, we initially propose an optimal multi-objective quantum-assisted algorithm, namely the Non-dominated Quantum Optimization (NDQO) algorithm, which evaluates the legitimate routes using the concept of Pareto Optimality at a reduced complexity. We then compare the performance of the NDQO algorithm to the state-of-the-art evolutionary algorithms, demonstrating that the NDQO algorithm achieves a near-optimal performance. Furthermore, we analytically derive the upper and lower bounds of the NDQO’s algorithmic complexity, which is of the order of O(N) and O(N√N) in the best- and worst-case scenario, respectively. This corresponds to a substantial complexity reduction of the NDQO from the order of O(N2)imposed by the brute-force (BF) method. However again, as the number of nodes increases, the total number of routes increases exponentially, making its employment infeasible despite the complexity reduction offered. Therefore, we propose a novel optimal quantum-assisted algorithm, namely the Non-Dominated Quantum Iterative Optimization (NDQIO) algorithm, which exploits the synergy between the hardware parallelism and the quantum parallelism for the sake of achieving a further complexity reduction, which is on the order of O(√N) and O(N√N)in the best- and worst-case scenarios, respectively. Additionally, we provide simulation results for demonstrating that our NDQIO algorithm achieves an average complexity reduction of almost an order of magnitude compared to the near-optimal NDQO algorithm,while activating the same order of comparison operators. Apart from the traditional QoS requirements, the network design also has to consider the nodes’ user-centric social behavior. Hence, the employment of socially-aware load balancing becomes imperative for avoiding the potential formation of bottlenecks in the network’s packet-flow. Therefore, we also propose a novel algorithm, referred to as the Multi-Objective Decomposition Quantum Optimization (MODQO) algorithm, which exploits the quantum parallelism to its full potential by exploiting the database correlations for performing multi-objective routing optimization, while at the same time balancing the tele-traffic load among the nodes without imposing a substantial degradation on the network’s delay and power consumption. Furthermore, we introduce a novel socially-aware load balancing metric, namely the normalized entropy of the normalized composite betweenness of the associated socially-aware network, for striking a better trade-off between the network’s delay and power consumption. We analytically prove that the MODQO algorithm achieves the full-search based accuracy at a significantly reduced complexity, which is several orders of magnitude lower than that of the full-search. Finally, we compare the MODQO algorithm to the classic NSGA-II evolutionary algorithm and demonstrate that the MODQO succeeds in halving the network’s average delay, whilst simultaneously reducing the network’s average power consumption by 6 dB without increasing the computational complexity.
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Titiloye, Olawale. "Optimization by quantum annealing for the graph colouring problem." Thesis, Manchester Metropolitan University, 2013. http://e-space.mmu.ac.uk/324247/.
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Quantum annealing is the quantum equivalent of the well known classical simulated annealing algorithm for combinatorial optimization problems. Despite the appeal of the approach, quantum annealing algorithms competitive with the state of the art for specific problems hardly exist in the literature. Graph colouring is a difficult problem of practical significance that can be formulated as combinatorial optimization. By introducing a symmetry-breaking problem representation, and finding fast incremental techniques to calculate energy changes, a competitive graph colouring algorithm based on quantum annealing is derived. This algorithm is further enhanced by tuning simplification techniques; replica spacing techniques to increase robustness; and a messaging protocol, which enables quantum annealing to efficiently take advantage of multiprocessor environments. Additionally, observations of some patterns in the tuning for random graphs led to a more effective algorithm able to find new upper bounds for several widely-used benchmark graphs, some of which had resisted improvement in the last two decades.
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Khalus, Vladislav Ivanovich. "T-COUNT OPTIMIZATION OF QUANTUM CARRY LOOK-AHEAD ADDER." UKnowledge, 2019. https://uknowledge.uky.edu/ece_etds/141.
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With the emergence of quantum physics and computer science in the 20th century, a new era was born which can solve very difficult problems in a much faster rate or problems that classical computing just can't solve. In the 21st century, quantum computing needs to be used to solve tough problems in engineering, business, medical, and other fields that required results not today but yesterday. To make this dream come true, engineers in the semiconductor industry need to make the quantum circuits a reality.
To realize quantum circuits and make them scalable, they need to be fault tolerant, therefore Clifford+T gates need to be implemented into those circuits. But the main issue is that in the Clifford+T gate set, T gates are expensive to implement.
Carry Look-Ahead addition circuits have caught the interest of researchers because the number of gate layers encountered by a given qubit in the circuit (or the circuit's depth) is logarithmic in terms of the input size n. Therefore, this thesis focuses on optimizing previous designs of out-of-place and in-place Carry Look-Ahead Adders to decrease the T-count, sum of all T and T Hermitian transpose gates in a quantum circuit.
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Rava, Andrea Basilio. "Quantum approximate optimization algorithm: combinatorial problems and classical statistical models." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2021. http://amslaurea.unibo.it/23113/.
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The Quantum Approximate Optimization Algorithm (QAOA) is a hybrid quantum-classical algorithm for solving combinatorial optimization problems. Since most of combinatorial optimization problems may be thought as particular instances of Ising Hamiltonians, the study of the QAOA is very relevant from the physical point of view for its potential applications in describing physical systems. In the QAOA a quantum state is prepared and, through 2p parameterized quantum evolutions, a final state which represents an extreme of cost function and encodes the approximate solution of the problem is obtained. The 2p parameters are determined through a classical parameter optimization process. In this work we apply QAOA to two different problems, the Max Cut and the random bond Ising Model (RBIM). For both problems we perform an analysis of the optimization efficiency, verifying that the quality of the approximation increases with p. For the Max Cut we perform a further analysis of the p=1 case for which we obtain an analytical expression for the cost function and make observations regarding the choice of the initial parameters in the optimization procedure. For the RBIM, for different disordered configurations we obtain the ground states energies and magnetizations for different lattice sizes and different level p of the optimisation. We observe that, even if the magnetisation is obtained for small lattice sizes, its behaviour suggests the presence of a transition separating a ferromagnetic from a paramagnetic phase.
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Al, Baity Heyam. "A quantum behaved particle swarm approach to multi-objective optimization." Thesis, University of Birmingham, 2015. http://etheses.bham.ac.uk//id/eprint/5857/.
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Many real-world optimization problems have multiple objectives that have to be optimized simultaneously. Although a great deal of effort has been devoted to solve multi-objective optimization problems, the problem is still open and the related issues still attract significant research efforts. Quantum-behaved Particle Swarm Optimization (QPSO) is a recently proposed population based metaheuristic that relies on quantum mechanics principles. Since its inception, much effort has been devoted to develop improved versions of QPSO designed for single objective optimization. However, many of its advantages are not yet available for multi-objective optimization. In this thesis, we develop a new framework for multi-objective problems using QPSO. The contribution of the work is threefold. First a hybrid leader selection method has been developed to compute the attractor of a given particle. Second, an archiving strategy has been proposed to control the growth of the archive size. Third, the developed framework has been further extended to handle constrained optimization problems. A comprehensive investigation of the developed framework has been carried out under different selection, archiving and constraint handling strategies. The developed framework is found to be a competitive technique to tackle this type of problems when compared against the state-of-the-art methods in multi-objective optimization.
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Al-Ta'ani, Ola. "Quantum circuit synthesis using Solovay-Kitaev algorithm and optimization techniques." Diss., Kansas State University, 2015. http://hdl.handle.net/2097/19190.
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Doctor of PhilosophyElectrical and Computer Engineering
Sanjoy Das
Quantum circuit synthesis is one of the major areas of current research in the field of quantum computing. Analogous to its Boolean counterpart, the task involves constructing arbitrary quantum gates using only those available within a small set of universal gates that can be realized physically. However, unlike the latter, there are an infinite number of single qubit quantum gates, all of which constitute the special unitary group SU(2). Realizing any given single qubit gate using a given universal gate family is a complex task. Although gates can be synthesized to arbitrary degree of precision as long as the set of finite strings of the gate family is a dense subset of SU(2), it is desirable to accomplish the highest level of precision using only the minimum number of universal gates within the string approximation. Almost all algorithms that have been proposed for this purpose are based on the Solovay-Kitaev algorithm. The crux of the Solovay-Kitaev algorithm is the use of a procedure to decompose a given quantum gate into a pair of group commutators with the pair being synthesized separately. The Solovay-Kitaev algorithm involves group commutator decomposition in a recursive manner, with a direct approximation of a gate into a string of universal gates being performed only at the last level, i.e. in the leaf nodes of the search tree representing the execution of the Solovay-Kitaev algorithm. The main contribution of this research is in integrating conventional optimization procedures within the Solovay-Kitaev algorithm. Two specific directions of research have been studied. Firstly, optimization is incorporated within the group commutator decomposition, so that a more optimal pair of group commutators are obtained. As the degree of precision of the synthesized gate is explicitly minimized by means of this optimization procedure, the enhanced algorithm allows for more accurate quantum gates to be synthesized than what the original Solovay-Kitaev algorithm achieves. Simulation results with random gates indicate that the obtained accuracy is an order of magnitude better than before. Two versions of the new algorithm are examined, with the optimization in the first version being invoked only at the bottom level of Solovay-Kitaev algorithm and when carried out across all levels of the search tree in the next. Extensive simulations show that the second version yields better results despite equivalent computation times. Theoretical analysis of the proposed algorithm is able to provide a more formal, quantitative explanation underlying the experimentally observed phenomena. The other direction of investigation of this research involves formulating the group commutator decomposition in the form of bi-criteria optimization. This phase of research relaxed the equality constraint in the previous approach and with relaxation, a bi-criteria optimization is proposed. This optimization algorithm is new and has been devised primarily when the objective needs to be relaxed in different stages. This bi-criteria approach is able to provide comparably accurate synthesis as the previous approach.
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Kim, Dong Kwon. "Optical properties of asymmetric double quantum wells and optimization for optical modulators." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/22649.
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Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008.Committee Chair: Citrin, David; Committee Member: Dupuis, Russell; Committee Member: Gaylord, Thomas; Committee Member: Rhodes, William; Committee Member: Zhang, Zhuomin.
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Semerjian, Guilhem. "Mean-field disordered systems : glasses and optimization problems, classical and quantum." Habilitation à diriger des recherches, Ecole Normale Supérieure de Paris - ENS Paris, 2013. http://tel.archives-ouvertes.fr/tel-00785924.
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Ce mémoire présente mes activités de recherche dans le domaine de la mécanique statistique des systèmes désordonnés, en particulier sur les modèles de champ moyen à connectivité finie. Ces modèles présentent de nombreuses transitions de phase dans la limite thermodynamique, avec des applications tant pour la physique des verres que pour leurs liens avec des problèmes d'optimisation de l'informatique théorique. Leur comportement sous l'effet de fluctuations quantiques est aussi discuté, en lien avec des perspectives de calcul quantique.
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Hadavi, Hamid. "Isometry Registration Among Deformable Objects, A Quantum Optimization with Genetic Operator." Thèse, Université d'Ottawa / University of Ottawa, 2013. http://hdl.handle.net/10393/24286.
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Non-rigid shapes are generally known as objects whose three dimensional geometry may deform by internal and/or external forces. Deformable shapes are all around us, ranging from protein molecules, to natural objects such as the trees in the forest or the fruits in our gardens, and even human bodies. Two deformable shapes may be related by isometry, which means their intrinsic geometries are preserved, even though their extrinsic geometries are dissimilar. An important problem in the analysis of the deformable shapes is to identify the three-dimensional correspondence between two isometric shapes, given that the two shapes may be deviated from isometry by intrinsic distortions. A major challenge is that non-rigid shapes have large degrees of freedom on how to deform. Nevertheless, irrespective of how they are deformed, they may be aligned such that the geodesic distance between two arbitrary points on two shapes are nearly equal. Such alignment may be expressed by a permutation matrix (a matrix with binary entries) that corresponds to every paired geodesic distance in between the two shapes. The alignment involves searching the space over all possible mappings (that is all the permutations) to locate the one that minimizes the amount of deviation from isometry. A brute-force search to locate the correspondence is not computationally feasible. This thesis introduces a novel approach created to locate such correspondences, in spite of the large solution space that encompasses all possible mappings and the presence of intrinsic distortion.
In order to find correspondences between two shapes, the first step is to create a suitable descriptor to accurately describe the deformable shapes. To this end, we developed deformation-invariant metric descriptors. A descriptor constitutes pair-wise geodesic distances among arbitrary number of discrete points that represent the topology of the non-rigid shape. Our descriptor provides isometric-invariant representation of the shape irrespective of its circumstantial deformation. Two isometric-invariant descriptors, representing two candidate deformable shapes, are the input parameters to our optimization algorithm. We then proceed to locate the permutation matrix that aligns the two descriptors, that minimizes the deviation from isometry.
Once we have developed such a descriptor, we turn our attention to finding correspondences between non deformable shapes. In this study, we investigate the use of both classical and quantum particle swarm optimization (PSO) algorithms for this task. To explore the merits of variants of PSO, integer optimization involving test functions with large dimensions were performed, and the results and the analysis suggest that quantum PSO is more effective optimization method than its classical PSO counterpart. Further, a scheme is proposed to structure the solution space, composed of permutation matrices, in lexicographic ordering. The search in the solution space is accordingly simplified to integer optimization to find the integer rank of the targeted permutation matrix. Empirical results suggest that this scheme improves the scalability of quantum PSO across large solution spaces. Yet, quantum PSO's global search capability requires assistance in order to more effectively manoeuvre through the local extrema prevalent in the large solution spaces. A mutation based genetic algorithm (GA) is employed to augment the search diversity of quantum PSO when/if the swarm stagnates among the local extrema. The mutation based GA instantly disengages the optimization engine from the local extrema in order to reorient the optimization energy to the trajectories that steer to the global extrema, or the targeted permutation matrix.
Our resultant optimization algorithm combines quantum Particle Swarm Optimization (PSO) and mutation based Genetic Algorithm (GA). Empirical results show that the optimization method presented is scalable and efficient on standard hardware across different solution space sizes. The performance of the optimization method, in simulations and on various near-isometric shapes, is discussed. In all cases investigated, the method could successfully identify the correspondence among the non-rigid deformable shapes that were related by isometry.
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Momeni, Pakdehi Davood [Verfasser]. "Optimization of epitaxial graphene growth for quantum metrology / Davood Momeni Pakdehi." Hannover : Gottfried Wilhelm Leibniz Universität Hannover, 2020. http://d-nb.info/1222160439/34.
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Song, Katherine Wei. "Fabrication and optimization of light emitting devices with core-shell quantum dots." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/82185.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 79-84).
Quantum dot light emitting devices (QD-LEDs) are promising options for the next generation of solid state lighting, color displays, and other optoelectronic applications. Overcoating quantum dots (QDs) -- semiconducting nanocrystals of CdSe, PbS, or another similar compound -- with a wide band-gap "shell" has recently been shown to significantly boost QD-LED performance and yield the most efficient accent QD-LEDs to date. This thesis studies fabrication techniques to make bright, efficient QD-LEDs with these "core-shell" QDs. The first part studies the electrophoretic deposition (EPD) of CdSe/ZnS QDs. QD-LEDs conventionally utilize a QD lm that is deposited via spin-casting, a reliable but highly unscalable technique for the deposition of thin, smooth films of QDs for QD-LED applications. Potential advantages of EPD include the ability for deposition onto a variety of substrate shapes and more energetically favorable QD packing. Devices made with EPD QD films exhibit peak efficiencies comparable to those of devices with a spun-cast QD layer and turn-on voltages surprisingly lower than the optical band-gap of the QDs. These results suggest that EPD is a viable alternative to spin-casting for the processing of QD-LEDs. The second part of this thesis explores the role of core-shell QDs in creating bright, efficient LEDs in the near-infrared ([lambda] >1 [mu]m) regime. Infrared QD-LEDs with record brightness and efficiencies are obtained by using QDs in which lead sulfide (PbS) cores are overcoated with a cadmium sulfide (CdS) shell. In situ photoluminescence quantum yield measurements confirm that the QD shell plays a significant role in shielding the emissive QD core from external quenching mechanisms. Finally, fabrication and material considerations for the non-QD layers in the modern QD-LED structure are also discussed. This thesis analyzes different film formation techniques for zinc oxide (ZnO), the electron transport layer in the QD-LEDs, and different materials and thicknesses for the organic hole transport layer.
by Katherine Wei Song.
S.M.
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Borneman, Troy W., David G. Cory, and Martin D. Hürlimann. "Signal optimization in inhomogeneous fields: application of quantum optimal control theory troy." Diffusion fundamentals 10 (2009) 12, S. 1-3, 2009. https://ul.qucosa.de/id/qucosa%3A14103.
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We demonstrate that pulses derived using Optimal Control Theory (OCT) techniques can be used to significantly enhance the robustness of the Carr-Purcell-Meiboom-Gill sequence (CPMG) [1,2] to inhomogeneities in the static BB0 field. By numerically inverting the Liouville - von Neumann equation, OCT pulses were derived that can be used directly in place of hard pulses in the CPMG sequence to greatly improve the bandwidth of refocusing. To retain the echo stability achieved by the Meiboom-Gill correction to the Carr-Purcell sequence, the refocusing pulses were designed to perform a unitary π-rotation as opposed to just a state inversion transfer. To illustrate this approach we present an example of optimized pulses that show an improved CPMG-like behavior with complete excitation and multiple refocusing over a bandwidth of +/- 2.6 γB1,max B with a pulse duration limited to 10 t180.
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Mori, Yuto. "Path optimization with neural network for sign problem in quantum field theories." Doctoral thesis, Kyoto University, 2021. http://hdl.handle.net/2433/263466.
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Bryngelsson, Erik. "Manufacturing optimization and film stability analysis of PbS quantum dot solar cells." Thesis, KTH, Skolan för kemi, bioteknologi och hälsa (CBH), 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-260053.
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Semiconductor colloidal quantum dots have an interesting potential to increase solar cell efficiency, with strong absorption in the infrared region and a tunable band gap. In this work an attempt was made to adopt a manufacturing process for PbS quantum dot solar cells, proven successful at Uppsala University. Two optimizations were investigated and the stability of the quantum dot films was analyzed with regards to three storage conditions, varying oxygen accessibility and light exposure, and measured with UV-Vis spectroscopy and X-ray photoelectron spectroscopy. Functioning solar cells were obtained but with lower performance than the results from Uppsala. Optimizations were partly successful with regards to improved spreading of the EDT solution on the PbS quantum dot film using ethanol and methanol as solvents. No improved cell performance was observed by applying both QD films inside argon atmosphere, as opposed to only the first one. Clear differences in oxidization of the films and loss of iodine ligand could be identified for the different storage conditions, with best stability exhibited by films stored under argon atmosphere.Kvantprickar av halvledande material har en intressant potential att förbättra solcellers verkningsgrad genom en stark absorption inom de infraröda spektrat och ett justerbart bandgap. I detta arbete gjordes ett försök att återskapa en tillverkningsprocess av kvantprickssolceller av PbS, som visat sig framgångsrik vid Uppsala universitet. Två optimeringar undersöktes och stabiliteten av kvantpricksfilmerna analyserades med avseende på tre förvaringsmiljöer med olika exponering för ljus och syre, och mättes med UV-visspektroskopi samt röntgenfotoelektronspektroskopi. Fullt fungerande solceller framställdes men med en lägre prestanda jämfört med resultaten i Uppsala. Optimeringarna var delvis lyckade gällande spridning av EDTlösningen på kvantpricksfilmen av PbS genom att använda etanol och metanol som lösningsmedel. Ingen förbättrad prestanda observerades hos cellerna genom att applicera båda kvantpricksfilmerna i argonatmosfär, jämfört med endast den första. Tydliga skillnader i oxidation för filmerna samt förluster av jodligand kunde identifieras för de olika förvaringsmiljöerna, med bäst stabilitet uppvisad av filmerna som förvarades i argonatmosfär.
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Maisel, Sascha B. [Verfasser]. "Characterization and optimization of multicomponent alloys based on quantum mechanics / Sascha B. Maisel." München : Verlag Dr. Hut, 2015. http://d-nb.info/1070123854/34.
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Hülsen, Michael [Verfasser]. "Optimization of Integral Transformations and Quantum Chemical Calculations on Organometallic Templates / Michael Hülsen." München : Verlag Dr. Hut, 2012. http://d-nb.info/1025821149/34.
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Brandeho, Mathieu. "New bounds for information complexity and quantum query complexity via convex optimization tools." Doctoral thesis, Universite Libre de Bruxelles, 2018. https://dipot.ulb.ac.be/dspace/bitstream/2013/277139/4/Main.pdf.
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Cette thèse rassemble trois travaux sur la complexité d'information et sur la complexité en requête quantique. Ces domaines d'études ont pour points communs les outils mathématiques pour étudier ces complexités, c'est-à-dire les problèmes d'optimisation.Les deux premiers travaux concernent le domaine de la complexité en requête quantique, en généralisant l'important résultat suivant: dans l'article cite{LMRSS11}, leurs auteurs parviennent à caractériser la complexité en requête quantique, à l'aide de la méthode par adversaire, un programme semi-définie positif introduit par A. Ambainis dans cite{Ambainis2000}. Cependant, cette caractérisation est restreinte aux modèles à temps discret, avec une erreur bornée. Ainsi, le premier travail consiste à généraliser leur résultat aux modèles à temps continu, tandis que le second travail est une démarche, non aboutie, pour caractériser la complexité en requête quantique dans le cas exact et pour erreur non bornée.Dans ce premier travail, pour caractériser la complexité en requête quantique aux modèles à temps discret, nous adaptons la démonstration des modèles à temps discret, en construisant un algorithme en requête adiabatique universel. Le principe de cet algorithme repose sur le théorème adiabatique cite{Born1928}, ainsi qu'une solution optimale du dual de la méthode par adversaire. À noter que l'analyse du temps d'exécution de notre algorithme adiabatique est basée sur preuve qui ne nécessite pas d'écart dans le spectre de l'Hamiltonien.Dans le second travail, on souhaite caractériser la complexité en requête quantique pour une erreur non bornée ou nulle. Pour cela on reprend et améliore la méthode par adversaire, avec une approche de la mécanique lagrangienne, dans laquelle on construit un Lagrangien indiquant le nombre de requêtes nécessaires pour se déplacer dans l'espace des phases, ainsi on peut définir l'``action en requête''. Or ce lagrangien s'exprime sous la forme d'un programme semi-defini, son étude classique via les équations d'Euler-Lagrange nécessite l'utilisation du théorème de l'enveloppe, un puissant outils d'économathématiques. Le dernier travail, plus éloigné, concerne la complexité en information (et par extension la complexité en communication) pour simuler des corrélations non-locales. Ou plus précisement la quantitié d'information (selon Shannon) que doive s'échanger deux parties pour obtenir ses corrélations. Dans ce but, nous définissons une nouvelle complexité, denommée la zero information complexity IC_0, via le modèle sans communication. Cette complexité a l'avantage de s'exprimer sous la forme d'une optimization convexe. Pour les corrélations CHSH, on résout le problème d'optimisation pour le cas à une seule direction où nous retrouvons un résultat connu. Pour le scénario à deux directions, on met numériquement en évidence la validité de cette borne, et on résout une forme relaxée de IC_0 qui est un nouveau résultat.Doctorat en Sciences de l'ingénieur et technologie
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Maisel, Sascha [Verfasser]. "Characterization and optimization of multicomponent alloys based on quantum mechanics / Sascha B. Maisel." München : Verlag Dr. Hut, 2015. http://d-nb.info/1070123854/34.
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Pang, Bo. "Handwriting Chinese character recognition based on quantum particle swarm optimization support vector machine." Thesis, University of Macau, 2018. http://umaclib3.umac.mo/record=b3950620.
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Junique, Stéphane. "Surface-normal multiple quantum well electroabsorption modulators based on GaAs-related materials." Licentiate thesis, KTH, Microelectronics and Information Technology, IMIT, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-292.
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Goubault, de Brugière Timothée. "Methods for optimizing the synthesis of quantum circuits Quantum CNOT Circuits Synthesis for NISQ Architectures Using the Syndrome Decoding Problem Quantum circuits synthesis using Householder transformations Synthesizing quantum circuits via numerical optimization Reuse method for quantum circuit synthesis." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASG018.
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Pour exécuter un algorithme abstrait sur un ordinateur quantique il faut compiler l'algorithme en une séquence d'instructions bas niveau exécutables par le processeur. L'étape de compilation est cruciale car elle détermine la quantité de ressources nécessaire pour l'exécution d'un algorithme ; par conséquent elle se doit d'être optimisée. Dans cette thèse nous nous intéressons à une brique de la compilation~: la synthèse de circuits quantiques à partir d'une spécification abstraite d'un opérateur. Dans un premier temps nous étudions le cas où la matrice unitaire d'un opérateur quantique nous est donnée et nous explorons la minimisation des ressources quantiques et la minimisation des ressources classiques. Même si l'optimisation simultanée de ces deux types de ressources semble difficile, nous proposons de meilleurs compromis améliorant la littérature.Dans un second temps nous nous intéressons à la classe des opérateurs dits linéaires réversibles. Nous nous intéressons cette fois-ci exclusivement à l'optimisation des ressources quantiques et nous améliorons l'état de l'art dans diverses cas de métriques (taille et profondeur du circuit) et de processeurs quantiques (processeurs NISQ, ou à connectivité complète)To run an abstract algorithm on a quantum computer, the algorithm must be compiled into a sequence of low-level instructions that can be executed by the processor. The compilation step is crucial because it determines the quantity of resources necessary for the execution of an algorithm. Therefore, the compilation stage must be optimized. In this thesis, we are interested in a brick of compilation: the synthesis of quantum circuits from an abstract specification of an operator.First, we study the case where the unitary matrix of a quantum operator is given to us and we explore the minimization of both quantum resources and classical resources. Even if the simultaneous optimization of these two types of resources seems difficult, we propose better compromises improving the literature.Secondly, we are interested in the class of so-called reversible linear operators. This time we are exclusively interested in the optimization of quantum resources and we improve the state of the art in various cases of quantum metrics (circuit size, circuit depth) and processors (NISQ, fully-connected processors)
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Perdomo, Alejandro. "Designing and Probing Open Quantum Systems: Quantum Annealing, Excitonic Energy Transfer, and Nonlinear Fluorescence Spectroscopy." Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10290.
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The 20th century saw the first revolution of quantum mechanics, setting the rules for our understanding of light, matter, and their interaction. The 21st century is focused on using these quantum mechanical laws to develop technologies which allows us to solve challenging practical problems. One of the directions is the use quantum devices which promise to surpass the best computers and best known classical algorithms for solving certain tasks. Crucial to the design of realistic devices and technologies is to account for the open nature of quantum systems and to cope with their interactions with the environment. In the first part of this dissertation, we show how to tackle classical optimization problems of interest in the physical sciences within one of these quantum computing paradigms, known as quantum annealing (QA). We present the largest implementation of QA on a biophysical problem (six different experiments with up to 81 superconducting quantum bits). Although the cases presented here can be solved on a classical computer, we present the first implementation of lattice protein folding on a quantum device under the Miyazawa-Jernigan model. This is the first step towards studying optimization problems in biophysics and statistical mechanics using quantum devices. In the second part of this dissertation, we focus on the problem of excitonic energy transfer. We provide an intuitive platform for engineering exciton transfer dynamics and we show that careful consideration of the properties of the environment leads to opportunities to engineer the transfer of an exciton. Since excitons in nanostructures are proposed for use in quantum information processing and artificial photosynthetic designs, our approach paves the way for engineering a wide range of desired exciton dy- namics. Finally, we develop the theory for a two-dimensional electronic spectroscopic technique based on fluorescence (2DFS) and challenge previous theoretical results claiming its equivalence to the two-dimensional photon echo (2DPE) technique which is based on polarization. Experimental realization of this technique confirms our the- oretical predictions. The new technique is more sensitive than 2DPE as a tool for conformational determination of excitonically coupled chromophores and o↵ers the possibility of applying two-dimensional electronic spectroscopy to single-molecules.
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Zhang, Huaijian. "Utilization of Symmetry in Optimization of Tensor Contraction Expressions." The Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1285047367.
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Hörnedal, Niklas. "Generalizations of the Mandelstam-Tamm Quantum Speed Limit." Thesis, Stockholms universitet, Fysikum, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-193265.
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Quantum speed limits are lower bounds on the evolution time for quantum systems. In this thesis, we consider closed quantum systems. We investigate how different principal bundles offers a geometrical method for obtaining generalizations of the Mandelstam-Tamm quantum speed limit for mixed states. We look at three different principal bundles from which we derive two already known quantum speed limits, the Uhlmann and Andersson QSLs, and one which is new, the Grassmann QSL. We also investigate the tightness of these quantum speed limits and how they compare with each other.
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Abreu, Jean Faber Ferreira de. "Quantum games from biophysical Hamiltonians and a sub-neuronal optimization criterion of the information." Laboratório Nacional de Computação Científica, 2006. http://www.lncc.br/tdmc/tde_busca/arquivo.php?codArquivo=108.
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The Theory of Games is a mathematical formalism used to analyze conflicts between two or more parts. In those conflicts, each part has a group of actions (strategies) that aids them in the optimization of their objectives. The objectives of the players are the rewards (payoffs) given according to their chosen strategy. By quantizing a game, advantages in operational efficiency and in the stability of the game solutions are demonstrated. In a quantum game, the strategies are operators that act on an isolated system. A natural issue is to consider a game in an open system. In this case the strategies are changed by Kraus operators which represent a natural measurement of the environment. We want to find the necessary physical conditions to model a quantum open system as a game. To analyze this issue we applied the formalism of Quantum Operations on the Fröhlich system and we described it as a model of Quantum Game. The interpretation is a conflict among different configurations of the environment which, by inserting noise in the main system exhibits regimes of minimum loss of information. On the other hand, the model of Fröhlich has been used to describe the biophysical dynamics of the neuronal microtubules. By describing the model of Fröhlich in the Quantum Game formalism, we have shown that regimes of stability may exist even under physiological conditions. From the evolutionary point of view, the Theory of Games can be the key to describe the natural optimization at sub-neuronal levels.A Teoria de Jogos (TJs) é um formalismo matemático usado para analisar situações de conflitos entre duas ou mais partes. Nesses conflitos, cada parte possui um conjunto de ações (estratégias) que auxilia na otimização de seus objetivos. Os objetivos dos jogadres são as recompensas (payoffs) que cada um recebe de acordo com a estratégia adotada. Ao se quantizar um jogo, mostra-se ganhos em eficiência operacional e ganhos na estabilidade das soluções. Em um jogo quântico (JQ), as estratégias são operadores que atuam num sistema isolado. Uma questão natural é considerar um jogo num sistema aberto. Nesta situação as estratégias são trocadas por operadores de Kraus que representam uma medida natural do ambiente. Nosso interesse é encontrar as condições físicas necessáriaas para modelarmos um sistema quântico aberto como um jogo. Para analisar essa questão aplicamos o formalismo de Operações Quânticas (OQs) sobre o sistema de Fröhlich e o apresentamos como um modelo de JQ. A interpretação é um conflito entre diferentes configurações do ambiente que, ao inserirem ruído no sistema principal, exibem regiões de mínima perda de informação. O modelo de Fröhlich vem sendo usado para descrever a dinâmica biofísica dos microtúbulos neuronais. Ao estruturamos o modelo de Fröhlich nos JQs, mostramos que as regiões de estabilidade podem existir sob condições fisiológicas. Usando o aspecto evolucionista, a TJs pode ser a chave para a descrição de processos de otimização da informação em nível sub-neuronal.
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Lee, Sangjin Ph D. Massachusetts Institute of Technology. "Design and optimization of colloidal quantum dot solids for enhanced charge transport and photovoltaics." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/104152.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 93-102).
Colloidal quantum dots (CQDs) have attracted much attention due to their distinctive optical properties such as wide spectral responses and tunable absorption spectra with simple size control. These properties, together with the advantages of solution processing and superior robustness to organic materials, have motivated the recent investigation of CQD-based solar cells, which have seen rapid growth in power conversion efficiency in just the last 10 years, to a current record of over 10%. However, in order to continue to push the efficiencies higher, a better understanding of the charge transport phenomena in CQD films is needed. While the carrier transport mechanisms between isolated molecules have been explored theoretically and the device-scale mobility of CQD layers has been characterized using experimental measurements such as time-of-flight analysis and field-effect-transistor measurements, a systematic study of the connection between these two distinct scales is required in order to provide crucial information regarding how CQD layers with higher charge carrier mobility can be achieved. While a few strategies such as ligand exchanges, band-like transport, and trap-state-mediated transport have been suggested to enhance the charge carrier mobility, inhomogeneity in CQD solids has been considered a source of the mobility degradation because the electronic properties in individual CQDs may have dispersions introduced in the synthesis and/or in the deposition process, leading to the deviations of the localized energy states from the regular positions or the average energy levels. Here, we suggest that control over such design factors in CQD solids can provide important pathways for improvements in device efficiencies as well as the charge carrier mobility. In particular, we have focused on the polydispersity in CQDs, which normally lies in the range of 5-15%. The effect of size-dispersion in CQD solids on the charge carrier mobility was computed using charge hopping transport models. The experimental film deposition processes were replicated using a molecular dynamics simulation where the equilibrium positions of CQDs with a given radii distribution were determined under a granular potential. The radii and positions of the CQDs were then used in the charge hopping transport simulator where the carrier mobility was estimated. We observed large decreases (up to 70%) in electron mobility for typical experimental polydispersity (about 10%) in CQD films. These large degradations in hopping charge transport were investigated using transport vector analysis with which we suggested that the site energy differences raised the portion of the off-axis rate of charge transport to the electric field direction. Furthermore, we have shown that controlling the size distribution remarkably impacts the charge carrier mobility and we suggested that tailored and potentially experimentally achievable re-arrangement of the CQD size ensemble can mediate the mobility drops even in highly dispersive cases, and presents an avenue towards improved charge transport. We then studied the degradation in CQD solar cells with respect to the polydispersity and how these enhanced charge transport from re-design of CQD solids can boost the photovoltaic performances. In addition, we estimated the potential in the binary CQD solids in terms of their improved charge transport and efficient light absorption. Combined with the accurate size-dependent optical absorption model for CQDs, our hopping model confirmed that the inclusion of smaller CQDs could enhance both the charge transport and the solar light absorption, leading to the enhanced average charge generation rates and solar cell performance.
by Sangjin Lee.
Ph. D.
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Malmberg, Olle, and Joakim Wellenstam. "Quantum Portfolio Optimization : a Multi-Level Perspective Study of the Swedish Fund Management Industry." Thesis, KTH, Industriell ekonomi och organisation (Inst.), 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-296676.
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In recent years, quantum computers have achieved new levels of sophistication and are by some estimates only a few years from being used in production. A growing body of literature points toward their potential uses within various industries, with the finance industry identified as exceptionally full of prospective applications. One application that has seen recent experimental success is Quantum Portfolio Optimization (QPO), where researchers have successfully mapped the optimization task onto existing quantum hardware. As classical Portfolio Optimization (PO) algorithms often have high computational costs, achieving a quantum speed-up could bring major benefits for various stakeholders. One industry that makes use of PO is fund management. Considering the diversity that exists between funds, with regard to strategy and methods, it is likely that sociological factors play a part in selecting which technical solutions are employed. The interplay between sociological and technical factors will be central to this study, as it investigates the conditions for QPO within the Swedish fund management through a socio-technical perspective. First, semi-structured qualitative interviews are conducted with industry actors and quantum computer stakeholders. This is followed by a two-step thematic analysis, structured on the socio-technical dimensions of Geels’ Multi-Level Perspective (MLP). After coding statements, four key themes are created to describe characteristics of the industry. Second, these themes are analyzed together with the niche of QPO through the lens of System Innovations theories, to evaluate the readiness of QPO and its potential socio-technical effects on the industry. The key findings characterizing the fund management industry are summarized through four themes. In particular, the trade-off between quantitative and qualitative methods, the inadequacy of historical data, the importance of third-party suppliers, and the poorly functioning and rigid competitive landscape are key characteristics. The current state of QPO is not considered to have reached a commercially viable price-to-performance ratio but has built a strong support that will help it improve over time. Furthermore, the results point toward QPO being introduced in a limited fashion, but may over time cause significant disruption, as it contributes to reshaping the socio-technical architecture of the industry.Under de senaste åren har kvantdatorer uppnått nya nivåer av sofistikation och är enligt vissa uppskattningar bara några år från att användas i produktion. Alltmer litteratur pekar ut potentiella användningsområden inom olika branscher och finansbranschen har identifierats som ett område med särskilt stor potential. En applikation som nyligen sett experimentell framgång är 'Quantum Portfolio Optimization' (QPO), där forskare framgångsrikt har översatt och beräknat optimeringsuppgiften på existerande kvantdatorer. Eftersom klassiska portföljoptimeringsalgoritmer ofta är krävande i datorkraft så kan kvantdatorers snabbhet potentiellt ge stora fördelar för olika intressenter. En bransch som använder portföljoptimering är fondförvaltning. Med tanke på den mångfald som finns bland fonder, med avseende på strategi och metoder, är det troligt att sociologiska faktorer spelar en roll i valet av vilka tekniska lösningar som används. Samspelet mellan sociologiska och tekniska faktorer kommer att vara centralt i denna studie, då den ämnar undersöka villkoren förutsättningarna för QPO inom svenska fondindustrin genom ett sociotekniskt perspektiv. Först genomförs halvstrukturerade kvalitativa intervjuer med branschaktörer och kvantdatoraktörer. Detta följs av en tematisk analys i två steg, strukturerad efter de sociotekniska dimensionerna i Geels 'Multi-Level Perspective' (MLP). Baserat på kodning av uttalanden så skapas fyra nyckelteman för att beskriva branschens egenskaper. Sedan analyseras dessa teman tillsammans med nischen, QPO, utifrån perspektiv från systeminnovationsteorier för att utvärdera QPO:s beredskap och dess potentiella sociotekniska effekter på branschen. De viktigaste resultaten som kännetecknar fondindustrin sammanfattas genom fyra teman. I synnerhet är avvägningen mellan kvantitativa och kvalitativa metoder, bristfälligheten i historiska data, betydelsen av tredjepartsleverantörer och det dåligt fungerande och styva konkurrenslandskapet viktiga egenskaper. Det nuvarande läget för QPO anses inte ha nått en kommersiellt lönsam nivå sett till förhållandet mellan pris och prestanda, men nischen har byggt ett starkt stödnätverk som kommer hjälpa den att förbättras över tid. Dessutom pekar resultaten mot att nischteknologin QPO kommer att introduceras i begränsad utsträckning, men kan med tiden ändå komma att orsaka betydande störningar eftersom den bidrar till att omforma branschens socio-tekniska arkitektur.
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Fišer, Petr. "Simplification of quantum circuits for modular exponentiation." Master's thesis, Vysoká škola ekonomická v Praze, 2015. http://www.nusl.cz/ntk/nusl-261826.
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This thesis is based on top of the previous thesis "Security of modern encryption protocols" where we introduced a new paradigm for constructing quantum circuits. We have built circuits for modular arithmetic (addition, multiplication and exponentiation) in order to break El-Gamal asymmetric cryptosystem. Current thesis reviews all proposed circuits and discusses possibilities of their further optimization in goal of lowering the number of used qbits at least by an order of magnitude. It also shows that this is not possible due to existence of COPY gates which make the design inherently unoptimizable. Getting rid of COPY gates is, however, not possible without substantial rewrite of the whole paradigm. The overall estimate of number of qbits used in circuits thus remains O(log(m)log^2(N)) where m is a processed number and N is a modulus. The thesis also proposes optimization of the modular multiplication circuit that, if issues with COPY gates are resolved, allows us to lower the number of used qbits by about O(log(m)) at the price of a longer execution time.
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Corn, John Russell. "Optimization Problems in Hilbert Space with POSS Complexes." Digital Commons @ East Tennessee State University, 2011. https://dc.etsu.edu/etd/1381.
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Beginning with a survey of functional variation methods in classical physics, we derive the Hartree-Fock theory from canonical quantization. Following a development of density functional theory, many-body perturbation theory, and other techniques of computational condensed matter physics, we perform a systematic study of metal-polyhydride impurities in T8 and T12 polyhedral oligomeric silsesquioxane (POSS) cage molecules. Second-quantized methods motivate the derivations throughout.
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Ni, Xiaotong Verfasser], J. I. [Gutachter] [Akademischer Betreuer] [Cirac, and Alejandro [Gutachter] Ibarra. "Quantum memory: design and optimization / Xiaotong Ni ; Gutachter: Ignacio Cirac, Alejandro Ibarra ; Betreuer: Ignacio Cirac." München : Universitätsbibliothek der TU München, 2017. http://d-nb.info/1128309939/34.
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Burger, Anat. "Optimization of a microwave resonator cavity to perform electron spin resonance measurements on quantum dots." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/36131.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2006.Includes bibliographical references (p. 51).
This thesis attempts to improve on an ongoing experiment of detecting electron spin resonance (ESR) on AlGaAs/GaAs lateral quantum dots. The experiment is performed in a 2.5 Tesla magnetic field at temperatures around 100mK. A resonator cavity is used to expose the quantum dot to a perturbational microwave magnetic field pulse that induces electron spin flip transitions. The statistics for measuring the probabilities of these transitions can be improved by increasing the strength and/or the duration of the pulsed magnetic field. The drawback is that both of these improvements lead to thermal heating which diminishes the quantum nature of the dot. I used electromagnetic field calculations and simulation software to explore different resonant modes, geometries, materials, and methods of excitation and optimize the design of potential new cavities. Two cavities were built specifically to test the TM010 and TE011 cylindrical modes. Although they did not perform as well as was theoretically expected, these cavities provide a better magnetic field magnitude per heating power than the current cavity.
by Anat Burger.
S.B.
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Simnacher, Timo Yannick [Verfasser], and Otfried [Gutachter] Gühne. "The interplay between quantum entanglement, coherence, and convex optimization / Timo Yannick Simnacher ; Gutachter: Otfried Gühne." Siegen : Universitätsbibliothek der Universität Siegen, 2021. http://nbn-resolving.de/urn:nbn:de:hbz:467-19677.
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Zhang, Zhenzhong. "Epitaxial growth optimization for 1.3-um InGaAs/GaAs Vertical-Cavity Surface-Emitting lasers." Licentiate thesis, KTH, Microelectronics and Applied Physics, MAP, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4648.
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Long-wavelength (1.3-μm) vertical-cavity surface-emitting lasers (VCSELs) are of great interest as low-cost, high performance light sources for fiber-optic metro and access networks. During recent years the main development effort in this field has been directed towards all epitaxial GaAs-based structures by employing novel active materials. Different active region candidates for GaAs-based 1.3-μm VCSELs such as GaInNAs/GaAs QWs, GaAsSb QWs or InAs/InGaAs QDs have been investigated. However, the difficult growth and materials properties of these systems have so far hampered any real deployment of the technology. More recently, a new variety of VCSELs have been developed at KTH as based on highly strained InGaAs QWs and negative gain cavity detuning to reach the 1.3-μm wavelength window. The great benefit of this approach is that it is fully compatible with standard materials and processing methods.
The aim of this thesis is to investigate long-wavelength (1.3-μm) VCSELs using ~1.2-μm In0.4GaAs/GaAs Multiple Quantum Wells (MQWs). A series of QW structures, DBR structures and laser structures, including VCSELs and Broad Area lasers (BALs) were grown by metal-organic vapor phase epitaxy (MOVPE) and characterized by various techniques: Photoluminescence (PL), high-resolution x-ray diffraction (XRD), atomic force microscopy (AFM), high accuracy reflectance measurements as well as static and dynamic device characterization. The work can be divided into three parts. The first part is dedicated to the optimization and characterization of InGaAs/GaAs QWs growth for long wavelength and strong luminescence. A strong sensitivity to the detailed growth conditions, such as V/III ratio and substrate misorientation is noted. Dislocations in highly strained InGaAs QW structure and Sb as surfactant assisted in InGaAs QW growth are also discussed here. The second part is related to the AlGaAs/GaAs DBR structures. It is shown that the InGaAs VCSELs with doped bottom DBRs have significantly lower slope efficiency, output power and higher threshold current. By a direct study of buried AlGaAs/GaAs interfaces, this is suggested to be due to doping-enhanced Al-Ga hetero-interdiffusion. In the third part, singlemode, high-performance 1.3-μm VCSELs based on highly strained InGaAs QWs are demonstrated. Temperature stable singlemode performance, including mW-range output power and 10 Gbps data transmission, is obtained by an inverted surface relief technique.
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Garduno, Nolasco Edson. "Nano-scale approaches for the development and optimization of state-of-the-art semiconductor photovoltaic devices." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/nanoscale-approaches-for-the-development-and-optimization-of-stateoftheart-semiconductor-photovoltaic-devices(927e70db-03ff-43e0-8b27-5472bc4a293f).html.
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This project is concerned with both the study of different Multiple Quantum Wells (MQWs) structures using the In0.53Ga0.47As/In0.52Al0.48As material system lattice matched to InP and a systematic investigation of the properties of InAs QD systems within GaAs with the aim of achieving enhancements of solar cell performance. The key challenge is the growth of QDs solar cell structures which exhibit sufficient absorption (enhanced infrared absorption) to increase short circuit current density (Jsc) but which can still maintains a high open circuit voltage (Voc). The research consists of epitaxial growth using state-of–the-art MBE, optical absorption, photoluminescence and high resolution x-ray diffraction measurements as well as device fabrication and characterization of novel solar cell structures. Optimization was performed on these novel cells to further improve their efficiency by inserting stacks of QD into different regions of the device. The effect of localized doping of such structures was used in an attempt to maintain and enhance the open-circuit voltage which in turn increases the device efficiency. The fabricated devices were characterized using measurements of the dark/light current-voltage (I-V) characteristics and spectral response (50-480 K). Solar cell external quantum efficiencies under standard air mass (AM) 1.5 spectrum were determined and the suitability of these new cells under solar concentration were assessed. Full physical simulations are performed using SILVACO semiconductors modelling software to generate models of multi-junction solar cells that were crucial in informing iterations to growth and fabrication and help to reconcile theory with experiment. One of the key findings, of this thesis, is the fact that Intermediate band photovoltaic devices using material based on InAs/GaAs vertically stacked quantum dot arrays, can be used in applications according to specific configuration criteria such as high temperature operation conditions. The intermediate band cell, including an inter-dot doped configuration, has been found to be a potential candidate as the inter dot doping profile reduces the efficiency degradation below the GaAs values including an enhancement in the open circuit voltage. It has been proved that these devices not only have a good performance at high temperatures but also by changing the vertical stacking QD layer periodicity can enhance the short circuit current density while keeping a large open circuit voltage. It was confirmed in practical device operation that thermal energy is required to enable the intermediate band in InAs/GaAs QD materials. The impact of this works can help in the future improvements of the intermediate band solar cells based on InAs on GaAs QD. The best overall efficiency of 11.6 % obtained in this work is an excellent value for so simple devices configuration. The Si3N4, tested for the first time on InAs/GaAs QD materials, reduces the reflectance on the device surface to a value of 2% and the operational wavelength can be tuned by controlling the layer thickness. A 100 nm Si3N4 antireflective coating proved to be an excellent coating from 700 to 1000 nm. In terms of short circuit current density a 37% enhancement was achieved.
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Olofsson, Karl-Johan. "Black-box optimization of simulated light extraction efficiency from quantum dots in pyramidal gallium nitride structures." Thesis, Linköpings universitet, Matematiska institutionen, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-162235.
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Microsized hexagonal gallium nitride pyramids show promise as next generation Light Emitting Diodes (LEDs) due to certain quantum properties within the pyramids. One metric for evaluating the efficiency of a LED device is by studying its Light Extraction Efficiency (LEE). To calculate the LEE for different pyramid designs, simulations can be performed using the FDTD method. Maximizing the LEE is treated as a black-box optimization problem with an interpolation method that utilizes radial basis functions. A simple heuristic is implemented and tested for various pyramid parameters. The LEE is shown to be highly dependent on the pyramid size, the source position and the polarization. Under certain circumstances, a LEE over 17% is found above the pyramid. The results are however in some situations very sensitive to the simulation parameters, leading to results not converging properly. Establishing convergence for all simulation evaluations must be done with further care. The results imply a high LEE for the pyramids is possible, which motivates the need for further research.
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Reimpell, Michael. "Quantum information and convex optimization /." 2008. http://www.gbv.de/dms/bs/toc/559790392.pdf.
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Sikora, Jamie William Jonathon. "Applications of Semidefinite Programming in Quantum Cryptography." Thesis, 2007. http://hdl.handle.net/10012/3056.
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Coin-flipping is the cryptographic task of generating a random coin-flip between two mistrustful parties. Kitaev discovered that the security of quantum coin-flipping protocols can be analyzed using semidefinite programming. This lead to his result that one party can force a desired coin-flip outcome with probability at least 1/√2.
We give sufficient background in quantum computing and semidefinite programming to understand Kitaev's semidefinite programming formulation for coin-flipping cheating strategies. These ideas are specialized to a specific class of protocols singled out by Nayak and Shor. We also use semidefinite programming to solve for the maximum cheating probability of a particular protocol which has the best known security.
Furthermore, we present a family of protocols where one party has a greater probability of forcing an outcome of 0 than an outcome of 1. We also discuss a computer search to find specific protocols which minimize the maximum cheating probability.
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Mattingly, Alan Charles. "'Variational' optimization in quantum field theory." Thesis, 1993. http://hdl.handle.net/1911/16649.
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We examine two different techniques for studying quantum field theories in which a 'variational' optimization of parameters plays a crucial role.
In the context of the O(N)-symmetric $\lambda\phi\sp4$ theory we discuss variational calculations of the effective potential that go beyond the Gaussian approximation. Trial wavefunctionals are constructed by applying a unitary operator $U = e\sp{-is\pi\sb{R}\phi\sbsp{T}{2}}$ to a Gaussian state. We calculate the expectation value of the Hamiltonian using the non-Gaussian trial states generated, and thus obtain optimization equations for the variational-parameter functions of the ansatz. At the origin, $\varphi\sb{c} = 0,$ these equations can be solved explicitly and lead to a nontrivial correction to the mass renormalization, with respect to the Gaussian case. Numerical results are obtained for the (0 + 1)-dimensional case and show a worthwhile quantitative improvement over the Gaussian approximation.
We also discuss the use of optimized perturbation theory (OPT) as applied to the third-order quantum chromodynamics (QCD) corrections to $R\sb{e\sp+e\sp-}.$ The OPT method, based on the principle of minimal sensitivity, finds an effective coupling constant that remains finite down to zero energy. This allows us to apply the Poggio-Quinn-Weinberg smearing method down to energies below 1 GeV, where we find good agreement between theory and experiment. The couplant freezes to a zero-energy value of $\alpha\sb{s}/\pi = 0.26,$ which is in remarkable concordance with values obtained phenomenologically.
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Pye, Cory Christopher. "Applications of optimization to quantum chemistry /." 1997.
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Silva, Hugo Gonçalves. "Quantum Effects for Spintronic Devices Optimization." Doctoral thesis, 2010. http://hdl.handle.net/10174/2242.
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This work is mainly dedicated to the study of spin dependent transport in mag-
netic nanostructures. The principal objective is the optimization of the magnetoresistive
performance of such structures, in order to built high density Magnetic Random Access
Memories (MRAM). Nevertheless, new resistive properties are also found, that could be
useful for another type of non-volatile memory device, in this case, Resistive Random
Access Memories (ReRAM). The thesis is basically divided into two parts, the ¯rst one
considers the theoretical analysis of multilayered magnetic junctions and the second one
is dedicated to the experimental study of magnetic granular multilayers.
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Amy, Matthew. "Algorithms for the Optimization of Quantum Circuits." Thesis, 2013. http://hdl.handle.net/10012/7818.
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This thesis investigates techniques for the automated optimization of quantum circuits. In the first part we develop an exponential time algorithm for synthesizing minimal depth quantum circuits. We combine this with effective heuristics for reducing the search space, and show how it can be extended to different optimization problems. We then use the algorithm to compute circuits over the Clifford group and T gate for many of the commonly used quantum gates, improving upon the former best known circuits in many cases.
In the second part, we present a polynomial time algorithm for the re-synthesis of CNOT and T gate circuits while reducing the number of phase gates and parallelizing them. We then describe different methods for expanding this algorithm to optimize circuits over Clifford and T gates.
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Rao, M. V. Panduranga. "Bounds On Augmented Automata And Quantum Adiabatic Optimization." Thesis, 2007. http://hdl.handle.net/2005/518.
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Quantum computing has generated a lot of interested in the past two decades. Research into powerful models of quantum computation has yielded important and elegant results like an efficient algorithm for factoring and a quadratic speed-up for unordered search. At the same time, given the current difficulty in the physical implementation of these general purpose models, considerable effort has also been made in estimating the power of weaker models of quantum computation: models that have a small quantum component.
The first part of this thesis is an investigation into the power of interference in quantum computing. While interference in probability amplitudes is a central feature even in powerful models, it is the only extra resource available to quantum finite automata. Of particular interest is interference in automata that have both classical and quantum states (2QCFA) as proposed by Ambainis and Watrous, since it inquires into the power of a classical deterministic finite automaton when augmented with a quantum component of constant size. Our contributions in this part are as follows:
• To abstract out the phenomenon of interference in quantum computing, we propose a model called the 2-way Optical Interference Automata (2OIA). The model consists of a 2DFA augmented with a simple optical arrangement. We show different ways of harnessing the power of interference in the form of algorithms on this model to recognize some non-trivial languages. We then go on to show a language recognizable by a Turing machine using O(n2) space but by no 2OIA.
• A natural classical model for comparison with 2QCFA is the weighted automaton, since it has the potential to capture interference in sum of path weights. Using the Cortes-Mohri definition of language recognition, we give an efficient simulation of 2QCFAwith algebraic amplitudes by weighted automata over the complex semi ring.
• We introduce quantum non-determinism to the Measure-Once 1-way Quantum Finite Automata of Moore and Crutchfield and Kondacs and Watrous and show that even then, the model can recognize only regular languages with bounded error.
• We propose a group theoretic generalization of counter automata that allows a notion of counter reversal complexity. To obtain this generalization, we combine concepts from classical counter automata theory with results in 2QCFA. We examine specific instances of this generalization and compare their ii iii powers. We also show an instance recognizing a language that is not recognized by conventional 2-way counter automata. Finally, we show a strict hierarchy among the 1-way versions of the instances Discussed.
The second part of the thesis deals with Quantum Adiabatic Optimization algorithms. A common trick for designing faster quantum adiabatic algorithms is to apply the adiabatic condition locally at every instant. However it is often difficult to determine the instantaneous gap between the lowest two eigen values, which is an essential ingredient in the adiabatic condition. We present a simple linear algebraic technique for obtaining a lower bound on the instantaneous gap even in such a situation. As an illustration, we investigate the adiabatic unordered search of van Dam et al. and Roland and Cerf when the non-zero entries of the diagonal final Hamiltonian are perturbed by a polynomial (in logN, where N is the length of the unordered list) amount. We use our technique to derive a bound on the running time of a local adiabatic schedule in terms of the minimum gap between the lowest two eigenvalues.
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Sikora, Jamie. "Analyzing Quantum Cryptographic Protocols Using Optimization Techniques." Thesis, 2012. http://hdl.handle.net/10012/6760.
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This thesis concerns the analysis of the unconditional security of quantum cryptographic protocols using convex optimization techniques. It is divided into the study of coin-flipping and oblivious transfer. We first examine a family of coin-flipping protocols. Almost all of the handful of explicitly described coin-flipping protocols are based on bit-commitment. To explore the possibility of finding explicit optimal or near-optimal protocols, we focus on a class which generalizes such protocols. We call these $\BCCF$-protocols, for bit-commitment based coin-flipping. We use the semidefinite programming (SDP) formulation of cheating strategies along the lines of Kitaev to analyze the structure of the protocols.
In the first part of the thesis, we show how these semidefinite programs can be used to simplify the analysis of the protocol. In particular, we show that a particular set of cheating strategies contains an optimal strategy. This reduces the problem to optimizing a linear combination of fidelity functions over a polytope which has several benefits. First, it allows one to model cheating probabilities using a simpler class of optimization problems known as second-order cone programs (SOCPs). Second, it helps with the construction of point games due to Kitaev as described in Mochon's work.
Point games were developed to give a new perspective for studying quantum protocols. In some sense, the notion of point games is dual to the notion of protocols.
There has been increased research activity in optimization concerning generalizing theory and algorithms for linear programming to much wider classes of optimization problems such as semidefinite programming. For example, semidefinite programming provides a tool for potentially improving results based on linear programming or investigating old problems that have eluded analysis by linear programming. In this sense, the history of semidefinite programming is very similar to the history of quantum computation.
Quantum computing gives a generalized model of computation to tackle new and old problems, improving on and generalizing older classical techniques. Indeed, there are striking differences between linear programming and semidefinite programming as there are between classical and quantum computation. In this thesis, we strengthen this analogy by studying a family of classical coin-flipping protocols based on classical bit-commitment. Cheating strategies for these ``classical $\BCCF$-protocols'' can be formulated as linear programs (LPs) which are closely related to the semidefinite programs for the quantum version. In fact, we can construct point games for the classical protocols as well using the analysis for the quantum case.
Using point games, we prove that every classical $\BCCF$-protocol allows exactly one of the parties to entirely determine the outcome. Also, we rederive Kitaev's lower bound to show that only ``classical'' protocols can saturate Kitaev's analysis. Moreover, if the product of Alice and Bob's optimal cheating probabilities is $1/2$, then at least one party can cheat with probability $1$.
The second part concerns the design of an algorithm to search for $\BCCF$-protocols with small bias. Most coin-flipping protocols with more than three rounds have eluded direct analysis. To better understand the properties of optimal $\BCCF$-protocols with four or more rounds, we turn to computational experiments. We design a computational optimization approach to search for the best protocol based on the semidefinite programming formulations of cheating strategies. We create a protocol filter using cheating strategies, some of which build upon known strategies and others are based on convex optimization and linear algebra. The protocol filter efficiently eliminates candidate protocols with too high a bias. Using this protocol filter and symmetry arguments, we perform searches in a matter of days that would have otherwise taken millions of years. Our experiments checked $10^{16}$ four and six-round $\BCCF$-protocols and suggest that the optimal bias is $1/4$.
The third part examines the relationship between oblivious transfer, bit-commitment, and coin-flipping. We consider oblivious transfer which succeeds with probability $1$ when the two parties are honest and construct a simple protocol with security provably better than any classical protocol. We also derive a lower bound by constructing a bit-commitment protocol from an oblivious transfer protocol. Known lower bounds for bit-commitment then lead to a constant lower bound on the bias of oblivious transfer. Finally, we show that it is possible to use Kitaev's semidefinite programming formulation of cheating strategies to obtain optimal lower bounds on a ``forcing'' variant of oblivious transfer related to coin-flipping.
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Paetznick, Adam. "Resource optimization for fault-tolerant quantum computing." Thesis, 2013. http://hdl.handle.net/10012/8111.
Full textAbstract:
Quantum computing offers the potential for efficiently solving otherwise classically difficult problems, with applications in material and drug design, cryptography, theoretical physics, number theory and more. However, quantum systems are notoriously fragile; interaction with the surrounding environment and lack of precise control constitute noise,
which makes construction of a reliable quantum computer extremely challenging. Threshold theorems show that by adding enough redundancy, reliable and arbitrarily long quantum computation is possible so long as the amount of noise is relatively low---below a ``threshold'' value. The amount of redundancy required is reasonable in the asymptotic sense, but in absolute terms the resource overhead of existing protocols is enormous when compared to current experimental capabilities.
In this thesis we examine a variety of techniques for reducing the resources required for fault-tolerant quantum computation. First, we show how to simplify universal encoded computation by using only transversal gates and standard error correction procedures, circumventing existing no-go theorems. The cost of certain error correction procedures is dominated by preparation of special ancillary states. We show how to simplify ancilla preparation, reducing the cost of error correction by more than a factor of four. Using this optimized ancilla preparation, we then develop improved techniques for proving rigorous lower bounds on the noise threshold. The techniques are specifically intended for analysis of relatively large codes such as the 23-qubit Golay code, for which we compute a lower bound on the threshold error rate of 0.132 percent per gate for depolarizing noise. This bound is the best known for any scheme.
Additional overhead can be incurred because quantum algorithms must be translated into sequences of gates that are actually available in the quantum computer. In particular, arbitrary single-qubit rotations must be decomposed into a discrete set of fault-tolerant gates. We find that by using a special class of non-deterministic circuits, the cost of decomposition can be reduced by as much as a factor of four over state-of-the-art techniques, which typically use deterministic circuits.
Finally, we examine global optimization of fault-tolerant quantum circuits. Physical connectivity constraints require that qubits are moved close together before they can interact, but such movement can cause data to lay idle, wasting time and space. We adapt techniques from VLSI in order to minimize time and space usage for computations in the surface code, and we develop a software prototype to demonstrate the potential savings.