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Dissertations / Theses on the topic 'Electronic quantum coherence'
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1
Acton, J. M. "Quantum coherence effects in electronic, photonic and atomic structures." Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.595334.
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Light propagating through a disordered dielectric exhibits mesoscopic phenomena, such as coherent back scattering. In electronic systems, equivalent phenomena have been successfully described by the non-linear s-model. Starting from Maxwell’s equation in its full vector form, it is shown that disordered photonic systems, in two and three dimensions, can also be described by a non-linear s-model. The quasi classical approximation on which this theory is based is found to be valid in a window of frequencies. Numerical simulations of these systems are consistent with the s-model predictions. The mathematical equivalence between disordered photonic and electronic systems shows that the mechanisms for localization in the electronic and photonic band gaps are the same; numerical simulations to demonstrate this are presented. An investigation into the interplay of s-wave superconductivity and itinerant antiferromagnetism in disordered metals is presented. First, a s-model to describe this interplay is derived. It is used to obtain the phase diagram for the mean field phase transition between superconductivity and antiferromagnetism. The suppression of antiferromagnetism by disorder (which is analogous to the effect of magnetic disorder on superconductivity) culminates in a quantum critical point. The bilayer proximity effect is also investigated and the density of states inside an antiferromagnet coupled to a superconductor is obtained. Feshback resonance phenomena in ultracold Fermi gases are normally described by a Fermi-Bose model in which the Feshback molecule is treated as a point-like boson. We consider an alternative model of the 6Li system, in which a spin state is shared between the open and closed channels. In contrast to the Fermi-Bose model, a critical coupling in the open channel is required to induce a Feshback resonance, even at small detuning.
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Cabart, Clément. "Measurement and control of electronic coherences." Thesis, Lyon, 2018. http://www.theses.fr/2018LYSEN031/document.
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Ces dernières années, de considérables efforts expérimentaux ont été dévoués au développement d’outils de nanoélectronique quantique, dans le but d’atteindre un niveau de contrôle sur le transport électronique quantique à l’échelle de l’électron unique. Ces avancées ont poussé à un changement de paradigme dans le domaine du transport électronique cohérent et donné naissance à l’optique quantique électronique, domaine dans lequel cette thèse s’inscrit. Cette thèse est consacrée à deux problématiques. Tout d’abord, elle s’intéresse au problème des interactions Coulombiennes entre électrons, qui donnent lieu à un phénomène de décohérence qu’il est nécessaire de caractériser et de prédire au mieux afin de le contrôler. En utilisant une approche analytique et numérique, il a été possible de prédire l’effet de ces interactions sur un système expérimentalement accessible, prédiction qui a ensuite été confirmée par l’expérience. Dans la foulée de ce résultat, cette thèse présente des possibilités de contrôle de ces interactions, et propose un moyen de les mettre en œuvre qui devrait pouvoir être testé dans une expérience. Je me suis également confronté à la problématique de la caractérisation d’états quantiques complexes. En particulier, suite à la démonstration expérimentale d’un protocole de tomographie pour des états mono-électroniques, je me suis tourné vers l’extension de ce protocole à des états plus complexes, pouvant exhiber des propriétés de cohérence à deux électrons, voire plus. Ces états étant également sensibles aux interactions de Coulomb, une extension au cas multi-électronique des outils utilisés pour traiter ces interactions est proposée dans cette thèseOver the last few years, extensive experimental efforts have been devoted to thedevelopment of quantum nanoelectronics tools aiming at controlling electronic trans-port down to the single electron level. These advances led to a paradigm shift inthe domain of coherent electronic transport, giving birth to electron quantum optics,which is the domain of this work.This manuscript is devoted to two problems. The first of these is the one ofCoulomb interactions between electrons, which lead to a decoherence phenomenonthat must be characterized and predicted in order to be controlled. Using an analyt-ical and numerical approach, it became possible to predict the effect of interactionson an experimentally relevant system, a prediction that was then confirmed in the ex-periment. After this result, this manuscript displays some ideas aiming at controllinginteractions and proposes some ways to test them experimentally.In this work, I also took on the problem of characterizing complex quantum states.In particular, following the experimental demonstration of a tomography protocol forfirst order coherences, I tried to extend this protocol to more complex states thatcould exhibit two-electron coherences, or more. These states being also sensitive to Coulomb interactions, an extension of the tools used to treat interactions to thismulti-electronic state is also presented in this work
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Rebentrost, Frank. "Exciton Transfer in Photosynthesis and Engineered Systems: Role of Electronic Coherence and the Environment." Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10474.
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Recent experiments show evidence for long-lived electronic coherence in several photosynthetic complexes, for example in the Fenna-Matthews-Olson complex of green sulfur bacteria. The experiments raise questions about the microscopic reasons for this quantum coherence and its role to the functioning of these highly evolved biological systems. The present thesis addresses both questions. We find that an interplay of electronic coherence and the fluctuating phonon environment is responsible for the high exciton transport efficiency in these complexes and generalize this idea to the concept of environment-assisted quantum transport (ENAQT). In addition, we quantify the contribution of coherent dynamics to the efficiency and thus to the biological functioning. We determine the effect of temporal (non-Markovian) and spatial correlations and develop an ab initio propagation method based on atomistic detail which predicts the long-lived coherence. The research in photosynthetic energy transfer can inspire new designs for the control of excitons in engineered systems. We develop a method for computing the Forster coupling between semiconductor nanoparticle quantum dots. The focus is on the size and shape dependence and the presence of a spatially varying dielectric environment and metallic gates. A separation of the wavefunction into slowly and fast varying part provides the basis for an efficient computation on a real-space grid. Finally, the simulation of structured models of photosynthetic energy transfer is a challenging task using conventional computing resources. To this end, we propose a special-purpose superconducting device based on flux quantum bits and quantum LC resonators and show that parameters can be engineered such that this simulation becomes possible.Chemistry and Chemical Biology
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Peeks, Martin. "Electronic delocalisation in linear and cyclic porphyrin oligomers." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:58a35932-320c-47dc-828e-0d121d693fd8.
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This thesis presents a combined experimental and computational evaluation of the physical-organic properties of butadiyne-linked porphyrin oligomers. The principal result from the thesis is the synthesis and characterisation of the largest aromatic and antiaromatic systems to date, in the form of an oxidised [6]-porphyrin nanoring, with diameter 2.4 nm. This large electronically coherent system provides insight into the connection between aromatic ring currents and persistent currents in metal and semiconductor mesoscopic rings. Chapter 1 briefly reviews the concepts used in the remainder of the thesis, with a particular focus on aromaticity. In Chapter 2, the barrier to inter-porphyrin torsional rotation in a butadiyne-linked porphyrin dimer is determined computationally and experimentally to be 3 kJ mol-1. The barrier height is closely related to the resonance delocalisation energy between the porphyrin subunits. In Chapter 3 we show that by oxidising a butadiyne-linked [6]-porphyrin nanoring to its 4+ and 6+ oxidation states, the nanoring becomes antiaromatic and aromatic respectively. In contrast, the neutral oxidation state exhibits only local aromaticity for the six porphyrin units. The 12+ cation can also be generated, and exhibits local antiaromaticity for each porphyrin unit. The characterisation of (anti)aromaticity employs NMR and computational techniques. In Chapter 4, the properties of cation radicals of linear and cyclic porphyrin oligomers are explored. Cations generated by spectroelectrochemistry are measured by optical spectroscopies, and chemically generated radical monocations are examined by cw/pulsed EPR spectroscopies. EPR and optical spectroscopies agree that the dimer monocation radical is fully delocalised, in Robin-Day Class III, whereas the monocations of longer oligomers are localised over 2-3 porphyrin units (Class II). In Chapter 5, photophysical and computational investigations into excited state aromaticity in porphyrin nanorings are presented. The computational results suggest the presence of aromaticity in the triplet excited states, but experiment fails to convincingly demonstrate the effect. Computational results in Chapter 6 show that a butadiyne linked [6]-porphyrin nanoring in which one butadiyne (C≡C-C≡C) is truncated to an alkyne (C≡C) exhibits a reversal of aromaticity and antiaromaticity in its oxidised states, compared to the all-butadiyne linked nanoring, consistent with Hückel's law.
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Roussely, Grégoire. "Mesures résolues en temps dans un conducteur mésoscopique." Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAY013/document.
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Au cours de la dernière décennie, un important effort a été fait dans le domaine des conducteurs électroniques de basse dimensionnalité afin de réaliser une électronique à électrons uniques. Une idée particulièrement attractive étant de pouvoir contrôler complétement la phase d’un électron unique volant pour transporter et manipuler de l’information quantique dans le but de construire un qubit volant. L’injection contrôlée d’électrons uniques dans un système électronique bidimensionnel balistique peut être fait grâce à une source d’électrons uniques basée sur des pulses de tensions lorentziens sub-nanosecondes. Une telle source peut aussi être utilisée pour mettre en évidence de nouveaux phénomènes d’interférences électroniques. Lorsqu’un pulse de tension court est injecté dans un interféromètre électronique, de nouveaux effets d’interférences sont attendus du fait de l’interaction du pulse avec les électrons de la mer de Fermi. Pour la réalisation de cette expérience, il est important de connaître avec précision la vitesse de propagation du paquet d’onde électronique créé par le pulse.Dans cette thèse, nous présentons des mesures résolues en temps d’un pulse de tension court (<100 ps) injecté dans un fil quantique 1D formé dans gaz d’électron bidimensionnel qui nous ont permis de déterminer sa vitesse de propagation. Nous montrons que le pulse se propage bien plus vite que la vitesse de Fermi d’un système sans interaction. La vitesse de propagation est augmentée par les interactions électron-électron. Pour un fil quantique contenant un grand nombre de modes, la vitesse mesurée est en excellent accord avec la vitesse d’un plasmon dans un système 2D en présence de grilles métalliques. En modifiant le potentiel de confinement électrostatique et donc l’intensité des interactions, nous montrons qu’il est possible de contrôler la vitesse de propagation. Nous avons ensuite étudié un interféromètre électronique à deux chemins basé sur deux fils couplés par une barrière tunnel. Nos mesures préliminaires font ressortir une signature qui peut être attribuée à des oscillations tunnel cohérentes des électrons injectés dans ce système. Dans un future proche, cet interféromètre pourrait être utilisé pour mettre en évidence ces nouveaux effets spectaculaires dus à l’interaction du pulse avec les électrons de la mer de FermiOver the past decade, an important effort has been made in the field of low dimensional electronic conductors towards single electron electronics with the goal to gain full control of the phase of a single electron in a solid-state system. A particular appealing idea is to use a single flying electron itself to carry and manipulate the quantum information, the so-called solid state flying qubit. On demand single electron injection into such a ballistic two-dimensional electron system can be realized by employing the recently developed single electron source based on sub-nanosecond lorentzian voltage pulses. Such a source could also be used to reveal interesting new physics. When a short voltage pulse is injected in an electronic interferometer, novel interference effects are expected due to the interference of the pulse with the surrounding Fermi sea. For the realization of such experiments it is important to know with high accuracy the propagation velocity of the electron wave packet created by the pulse.In this thesis, we present time resolved measurements of a short voltage pulse (<100 ps) injected into a 1D quantum wire formed in a two-dimensional electron gas and determine its propagation speed. We show that the voltage pulse propagates much faster than the Fermi velocity of a non-interacting system. The propagation speed is enhanced due to electron interactions within the quantum wire. For a quantum wire containing a large number of modes, the measured propagation velocity agrees very well with the 2D plasmon velocity for a gated two-dimensional electron gas. Increasing the confinement potential allows to control the strength of the electron interactions and hence the propagation speed. We then have studied an electronic two-path interferometer based on two tunnel-coupled wires. Our preliminary measurements show a signature that can be attributed to the coherent tunneling of the electrons injected into this system. In the near future, this system could be used to reveal these new striking effects due to the interaction of the voltage pulse with the Fermi sea
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PEROSA, GIOVANNI. "Impact of the Electrons Dynamics on the Free-electron Lasers Radiation Coherence." Doctoral thesis, Università degli Studi di Trieste, 2023. https://hdl.handle.net/11368/3041022.
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Modern science advancements rely on the possibility of producing short laser-like coherent pulses in the XUV and in the X-rays wavelength ranges to probe electronic structure in atoms, molecules and solid-state matter. For this reason, light-sources including synchrotrons, inverse Compton scattering, high harmonic generation in gas (HHG) and free electron lasers (FELs) are invaluable tools for research in these fields.
In particular, they all have in common the exploitation of the radiating process resulting from electrons’ acceleration under the influence of an electromagnetic field.
The aim of this thesis is to explore the impact of electrons’ dynamics on the coherence of FELs seeded by an external laser. In this thesis I demonstrate that electrons’ dynamics plays a major role in the conversion and transformation of light’s features, such as coherence, which can be transmitted to electrons and "inherited" from the re-emitted light. To fulfill this purpose, both the theoretical and the experimental approaches have been used. Most of the models presented, derived or extended in this work are, in fact, supported by experimental evidence.
The interplay between electrons and light’s properties is investigated using both classical and quantum dynamics. While the former is routinely adopted to describe the FEL dynamics and collective phenomena in an electron bunch, the latter becomes mandatory to fully achieve a faithful description of the varieties of phenomena that involve the emission of photons.
From the classical point of view, a comprehensive analytical model for electron beam longitudinal dynamics is derived by including a new phenomenon, known as intrabeam scattering, and by investigating its effect on the electrons’ distribution. The predictions of this model can be directly compared with both beam and FEL measurements, showing a good agreement with both.
From the quantum-dynamical point of view, we start to explore the possibility to answer the following question: "is it possible to introduce quantum features, such as coherence, in any process of harmonic generation from a coherent light pulse?" In order to do so, we focus our attention on the characterization of quantum coherence via photon number distribution and the quantum electrodynamics of an electron in a laser field.
The practical aspect of my investigation is threefold: the prediction and characterization of electron beam quality; the optimization of seeded and unseeded FELs performances, that is possible through the mitigation of instabilities originated in the electron bunch; the investigation of unexplored FELs features and configurations that could be exploited for novel experiments.
Finally, although the results and discussions are directly applied to the FEL case, some of the theoretical results regarding the coherence can be applied, without loss of generality, to any process of electrons-light interaction.Modern science advancements rely on the possibility of producing short laser-like coherent pulses in the XUV and in the X-rays wavelength ranges to probe electronic structure in atoms, molecules and solid-state matter. For this reason, light-sources including synchrotrons, inverse Compton scattering, high harmonic generation in gas (HHG) and free electron lasers (FELs) are invaluable tools for research in these fields. In particular, they all have in common the exploitation of the radiating process resulting from electrons’ acceleration under the influence of an electromagnetic field. The aim of this thesis is to explore the impact of electrons’ dynamics on the coherence of FELs seeded by an external laser. In this thesis I demonstrate that electrons’ dynamics plays a major role in the conversion and transformation of light’s features, such as coherence, which can be transmitted to electrons and "inherited" from the re-emitted light. To fulfill this purpose, both the theoretical and the experimental approaches have been used. Most of the models presented, derived or extended in this work are, in fact, supported by experimental evidence. The interplay between electrons and light’s properties is investigated using both classical and quantum dynamics. While the former is routinely adopted to describe the FEL dynamics and collective phenomena in an electron bunch, the latter becomes mandatory to fully achieve a faithful description of the varieties of phenomena that involve the emission of photons. From the classical point of view, a comprehensive analytical model for electron beam longitudinal dynamics is derived by including a new phenomenon, known as intrabeam scattering, and by investigating its effect on the electrons’ distribution. The predictions of this model can be directly compared with both beam and FEL measurements, showing a good agreement with both. From the quantum-dynamical point of view, we start to explore the possibility to answer the following question: "is it possible to introduce quantum features, such as coherence, in any process of harmonic generation from a coherent light pulse?" In order to do so, we focus our attention on the characterization of quantum coherence via photon number distribution and the quantum electrodynamics of an electron in a laser field. The practical aspect of my investigation is threefold: the prediction and characterization of electron beam quality; the optimization of seeded and unseeded FELs performances, that is possible through the mitigation of instabilities originated in the electron bunch; the investigation of unexplored FELs features and configurations that could be exploited for novel experiments. Finally, although the results and discussions are directly applied to the FEL case, some of the theoretical results regarding the coherence can be applied, without loss of generality, to any process of electrons-light interaction.
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Mallet, François. "Cohérence quantique, diffusion magnétique et effets topologiques." Grenoble 1, 2006. https://theses.hal.science/tel-00546850.
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Dans mon mémoire de Thèse sont regroupés des résultats expérimentaux, centrés autour de la thématique de la cohérence quantique des électrons à très basse température, obtenus à partir de mesures très précises des corrections quantiques au transport classique dans les nanostructures métalliques. Nous avons tout d'abord étudié les effets de cohérence dans des réseaux de fils métalliques. Nous avons montré l'influence de la dimension du régime de diffusion sur la cohérence. En passant d'un conducteur macroscopique à un conducteur mésoscopique, on a observé un “crossover” dimensionnel pour l'amplitude des diverses corrections quantiques quand la longueur de cohérence de phase excède la taille typique du système, ce qui nous a permis de préciser exactement ce qu'est la moyenne d'ensemble en Physique Mésoscopique. Dans la deuxième partie de ce manuscrit, nous avons présenté des mesures du temps de cohérence de phase dans des fils métalliques contenant des impuretés magnétiques. Ces échantillons ont été fabriqués d'une fa¸con originale et contrôlée en utilisant une technologie nouvelle grâce à l'utilisation d'un faisceau d'ions focalisé. Nous avons mesuré un comportement universel sur 2 decades en temperature du déphasage par impuretés implantées, ceci étant la preuve que cette décohérence supplémentaire s'inscrit dans la Physique de l'effet Kondo. Nous avons montré que le taux de déphasage mesuré est en très bon accord avec de récents calculs du Groupe de Renormalisation Numérique. Plus particulièrement, nous avons montré de façon non équivoque que l'écrantage en dessous de TK induit une désaturation du temps de cohérence de phase linéaire en température jusqu'à 0, 1 TKIn this thesis are reported experimental results centered on the thematic of the electronic quantum coherence at very low temperatures, obtained by very precise measurements of the quantum correction to the classical electronic transport in metallic nanostructures. We have first studied the coherence effects in network of metallic one-dimensional wires. We have shown the influence on the coherence itself of the diffusion dimensionality. By going from a macroscopic conductor to a purely mesoscopic one, we measured a crossover in the scaling of the quantum corrections amplitudes when the phase coherence length exceed the typical size of the system. This has allowed us to really precise what the ensemble averaging is in Mesoscopic Physics. In the second part of this work, we have shown the temperature dependence of the phase coherence length in metallic wire with magnetic impurities. These samples were fabricated in a very new and controlled way, by using a new technics with a focus ion beam. We have measured a universal behavior over 2 decades in temperature for the dephasing due to one magnetic impurity. This was the direct prove that this added decoherence belongs to the physics of the generic many body problem named « Kondo Physics ». We have finally shown that the measured dephasing rate was in excellent agreement with recent theoretical calculations based one the numerical renormalization group technics. More precisely we have shown that the magnetic impurities screening induces a linear desaturation of the phase coherence time above 0,1 TK
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Flentje, Hanno. "Coherent transfer of electron spins in tunnel-coupled quantum dots." Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAY039/document.
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De récentes avancées technologiques laissent entrevoir le potentiel des spins électroniques uniques comme supports pour le stockage et la manipulation de l'information. En raison de leur nature quantique, les spins électroniques contrôlé à l’échelle de l’électron unique peuvent non seulement être utilisés pour stocker l'information classique, mais pourraient également être mis en œuvre pour réaliser des qubits dans un ordinateur quantique. Dans un tel dispositif, les superpositions de différents états de spin peuvent être utilisées pour calculer plus efficacement que les ordinateurs classiques.Une mise en œuvre prometteuse d'un tel système est un électron piégé dans une boite quantique latérale. Ce dispositif nanométrique défini dans des structures semiconductrices permet d'isoler et de manipuler le spin d’un électron de façon cohérente avec des potentiels électrostatiques. Dans cette thèse, nous manipulons les électrons dans des boites quantiques dans un régime dit « isolé». La manipulation de charges électroniques individuelles en plusieurs boites quantiques connectées entre elles apparaît alors être simplifiée. Cette manipulation de spin se fait grâce à l’échange cohérent d’un quantum de spin entre deux électrons piégés. Le contrôle du couplage tunnel entre ces deux boites quantiques rend cet échange contrôlé. De cette façon, la manipulation de spin peut se faire à un "sweet spot", un point insensible au bruit de charge, permettant ainsi d'obtenir des oscillations de spin de haute qualité.Le contrôle précis de la charge dans le régime isolé est ensuite utilisé pour contrôler le déplacement d’un électron dans un système circulaire de trois boites quantiques qui sont fortement couplées par effet tunnel. Ainsi la cohérence d'une superposition de deux spins électroniques déplacée le long d’une boucle fermée a été étudiée. Nos mesures montrent le transport cohérent de spins électroniques uniques sur des distances allant jusqu'à 5 μm. Pendant le transfert, le temps de cohérence se révèle être considérablement augmenté. Nous avons identifié le mécanisme sous-jacent à cette amélioration comme provenant d’un rétrécissement, lors du mouvement, des gradients de champ nucléaires générées par l'environnement cristallin. Les sources de décohérence sont discutées et permettent d’obtenir de nouvelles connaissances sur la dynamique interne du processus de transfert entre des boites quantiques couplées. Nos résultats sur le transport cohérent d'électrons peuvent être utilisés pour évaluer les possibilités d’intégration à grande échelle de qubits de spin dans des réseaux de boites quantiques à deux dimensionsRecent technological advances hint at the future possibility to use single electron spins as carriers and storage of information. Due to their quantum nature, individually controlled electron spins can not only be used to store classical information, but could also find implementation as quantum bits in a quantum computer. In this envisioned device, the superposition of different spin states could be used to perform novel calculation procedures more efficiently than their classical counterparts.A promising implementation of a controllable single electron spin system is an electron trapped in a lateral quantum dot. This nanoscale solid state device allows to isolate and coherently manipulate the spin of individual electrons with electrostatic potentials. In this thesis we study electrons in quantum dot structures using a manipulation technique which we call the "isolated regime". In this regime the manipulation of individual electron charges in several connected quantum dots is shown to be simplified. This allows to implement a novel spin manipulation scheme to induce coherent exchange of a quantum of spin between two electrons via a variation of the tunnel-coupling between adjacent quantum dots. This manipulation scheme is observed to lead to a reduced sensibility to charge noise at a "sweet spot" and thereby allows to obtain high quality spin oscillations.The improved charge control in the isolated regime is then used to achieve circular coupling in a triple quantum dot device with high tunnel-rates. This allows to directly probe the coherence of a superposition of two electron spins which are displaced on a closed loop in the three quantum dots. Our measurements demonstrate coherent electron transport over distances of up to 5 μm. During the transfer the coherence time is found to be significantly increased. We identify the underlying mechanism for the enhancement with a motional narrowing of the nuclear field gradients originating from the crystal environment. The limiting decoherence source is found to be single electron spin-flips induced by a real space motion of the electrons. Our results on the coherent transport of electrons can be used to asses the scaling possibilities of spin qubit implementations on two-dimensional lattices
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Kabir, Amin. "Phase coherent photorefractive effect in II-VI semiconductor quantum wells and its application for optical coherence imaging." University of Cincinnati / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1282315981.
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Schneider, Adam. "Coherent electron transport in triple quantum dots." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=32541.
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We use a quantum master equation approach to study the transport properties of a triple quantum dot ring. Unlike double quantum dots and triple quantum dot chains, this geometry gives two transport paths with a relative phase sensitive to magnetic flux via the Aharonov-Bohm effect. This gives rise to a coherent population trapping effect and what is known as a "dark state". Unlike other master equation techniques valid only in the high bias voltage limit, our treatment reproduces such results as well as giving an analytic zero-bias conductance formula. As well as providing a more robust signature of this "dark state" physics, our model further predicts a negative differential resistance in connection with high bias rectification already predicted.Nous utilisons une approche d´equation quantique maîtresse pour étudier les propriétés de transport des points quantiques triples en forme d'anneau. Contrairement aux points quantiques doubles et triples en forme de chaînes, cette géométrie offre deux chemins pour le transport avec une phase quantique relative qui est sensible au flux magnétique en raison de l'effet Aharonov-Bohm. Ceci méne à un effet de piégeage de population cohérent et cela est connu sous le nom d'un "état sombre". Contrairement à d'autres techniques d'équation maîtresse qui sont seulement valides dans la limite d'un potentiel électrique élevé, notre méthode reproduit les résultats de ces derniers en plus de donner une expression analytique pour la conductance différentielle de zéro potentiel électrique. En plus de donner une optique plus robuste de la physique "d´etats sombres", notre modèle prédit une résistance différentielle négative qui est reliée au phénomène déjà prédit de rectification à potentiel élevé.
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Fornieri, Antonio. "Coherent manipulation of electronic heat currents in superconducting quantum circuits." Doctoral thesis, Scuola Normale Superiore, 2017. http://hdl.handle.net/11384/85898.
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Shen, Yumin. "Coherent nonlinear optics of electron spins in semiconductors /." view abstract or download file of text, 2007. http://proquest.umi.com/pqdweb?did=1417814931&sid=1&Fmt=2&clientId=11238&RQT=309&VName=PQD.
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Thesis (Ph. D.)--University of Oregon, 2007.Typescript. Includes vita and abstract. Includes bibliographical references (leaves 157-164). Also available for download via the World Wide Web; free to University of Oregon users.
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Shen, Jianqi. "Quantum Coherence and Quantum-Vacuum Effects in Some Artificial Electromagnetic Media." Doctoral thesis, KTH, Elektroteknisk teori och konstruktion, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-10074.
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The author of this thesis concentrates his attention on quantum optical properties of some artificial electromagnetic media, such as quantum coherent atomic vapors (various multilevel electromagnetically induced transparency vapors) and negative refractive index materials, and suggests some possible ways to manipulate wave propagations inside the artificial electromagnetic materials based on quantum coherence and quantum vacuum effects. In Chapters 1 and 2, the author reviews the previous papers on quantum coherence as well as the relevant work such as electromagnetically induced transparency (EIT), atomic population trapping and their various applications. The basic concepts of quantum coherence (atomic phase coherence, quantum interferences within atomic energy levels) and quantum vacuum are introduced, and the theoretical formulations for treating wave propagations in quantum coherent media are presented. In Chapter 3, the author considers three topics on the manipulation of light propagations via quantum coherence and quantum interferences: i) the evolutional optical behaviors (turn-on dynamics) of a four-level N-configuration atomic system is studied and the tunable optical behavior that depends on the intensity ratio of the signal field to the control field is considered. Some typical photonic logic gates (e.g. NOT and NOR gates) are designed based on the tunable four-level optical responses of the N-configuration atomic system; ii) the destructive and constructive quantum interferences between two control transitions (driven by the control fields) in a tripod-type four-level system is suggested. The double-control quantum interferences can be utilized to realize some photonic devices such as the logic-gate devices, e.g., NOT, OR, NOR and EXNOR gates; iii) some new quantum coherent schemes (using EIT and dressed-state mixed-parity transitions) for realizing negative refractive indices are proposed. The most remarkable characteristic (and advantage) of the present scenarios is such that the isotropic left-handed media (with microscopic structure units at the atomic level) in the optical frequency band can be achieved. Quantum vacuum (the ground state of quantized fields) can exhibit many interesting effects. In Chapter 4, we investigate two quantum-vacuum effects in artificial materials: i) the anisotropic distribution of quantum-vacuum momentum density in a moving electromagnetic medium; ii) the angular momentum transfer between quantum vacuum and anisotropic medium. Such quantum-vacuum macroscopic mechanical effects could be detected by current technology, e.g., the so-called fiber optical sensor that can measure motion with nanoscale sensitivity. We expect that these vacuum effects could be utilized to develop sensitive sensor techniques or to design new quantum optical and photonic devices.In Chapter 5, the author suggests some interesting effects due to the combination of quantum coherence and quantum vacuum, i.e., the quantum coherent effects, in which the quantum-vacuum fluctuation field is involved. Two topics are addressed: i) spontaneous emission inhibition due to quantum interference in a three-level system; ii) quantum light-induced guiding potentials for coherent manipulation of atomic matter waves (containing multilevel atoms). These quantum guiding potentials could be utilized to cool and trap atoms, and may be used for the development of new techniques of atom fibers and atom chips, where the coherent manipulation of atomic matter waves is needed.In Chapter 6, we conclude this thesis with some remarks, briefly discuss new work that deserves further consideration in the future, and present a guide to the previously published papers by us.QC 20100810
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Khastehdel, Fumani Ahmad. "QUANTUM CONFINED STATES AND ROOM TEMPERATURE SPIN COHERENCE IN SEMICONDUCTOR NANOCRYSTAL QUANTUM DOTS." Case Western Reserve University School of Graduate Studies / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=case1449151739.
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Volpato, Andrea. "Innovative Strategies in Coherent Multidimensional Electronic Spectroscopy." Doctoral thesis, Università degli studi di Padova, 2017. http://hdl.handle.net/11577/3425369.
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Recent experimental evidences of long-lived quantum electronic coherences in photosynthetic systems have focused the attention on the possible role of quantum phenomena in enhancing biologically relevant functions and the performance of man-made energy devices.
Two-dimensional electronic spectroscopy (2DES) provided several compelling evidences that sparked the discussion, but we are far from achieving clear statements.
The technique is little more than a decade old and active research is progressing on setup development and data-analysis procedures.
A high-performance setup has been built and advanced calibration procedure and acquisition schemes have been designed, in order to tackle the challenges of current instrumental implementations.
Time-frequency decomposition techniques, borrowed from the signal-processing field, have been adapted and applied to the analysis of 2DES coherent oscillating signals. Moreover, a global analysis method based on the variable projection algorithm has been developed for robust and quantitative analysis of coherence signatures. The dynamics of the relevant beating components is resolved with unmatched clarity, supplying a valuable help in their interpretation.
An oligomeric porphyrin based model system has been investigated with the developed tools. Vibrational coherences with drastically different behaviors have been analyzed bringing out the role of the disorder in modulating the coherent dynamics.La recente osservazione di coerenze quantistiche elettroniche con lunga durata in sistemi fotosintetici ha stimolato l’interesse sul possibile ruolo dei fenomeni quantistici nel migliorare alcune funzioni di rilevanza biologica e nell'aumentare le prestazioni di dispositivi artificiali. Tale interesse è stato supportato da numerose evidenze sperimentali fornite dalla spettroscopia bidimensionale elettronica (2DES) ma la ricerca in questo ambito è ancora lontana dal raggiungere conclusioni definitive. La tecnica 2DES è nata da poco più di un decennio ed è ancora molto attiva la ricerca per lo sviluppo di un apparato strumentale ottimale e di efficienti metodi di elaborazione dati. Nell'ambito del progetto di dottorato, è stato costruito un setup sperimentale ad alte prestazioni accoppiato con avanzate procedure di calibrazione e di acquisizione dati, affrontando le sfide principali delle attuali implementazioni strumentali. Attingendo dal campo ingegneristico dell’elaborazione dei segnali, le tecniche di decomposizione tempo-frequenza sono state applicate allo studio di segnali oscillanti 2DES. Inoltre, un metodo di analisi globale basato sul variable projection algorithm è stato sviluppato, allo scopo di avere uno strumento robusto e quantitativo per lo studio dei responsi coerenti. La definizione della dinamica delle componenti oscillanti si è dimostrata un valido strumento per l’interpretazione del dato sperimentale. I metodi sviluppati sono stati utilizzati per l'analisi dei dati sperimentali ottenuti con un sistema modello costituito da un oligomero con catene laterali porfiriniche. Sono state analizzate coerenze vibrazionali con caratteristiche differenti evidenziando l’influenza del disordine nel modulare la risposta coerente.
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Fleming, Stephen. "Coherent behaviour of trapped electrons in a Coulomb glass." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708604.
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Sweeney, Timothy Michael 1978. "Coherent Control of Electron Spins in Semiconductor Quantum Wells." Thesis, University of Oregon, 2011. http://hdl.handle.net/1794/11975.
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xvii, 110 p. : ill. (some col.)Electron spin states in semiconductors feature long coherence lifetimes, which have stimulated intense interest in the use of these spins for applications in spin based electronics and quantum information processing (QIP). A principal requirement for these spins to be viable candidates in QIP is the ability to coherently control the spins on timescales much faster than the decoherence times. The ability to optically control the spin state can meet this requirement. The spin states of electrons exhibit strong radiative coupling to negatively charged exciton (trion) states, and this radiative coupling makes coherent optical control of spin states possible. This dissertation presents experimental demonstration of coherent control of an electron spin ensemble in a two-dimensional electron gas in a CdTe quantum well. We present two complimentary techniques to optically manipulate these electron spins using a Raman transition. The first demonstration is with a single off-resonant ultrafast optical pulse. This ultrafast pulse acts like an effective magnetic field in the propagation direction of the optical pulse. The second experiment utilizes phase-locked Raman resonant pulse pairs to coherently rotate the quantum state, where the relative phase of the pulse pair sets the axis of rotation. The Raman pulse pair acts like a microwave field driving the spin states. This research demonstrates two significant contributions to the field of coherent optical interactions with semiconductors. First, we have advanced the potential use of electron spin ensembles in semiconductors for optics based quantum information processing hardware through our demonstration of coherent spin flips and complete coherent control. Second, we have experimentally realized full coherent control through the use of phase-locked Raman pulse pairs that overcomes inherent limitations of the single-pulse optical rotation technique, which is the current standard technique used in coherent control. This dissertation includes previously published and unpublished co-authored material.
Committee in charge: Dr. Miriam Deutsch, Chairperson; Dr. Hailin Wang, Advisor; Dr. Steven van Enk, Member; Dr. Raghuveer Parthasarathy, Member; Dr. Catherine Page, Outside Member
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Meneghin, Elena. "Coherent multidimensional electronic spectroscopy: from bioinspired to biological systems." Doctoral thesis, Università degli studi di Padova, 2017. http://hdl.handle.net/11577/3425861.
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Could quantum phenomena affect biological processes? This question has always thrilled scientists but, only from the last decade, we have been witnessing determined and rapid strides in refinement of the experimental tools capable to unravel quantum dynamics. The unique property of assessing simultaneously distinct phenomena in the ultrafast time regime makes coherent two-dimensional electronic spectroscopy the leading technique in the novel field of quantum biology.
Artificial antennas, thanks to their lower degree of complexity, represent ideal test systems to clarify the design principles that allow quantum phenomena to survive in their natural counterparts. We studied different multi-chromophoric model systems of light-harvesting complexes by self-assembling pigment-peptide conjugates. Distinct chromophore and protein components were tested to clarify which of these two parts has a crucial role in preserving quantum phenomena.
When dealing with natural light-harvesting systems, one complicating factor in the interpretation of coherent signals is that vibronic coupling gives rise to additional oscillating patterns that might overlap with electronic features. We therefore provided a detailed investigation of the peculiar signatures of isolated pigments, such as chlorophyll a and bacteriochlorophyll a.
The expertise and the knowledge matured dealing with artificial antennas and isolated chromophores have been put to use in the interpretation of the ultrafast dynamics of a naturally occurring light-harvesting system, peridinin-chlorophyll-protein.
Quantum effects may contribute, not only to the photophysical, but also to photochemical behavior of multichromophores, such as proton transfer capability. H-tunnelling is an exquisitely quantum phenomenon which is very sensitive to distance fluctuations between donor and acceptor. In this work, we exploited again two-dimensional electronic spectroscopy to explore how H-tunnelling is affected by the motions of the surrounding and, therefore, if the coupling with nuclear motion can really fasten the overall kinetics of the reaction.Fenomeni di tipo quantistico possono contribuire in modo fondamentale in processi di tipo biologico? Questa domanda ha da sempre entusiasmato gli scienziati ma, solo a partire dall’ultima decade, stiamo assistendo ai passi da giganti fatti nella messa a punto di strumenti sperimentali sempre più efficienti nel rilevare dinamiche di tipo quantistico. La spettroscopia elettronica coerente bidimensionale, grazie alla sua peculiarità di dare accesso all’osservazione simultanea di fenomeni di diversa tipologia nel regime ultraveloce, si presenta come la tecnica principe nel nuovo campo della biologia quantistica. Le antenne artificiali, grazie al loro minor grado di complessità, sono sistemi modello ideali per delucidare quali siano i principi strutturali che permettono a fenomeni di tipo quantistico di sopravvivere nei loro analoghi naturali. Abbiamo studiato diversi sistemi multi-cromoforici, modello dei complessi antenna naturali, grazie all’auto-assemblaggio di sistemi coniugati pigmento-peptide. Sono state confrontate diverse tipologie di cromoforo e di componente proteica per determinare quale delle due parti costituenti avesse un ruolo cruciale nel preservare i fenomeni quantistici. Nello studio dei sistemi antenna naturali, un fattore che può complicare ulteriormente l’interpretazione dei segnali coerenti, è l’accoppiamento vibronico, il quale dà origine a componenti oscillanti che potrebbero sovrapporsi a quelle elettroniche. Per tale ragione abbiamo provveduto a condurre un’indagine dettagliata dei contributi peculiari di pigmenti isolati come la clorofilla a e la batterioclorofilla a. L’esperienza e la conoscenza maturate nello studio di sistemi antenna artificiali e di cromofori isolati hanno permesso l’interpretazione della dinamica ultraveloce di un complesso antenna naturale, la peridinin-chlorophyll-protein. Effetti di tipo quantistico possono influenzare non solo processi di tipo fotofisico, ma anche reazioni fotochimiche come, ad esempio, il trasferimento di protoni. Il tunnelling protonico, infatti, è un fenomeno di natura squisitamente quantistica e che risulta molto sensibile alle fluttuazioni della distanza tra donatore e accettore. In questo lavoro abbiamo utilizzato spettroscopia elettronica coerente bidimensionale per esplorare come il tunnelling protonico possa essere influenzato dai moti dell’intorno e, di conseguenza, come la cinetica globale della reazione possa essere velocizzata dall’accoppiamento con moti nucleari.
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Cooper-Roy, Alexandre. "Coherent control of electron spins in diamond for quantum information science and quantum sensing." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/111688.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear 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 115-122).
This thesis introduces and experimentally demonstrates coherent control techniques to exploit electron spins in diamond for applications in quantum information processing and quantum sensing. Specifically, optically-detected magnetic resonance measurements are performed on quantum states of single and multiple electronic spins associated with nitrogen-vacancy centers and other paramagnetic centers in synthetic diamond crystals. We first introduce and experimentally demonstrate the Walsh reconstruction method as a general framework to estimate the parameters of deterministic and stochastic fields with a quantum probe. Our method generalizes sampling techniques based on dynamical decoupling sequences and enables measuring the temporal profile of time-varying magnetic fields in the presence of dephasing noise. We then introduce and experimentally demonstrate coherent control techniques to identify, integrate, and exploit unknown quantum systems located in the environment of a quantum probe. We first locate and identify two hybrid electron-nuclear spins systems associated with unknown paramagnetic centers in the environment of a single nitrogen-vacancy center in diamond. We then prepare, manipulate, and measure their quantum states using cross-polarization sequences, coherent feedback techniques, and quantum measurements. We finally create and detect entangled states of up to three electron spins to perform environment-assisted quantum metrology of time-varying magnetic fields. These results demonstrate a scalable approach to create entangled states of many particles with quantum resources extracted from the environment of a quantum probe. Applications of these techniques range from real-time functional imaging of neural activity at the level of single neurons to magnetic resonance spectroscopy and imaging of biological complexes in living cells and characterization of the structure and dynamics of magnetic materials.
by Alexandre Cooper-Roy.
Ph. D.
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Rohr, Sven. "Hybrid spin-nanomechanical systems in parametric interaction." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENY046/document.
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L'exploration du monde quantique au moyen d'objets macroscopiques constitue l'un des défis centraux de ces dernières décennies pour la recherche en physique. Parmi les systèmes proposés pour atteindre cet objectif, les systèmes hybrides, qui couplent un résonateur nanomécanique à un qubit unique, font figure de paradigme.L'excitation cohérente d'un oscillateur mécanique macroscopique par un unique spin électronique ouvrirait en particulier de nouvelles perspectives pour la création d'états quantiques arbitraires du mouvement.Dans ce manuscrit, nous considérons un système hybride constitué d'un oscillateur nanomécanique et du spin électronique d'un unique centre NV, couplés entre eux par une interaction magnétique. Nous nous concentrons sur le cas d'une interaction paramétrique où la vibration mécanique module l'énergie du qubit, et plus précisément sur le cas où le qubit ainsi forcé et l'oscillateur mécanique évoluent sur des échelles de temps comparables.Dans cette situation, nos observations montrent une synchronisation de la dynamique du qubit sur l'oscillation mécanique. Le phénomène est dans un premier temps abordé par une expérience-test qui remplace le mouvement mécanique par un champ radiofréquence en couplage paramétrique avec le spin. Cette première implémentation permet de dégager les propriétés essentielles de l'effet paramétrique, qui est dans un second temps observé sur l'expérience principale.Dans cette seconde expérience, un centre NV est attaché à l'extrémité d'un nanofil de carbure de silicium en vibration placé dans un fort gradient de champ magnétique. Le caractère bidimensionnel des déformations du nanofil octroie alors à la synchronisation des signatures vectorielles encore inédites, qui peuvent aussi être interprétées comme la manifestation d'un triplet de Mollow phononique, ainsi qu'il a été observé dans les premières expériences d'électrodynamique quantique.Finalement, nous explorons la robustesse de la synchronisation vis-à-vis du mouvement Brownien du résonateur, et démontrons la possibilité de protéger le qubit de cette source de décohérence additionnelle grâce à une excitation mécanique de faible amplitudeProbing the quantum world with macroscopic objects has been a core challenge for research in physics during the past decades. Proposed systems to reach this goal include hybrid devices that couple a nanomechanical resonator to a single spin qubit. In particular, the coherent actuation of a macroscopic mechanical oscillator by a single electronic spin would open perspectives in the creation of arbitrary quantum states of motion.In this manuscript, we investigate a hybrid system coupling a nanomechanical oscillator and a single electronic spin of a NV defect in magnetic interaction. We focus on the parametric interaction case, when the mechanical motion modulates the qubit energy, and in particular when the driven qubit and mechanical oscillators evolves on similar timescales. In that situation a synchronization of the qubit dynamics onto the mechanical motion is observed. The phenomenon is first explored on a test experiment where mechanical motion is replaced by a parametrically coupled RF field. It allows to establish the main properties of the phenomenon, which is subsequently investigated on the core experiment. It consists of a NV defect attached at the vibrating extremity of a silicon carbide nanowire, immersed in a strong magnetic field gradient. The bidimensional character of the nanowire deformations is responsible for novel vectorial signatures in the synchronization, which can also be viewed as a phononic Mollow triplet as observed in early quantum electrodynamics experiments. We finally explore the robustness of the synchronization against the Brownian motion of the resonator and demonstrate the possibility to protect the qubit against this additional decoherence source by applying a small coherent mechanical drive
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Bode, Niels [Verfasser]. "Coherent electrons and collective modes in quantum-transport through nanostructures / Niels Bode." Berlin : Freie Universität Berlin, 2012. http://d-nb.info/1027815944/34.
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Dongol, Amit. "Carrier Dynamics and Application of the Phase Coherent Photorefractive Effect in ZnSe Quantum Wells." University of Cincinnati / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1396453493.
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Yang, Jamie Chiaming. "Coherent control of hyperfine-coupled electron and nuclear spins for quantum information processing." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/44789.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2008.Includes bibliographical references (p. 81-87).
Coupled electron-nuclear spins are promising physical systems for quantum information processing: By combining the long coherence times of the nuclear spins with the ability to initialize, control, and measure the electron spin state, the favorable properties of each spin species are utilized. This thesis discusses a procedure to initialize these nuclear spin qubits, and presents a vision of how these systems could be used as the fundamental processing unit of a quantum computer. The focus of this thesis is on control of a system in which a single electron spin is coupled to N nuclear spins via resolvable anisotropic hyperfine (AHF) interactions. High-fidelity universal control of this le-Nn system is possible using only excitations on a single electron spin transition. This electron spin actuator control is implemented by using optimal control theory to find the modulation sequences that generate the desired unitary operations. Decoherence and the challenge of making useful qubits from these systems are also discussed. Experimental evidence of control using an electron spin actuator was acquired with a custom-built pulsed electron spin resonance spectrometer. Complex modulation sequences found by the GRadient Ascent Pulse Engineering (GRAPE) algorithm were used to perform electron spin echo envelope modulation (ESEEM) experiments and simple preparation-quantum operation-readout experiments on an ensemble of 1e-1n systems. The data provided evidence that we can generate any unitary operation on an AHF-coupled 1e-1n system while sitting on a single transmitter frequency. The data also guided design of the next iteration of these experiments, which will include an improved spectrometer, bandwidth-constrained GRAPE, and samples with larger Hilbert spaces.
by Jamie Chiaming Yang.
Ph.D.
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Oleary, Shannon 1977. "Coherent optical manipulation of electron spins in semiconductor nanostructures." Thesis, University of Oregon, 2008. http://hdl.handle.net/1794/8449.
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xiv, 114 p. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.Electron spin coherence can arise through a coherent superposition of two spin states in the conduction band of a semiconductor and can persist over remarkably long time and length scales. The robust nature of electron spin coherence makes it an excellent model system for exploring coherent quantum phenomena in semiconductors. This dissertation presents both spectral- and time-domain nonlinear optical studies of electron spin coherence through Λ-type three-level systems in two- and zero-dimensional semiconductor systems. The spectral domain study focuses on the experimental realization of electromagnetically induced transparency (EIT), a phenomenon that exploits destructive interference induced by the spin coherence. Coherent Zeeman Resonance (CZR), a precursor to EIT, is demonstrated in two 2D systems, a GaAs mixed-type quantum well (MTQW) and a modulation doped CdTe quantum well (QW). For these studies, Λ-type three-level systems are formed via dipole coupling of a trion to two electron spin states. The CZR response can be described qualitatively by effective density matrix equations. In addition, effects of manybody Coulomb interactions on CZR are investigated by varying the electron density in the MTQW via optical carrier injection. Time-domain studies based on transient differential transmission (DT) are carried out to explore the excitation, manipulation, and detection of electron spin coherence and to better understand how manybody interactions affect coherent nonlinear optical processes in semiconductors. While electron spin coherence can be formed and detected via resonant excitation of excitons or trions, a surprising observation is that injecting excitons into the 2D electron gas in a modulation doped CdTe QW can significantly alter the oscillatory nonlinear response of the electron spin coherence, while the response remains qualitatively unchanged when trions are injected. These behaviors are attributed to an interplay between manybody effects and carrier heating generated by trion formation from excitons. Finally, donor-bound electrons in GaAs are used as a model of localized electron spins. Spin decoherence of order 10 ns, limited by nuclear hyperfine interactions, is observed. Electron spin rotation induced by a nearly resonant laser pulse is also observed, opening the door for further work on mitigating electron spin decoherence time through optical spin echoes.
Adviser: Hailin Wang
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Phelps, Carey E. 1982. "Ultrafast Coherent Electron Spin Control and Correlated Tunneling Dynamics of Two-Dimensional Electron Gases." Thesis, University of Oregon, 2011. http://hdl.handle.net/1794/11936.
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xvi, 143 p. : ill. (some col.)Electron spins form a two-level quantum system in which the remarkable properties of quantum mechanics can be probed and utilized for many applications. By learning to manipulate these spins, it may be possible to construct a completely new form of technology based on the electron spin degree of freedom, known as spintronics. The most ambitious goal of spintronics is the development of quantum computing, in which electron spins are utilized as quantum bits, or qubits, with properties that are not possible with classical bits. Before these ideas can become reality, a system must be found in which spin lifetimes are long enough and in which spins can be completely controlled. Semiconductors are an excellent candidate for electron spin control since they can be integrated into on-chip devices and produced on a scalable level. The focus of this dissertation is on electron spin control in two different semiconductor systems, namely a two-dimensional electron gas in a modulation-doped quantum well and donor-bound electrons in bulk semiconductors. Both systems have been studied extensively for a variety of purposes. However, the ability to manipulate spins has been elusive. In this dissertation, the first experimentally successful demonstration of electron spin control in a two-dimensional electron gas is presented, in which ultrafast optical pulses induce spin rotations via the optical Stark effect. Donor-bound electron spin manipulation in bulk semiconductors is also investigated in this dissertation. Important information was obtained on the limiting factors that serve to prohibit spin control in this system. By taking these new factors into account, it is our hope that full electron spin control can eventually be accomplished in this system. Finally, through the course of investigating electron spin dynamics, a strange nonlinear optical behavior was observed in a bilayer system, which was determined to result from a coupling of optical interactions with tunneling rates between layers. The data suggest that there is a strong interplay between interlayer and intralayer correlations in this system. Investigations into the nature of this interaction were undertaken and are presented in the last part of this dissertation. This dissertation includes previously published and unpublished co-authored material.
Committee in charge: Dr. Daniel Steck, Chair; Dr. Hailin Wang, Advisor; Dr. Jens Nockel, Inside; Dr. John Toner, Inside; Dr. Andrew Marcus, Outside
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Paul, Jagannath. "Coherent Response of Two Dimensional Electron Gas probed by Two Dimensional Fourier Transform Spectroscopy." Scholar Commons, 2017. http://scholarcommons.usf.edu/etd/6738.
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Advent of ultrashort lasers made it possible to probe various scattering phenomena in materials that occur in a time scale on the order of few femtoseconds to several tens of picoseconds. Nonlinear optical spectroscopy techniques, such as pump-probe, transient four wave mixing (TFWM), etc., are very common to study the carrier dynamics in various material systems. In time domain, the transient FWM uses several ultrashort pulses separated by time delays to obtain the information of dephasing and population relaxation times, which are very important parameters that govern the carrier dynamics of materials. A recently developed multidimensional nonlinear optical spectroscopy is an enhanced version of TFWM which keeps track of two time delays simultaneously and correlate them in the frequency domain with the aid of Fourier transform in a two dimensional map. Using this technique, the nonlinear complex signal field is characterized both in amplitude and phase. Furthermore, this technique allows us to identify the coupling between resonances which are rather difficult to interpret from time domain measurements. This work focuses on the study of the coherent response of a two dimensional electron gas formed in a modulation doped GaAs/AlGaAs quantum well both at zero and at high magnetic fields.
In modulation doped quantum wells, the excitons are formed as a result of the inter- actions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the formation of Mahan excitons, which is also referred to as Fermi edge singularity (FES). Polarization and temperature dependent rephasing 2DFT spectra in combination with TI-FWM measurements, provides insight into the dephasing mechanism of the heavy hole (HH) Mahan exciton. In addition to that strong quantum coherence between the HH and LH Mahan excitons is observed, which is rather surprising at this high doping concentration. The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence be destroyed as a result of the screening and electron-electron interactions. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum 2DFT spectra. Theoretical simulations based on the optical Bloch Equations (OBE) where many-body effects are included phenomenologically, corroborate the experimental results. Time-dependent density functional theory (TD-DFT) calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system. Furthermore, in semiconductors under the application of magnetic field, the energy states in conduction and valence bands become quantized and Landau levels are formed. We observe optical excitation originating from different Landau levels in the absorption spectra in an undoped and a modulation doped quantum wells. 2DFT measurements in magnetic field up to 25 Tesla have been performed and the spectra reveal distinct difference in the line shapes in the two samples. In addition, strong coherent coupling between landau levels is observed in the undoped sample. In order to gain deeper understanding of the observations, the experimental results are further supported with TD-DFT calculation.
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Brown, Richard Matthew. "Coherent transfer between electron and nuclear spin qubits and their decoherence properties." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:21e043b7-3b72-44d7-8095-74308a6827dd.
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Conventional computing faces a huge technical challenge as traditional transistors will soon reach their size limitations. This will halt progress in reaching faster processing speeds and to overcome this problem, require an entirely new approach. Quantum computing (QC) is a natural solution offering a route to miniaturisation by, for example, storing information in electron or nuclear spin states, whilst harnessing the power of quantum physics to perform certain calculations exponentially faster than its classical counterpart. However, QCs face many difficulties, such as, protecting the quantum-bit (qubit) from the environment and its irreversible loss through the process of decoherence. Hybrid systems provide a route to harnessing the benefits of multiple degrees of freedom through the coherent transfer of quantum information between them. In this thesis I show coherent qubit transfer between electron and nuclear spin states in a 15N@C60 molecular system (comprising a nitrogen atom encapsulated in a carbon cage) and a solid state system, using phosphorous donors in silicon (Si:P). The propagation uses a series of resonant mi- crowave and radiofrequency pulses and is shown with a two-way fidelity of around 90% for an arbitrary qubit state. The transfer allows quantum information to be held in the nuclear spin for up to 3 orders of magnitude longer than in the electron spin, producing a 15N@C60 and Si:P ‘quantum memory’ of up to 130 ms and 1.75 s, respectively. I show electron and nuclear spin relaxation (T1), in both systems, is dominated by a two-phonon process resonant with an excited state, with a constant electron/nuclear T1 ratio. The thesis further investigates the decoherence and relaxation properties of metal atoms encapsulated in a carbon cage, termed metallofullerenes, discovering that exceptionally long electron spin decoherence times are possible, such that these can be considered a viable QC candidate.
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Zeltzer, Gabriel. "Accessing electronic and vibronic quanta and their coherent interactions in atomically precise nanostructures /." May be available electronically:, 2007. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
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Thibierge, Étienne. "Cohérence à un et deux électrons en optique quantique électronique." Thesis, Lyon, École normale supérieure, 2015. http://www.theses.fr/2015ENSL0998/document.
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Cette thèse se place dans le domaine du transport quantique cohérent, et vise à développer un formalisme adapté à la modélisation d'expériences réalisées dans les canaux de bord de l'effet Hall quantique entier. Ce formalisme repose sur les analogies entre ces expériences et celles de l'optique quantique photonique.Le manuscrit commence par une introduction au contexte de la thèse qui propose un tour d'horizon des enjeux, des outils et des succès de l'optique quantique électronique.La première partie du travail traite des propriétés de cohérence mono-électronique et introduit la notion clé d'excès de cohérence à un électron. Plusieurs représentations sont proposées et analysées, permettant d’accéder aux informations physiques contenues dans la fonction de cohérence. Les états émis par des sources à électrons utilisées par plusieurs groupes expérimentaux sont ensuite analysés sous cet angle.Les effets à deux électrons sont au cœur de la seconde partie. L'excès de cohérence à deux électrons est défini en prenant en compte les effets de corrélation classique et d'échange quantique. Les conséquences de l'anti-symétrie fermionique sont également analysées en détail, montrant une redondance dans les informations encodées dans la cohérence à deux électrons. Enfin, un degré de cohérence normalisé est introduit pour étudier plus directement les effets d'indiscernabilité et d'anti-bunching.La mesure et la manipulation de la cohérence électronique par interférométrie sont abordées dans la troisième partie. Dans un premier temps, le lien entre les fonctions de cohérence électronique et les quantités directement accessibles dans les expériences est établi, ce qui justifie le besoin de protocoles plus complexes. La mesure d'excès de cohérence à un électron est alors envisagée par interférométrie Mach-Zehnder à un électron, puis par interférométrie Hong-Ou-Mandel à deux électrons, ce qui suggère une interprétation plus simple d'un protocole de tomographie électronique établi en 2011. Un protocole de mesure de l'excès de cohérence à deux électrons est ensuite proposé par interférométrie de type Franson, étendant les idées relatives à la mesure de cohérence à un électron par un interféromètre de Mach-Zehnder. Enfin, une vision complémentaire est apportée sur l'interféromètre de Franson, en utilisant celui-ci cette fois pour générer une cohérence à deux électrons non localeThis thesis deals with coherent quantum transport and aims at developing a formalism well suited to model experiments conducted in edge channels of integer quantum Hall effect. This formalism relies on analogies between these experiments and photon quantum optics ones.The manuscript begins with an introduction to the context of the thesis and an overview of issues, tools and successes of electron quantum optics.The first part of the work addresses the question of single electron coherence properties and introduces the key notion of excess of single electron coherence. Several representations are proposed and analyzed, giving access to physical informations encoded in the coherence function. The quantum states emitted by experimentally demonstrated electron sources are then analyzed under this perspective.Two electron effects are at the heart of the second part. The excess of two-electron coherence is defined taking into account both classical correction and quantum exchange effects. A detailed analysis of consequences of fermionic anti-symmetry is provided and shows that information encoded into two-electron coherence is redundant. Last, a normalized degree of coherence is introduced in view of a more direct study of indistinguishability and anti-bunching.The issue of measuring and manipulating electronic coherence by interferometry is addressed in the third part. First the relation between electronic coherence functions and directly measurable quantities in experiments is established, justifying the need for more involved measurement protocols. The measure of the excess of single electron coherence is envisioned through single electron Mach-Zehnder interferometry and two-electron Hong-Ou-Mandel interferometry, suggesting a simpler interpretation of a tomography protocol established in 2011. A protocol for measuring the excess of two-electron coherence is then proposed by Franson-like interferometry, which generalizes the ideas used for measuring single electron coherence with a Mach-Zehnder interferometer. Last, a complementary point of view on Franson interferometer is given, by using it to generate a non-local two-electron coherence
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Eickemeyer, Felix. "Ultrafast dynamics of coherent intersubband polarizations in quantum wells and quantum cascade laser structures." Doctoral thesis, [S.l.] : [s.n.], 2002. http://dochost.rz.hu-berlin.de/dissertationen/eickemeyer-felix-2002-07-03.
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Roussel, Benjamin. "Autopsy of a quantum electrical current." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSE1285/document.
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Les expériences de physique quantique ont atteint un niveau de contrôle permettant de préparer avec précision l'état quantique de nombreux systèmes physiques. Cela a mené à la naissance de l'optique quantique électronique, un sujet émergent qui vise à préparer, manipuler et caractériser l'état de courants électriques contenant quelques excitations électroniques se propageant dans un conducteur quantique ballistique. Ceci est un défi conséquent qui se heurte à la difficulté de caractériser un état quantique à N corps.Le sujet de cette thèse sera le développement de méthodes de traitement du signal quantique permettant d'accéder à une connaissance partielle d'un tel état pour des courants électriques quantiques. Une première méthode consiste à les analyser à nombre d'excitations fixé au travers des cohérences électroniques. Pour cela, nous élaborons une analyse de la cohérence à un électron en termes d'atomes de signaux électroniques. En combinant cela au protocole de tomographie par interférometrie HOM, nous présentons la première autopsie, fonction d'onde par fonction d'onde, d'un courant électrique quantique.Une autre approche consiste à examiner des indicateurs sondant directement l'état à N corps. Nous étudions le rayonnement émis par un conducteur quantique ainsi que la décohérence électronique d'une excitation à un électron. Ensuite nous analysons la distribution de probabilité de la chaleur dissipée par un système quantique mésoscopique. Dans ce cadre, nous développons une théorie de l'effet Joule en régime quantique et à explorons comment celle-ci pourrait permettre de sonder l'état à N corpsQuantum physics experiments have reached a level of precision and control that allows quantum state engineering for many systems. This has led to the birth of electron quantum optics, an emerging field which aims at generating, manipulating and characterizing quantum electrical currents built from few-electron excitations propagating within ballistic quantum conductors. This is challenging since it is generically impossible in practice to fully characterize the many-body state of a beam containing indistinguishable electrons. The thesis presents new quantum signal processing approaches for accessing, at least partially, to the quantum many-body state of quantum electrical currents.A first approach is to access such a state at few-particle levels through electronic coherences. We will thus present a new representation of single-electron coherence in terms of electronic "atoms of signal". Combining this signal processing algorithm to HOM tomography enables us to present the first autopsy, wavefunction by wavefunction, of an experimental electrical quantum current. Another method is to look for indicators giving information directly at the many-body level. We will investigate the radiation emitted by a quantum conductor and address the problem of decoherence of a general single-electron excitation. Finally, we will look at the heat deposited by a mesoscopic quantum system, leading to a quantum version of Joule heating and discuss how it gives an insight on the many-body state of the electron fluid
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Gamble, Stephanie Nicole. "Conical Intersections and Avoided Crossings of Electronic Energy Levels." Diss., Virginia Tech, 2021. http://hdl.handle.net/10919/101899.
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We study the unique phenomena which occur in certain systems characterized by the crossing or avoided crossing of two electronic eigenvalues. First, an example problem will be investigated for a given Hamiltonian resulting in a codimension 1 crossing by implementing results by Hagedorn from 1994. Then we perturb the Hamiltonian to study the system for the corresponding avoided crossing by implementing results by Hagedorn and Joye from 1998. The results from these demonstrate the behavior which occurs at a codimension 1 crossing and avoided crossing and illustrates the differences. These solutions may also be used in further studies with Herman-Kluk propagation and more.
Secondly, we study codimension 2 crossings by considering a more general type of wave packet. We focus on the case of Schrödinger equation but our methods are general enough to be adapted to other systems with the geometric conditions therein. The motivation comes from the construction of surface hopping algorithms giving an approximation of the solution of a system of Schrödinger equations coupled by a potential admitting a conical intersection, in the spirit of Herman-Kluk approximation (in close relation with frozen/thawed approximations). Our main Theorem gives explicit transition formulas for the profiles when passing through a conical crossing point, including precise computation of the transformation of the phase and its proof is based on a normal form approach.Doctor of Philosophy
We study energies of molecular systems in which special circumstances occur. In particular, when these energies intersect, or come close to intersecting. These phenomena give rise to unique physics which allows special reactions to occur and are thus of interest to study. We study one example of a more specific type of energy level crossing and avoided crossing, and then consider another type of crossing in a more general setting. We find solutions for these systems to draw our results from.
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LI, XIAOXU. "WAVELENGTH-DIVISION-MULTIPLEXED TRANSMISSION USING SEMICONDUCTOR OPTICAL AMPLIFIERS AND ELECTRONIC IMPAIRMENT COMPENSATION." Doctoral diss., University of Central Florida, 2009. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/4025.
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Over the last decade, rapid growth of broadband services necessitated research aimed at increasing transmission capacity in fiber-optic communication systems. Wavelength division multiplexing (WDM) technology has been widely used in fiber-optic systems to fully utilize fiber transmission bandwidth. Among optical amplifiers for WDM transmission, semiconductor optical amplifier (SOA) is a promising candidate, thanks to its broad bandwidth, compact size, and low cost. In transmission systems using SOAs, due to their large noise figures, high signal launching powers are required to ensure reasonable optical signal-to-noise ratio of the received signals. Hence the SOAs are operated in the saturation region and the signals will suffer from SOA impairments including self-gain modulation, self-phase modulation, and inter channel crosstalk effects such as cross-gain modulation, cross-phase modulation, and four-wave mixing in WDM. One possibility to circumvent these nonlinear impairments is to use constant-intensity modulation format in the 1310 nm window where dispersion is also negligible. In this dissertation, differential phase-shift keying (DPSK) WDM transmission in the 1310 nm window using SOAs was first considered to increase the capacity of existing telecommunication network. A WDM transmission of 4 x 10 Gbit/s DPSK signals over 540 km standard single mode fiber (SSMF) using cascaded SOAs was demonstrated in a recirculating loop. In order to increase the transmission reach of such WDM systems, those SOA impairments must be compensated. To do so, an accurate model for quantum-dot (QD) SOA must be established. In this dissertation, the QD-SOA was modeled with the assumption of overall charge neutrality. Static gain was calculated. Optical modulation response and nonlinear phase noise were studied semi-analytically based on small-signal analysis.
The quantitative studies show that an ultrafast gain recovery time of ~0.1 ps can be achieved when QD-SOAs are under high current injection, which leads to high saturation output power. However more nonlinear phase noise is induced when the QD-SOAs are used in the transmission systems operating at 10 Gbit/s or 40 Gbit/s. Electronic post-compensation for SOA impairments using coherent detection and digital signal processing (DSP) was investigated next in this dissertation. An on-off keying transmission over 100 km SSMF using three SOAs at 1.3 [micrometer] were demonstrated experimentally with direct detection and SOA impairment compensation. The data pattern effect of the signal was compensated effectively. Both optimum launching power and Q-factor were improved by 8 dB. For advanced modulation formats involving phase modulation or in transmission windows with large dispersion, coherent detection must be used and fiber impairments in WDM systems need to be compensated as well. The proposed fiber impairment compensation is based on digital backward propagation. The corresponding DSP implementation was described and the required calculations as well as system latency were derived. Finally joint SOA and fiber impairment compensations were experimentally demonstrated for an amplitude-phase-shift keying transmission.Ph.D.
Optics and Photonics
Optics and Photonics
Optics PhD
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Botzem, Tim [Verfasser], Jörg Hendrik [Akademischer Betreuer] Bluhm, and Ferdinand [Akademischer Betreuer] Kuemmeth. "Coherence and high fidelity control of two-electron spin qubits in GaAs quantum dots / Tim Botzem ; Jörg Hendrik Bluhm, Ferdinand Kuemmeth." Aachen : Universitätsbibliothek der RWTH Aachen, 2017. http://d-nb.info/1162498404/34.
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Ullah, Saeed. "Optical control and detection of spin coherence in multilayer systems." Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-10052017-163058/.
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Since a decade, spintronics and related physics have attracted considerable attention due to the massive research conducted in these areas. The main reason for growing interest in these fields is the expectation to use the electrons spin instead of or in addition to the charge for the applications in spin-based electronics, quantum information, and quantum computation. A prime concern for these spins to be possible candidates for carrying information is the ability to coherently control them on the time scales much faster than the decoherence times. This thesis reports on the spin dynamics in two-dimensional electron gases hosted in artificially grown III-V semiconductor quantum wells. Here we present a series of experiments utilizing the techniques to optically control the spin polarization triggered by either optical or electrical methods i.e. well known pump-probe technique and current-induced spin polarization. We investigated the spin coherence in high mobility dense two-dimensional electron gas confined in GaAs/AlGaAs double and triple quantum wells, and, it\'s dephasing on the experimental parameters like applied magnetic field, optical power, pump-probe delay and excitation wavelength. We have also studied the large spin relaxation anisotropy and the influence of sample temperature on the long-lived spin coherence in triple quantum well structure. The anisotropy was studied as a function sample temperature, pump-probe delay time, and excitation power, where, the coherent spin dynamics was measured in a broad range of temperature from 5 K up to 250 K using time-resolved Kerr rotation and resonant spin amplification. Additionally, the influence of Al concentration on the spin dynamics of AlGaAs/AlAs QWs was studied. Where, the composition engineering in the studied structures allows tuning of the spin dephasing time and electron g-factor. Finally, we studied the macroscopic transverse drift of long current-induced spin coherence using non-local Kerr rotation measurements, based on the optical resonant amplification of the electrically-induced polarization. Significant spatial variation of the electron g-factor and the coherence times in the nanosecond scale transported away half-millimeter distances in a direction transverse to the applied electric field was observed.Há uma década, a spintrônica e outras áreas relacionadas vêm atraindo considerável atenção, devido a enorme quantidade de pesquisa conduzidas por elas. A principal razão para o crescente interesse neste campo é a expectativa da aplicação do controle do spin do elétron no lugar ou em adição à carga, em dispositivos eletrônicos e informação e computação quânticas. A possibilidade destes spins carregarem informação depende, primeiramente, da habilidade de controlá-los coerentemente, em uma escala de tempo muito mais rápida do que o tempo de decoerência. Esta tese trata da dinâmica de spins em gases de elétrons bidimensionais, em poços quânticos de semicondutores III-V, crescidos artificialmente. Nós apresentamos uma série de experimentos, utilizando técnicas para o controle ótico da polarização de spin, desencadeadas por métodos óticos ou eletrônicos, ou seja, técnicas conhecidas de bombeio e prova e polarização de spin induzida por corrente. Nós investigamos a coerência de spin em gases bidimensionais, confinados em poços quânticos duplos e triplos de GaAs/AlGaAs e a dependência da defasagem com parâmetros experimentais, como campo magnético externo, potência ótica, tempo entre os pulsos de bombeio e prova e comprimento de onda da excitação. Também estudamos a grande anisotropia de relaxação de spin como função da temperatura da amostra, potência de excitação e defasagem entre bombeio e prova, medidos para uma vasta gama de temperatura, entre 5K e 250K, usando Rotação de Kerr com Resolução Temporal (TRKR) e Amplificação Ressonante de Spin (RSA). Além disso estudamos a influência da concentração de Al na dinâmica dos poços de AlGaAs/AlAs, para o qual a engenharia da composição da estrutura permite sintonizar o tempo de defasagem de spin e o fator $ g $ do elétron. Por fim, estudamos a deriva transversal macroscópica da longa coerência de spin induzida por corrente, através de medidas de Rotação de Kerr não-locais, baseadas na amplificação ressonante ótica da polarização eletricamente induzida. Observamos uma variação espacial significante do fator $ g $ e do tempo de vida da coerência, na escala de nanosegundos, deslocada distâncias de meio milímetro na direção transversa ao campo magnético aplicado.
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Dai, Zhenting. "Coherent and Dissipative Transport in Metallic Atomic-Size Contacts." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/19880.
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Thin-film niobium mechanically controlled break junctions and resistively shunted niobium mechanically-controlled break junctions were developed and successfully microfabricated. Using these devices, high-stability atomic size contacts were routinely produced and investigated both in the normal and superconducting states. Investigations of the two-level conductance fluctuations in the smallest contacts allowed the calculation of their specific atomic structure. Embedding resistive shunts close to the superconducting atomic-sized junctions affected the coherence of the electronic transport. Finally, point contact spectroscopy measurements provide evidence of the interaction of conduction electrons with the mechanical degrees of freedom of the atomic-size niobium contacts.
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Priebe, Katharina Elisabeth [Verfasser], Claus [Akademischer Betreuer] Ropers, Stefan [Gutachter] Mathias, and Thomas [Gutachter] Baumert. "Coherent Control and Reconstruction of Free-Electron Quantum States in Ultrafast Electron Microscopy / Katharina Elisabeth Priebe ; Gutachter: Stefan Mathias, Thomas Baumert ; Betreuer: Claus Ropers." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2018. http://d-nb.info/114995471X/34.
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Varwig, Steffen [Verfasser], Manfred [Akademischer Betreuer] Bayer, and Metin [Gutachter] Tolan. "Optical electron spin tomography and hole spin coherence studies in (In,Ga)As/GaAs quantum dots / Steffen Varwig. Betreuer: Manfred Bayer. Gutachter: Metin Tolan." Dortmund : Universitätsbibliothek Dortmund, 2014. http://d-nb.info/1100692487/34.
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Varwig, Steffen [Verfasser], Manfred Akademischer Betreuer] Bayer, and Metin [Gutachter] [Tolan. "Optical electron spin tomography and hole spin coherence studies in (In,Ga)As/GaAs quantum dots / Steffen Varwig. Betreuer: Manfred Bayer. Gutachter: Metin Tolan." Dortmund : Universitätsbibliothek Dortmund, 2014. http://nbn-resolving.de/urn:nbn:de:101:1-201605191564.
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Biggs, Jason Daniel 1978. "Theoretical studies of the external vibrational control of electronic excitation transfer and its observation using polarization- and optical phase-sensitive ultrafast spectroscopy." Thesis, University of Oregon, 2010. http://hdl.handle.net/1794/11074.
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xvi, 218 p. : ill. (some col.)Our theoretical studies involve the control of electronic energy transfer in molecular dimers through the preparation of specific vibrational coherences prior to electronic excitation. Our control strategy is based upon the fact that, following impulsive electronic excitation, nuclear motion acts to change the instantaneous energy difference between site-excited electronic states and thereby influences short-time electronic excitation transfer (EET). By inducing coherent intramolecular vibration in one of the chromophores prior to short-pulse electronic excitation, we exert external control over electronic dynamics. As a means to monitor this coherent control over EET, we propose using multidimensional wave-packet interferometry (md-WPI). Two pairs of polarized phase-related femtosecond pulses following the control pulse would generate superpositions of coherent nuclear wave packets in optically accessible electronic states. Interference contributions to the time- and frequency-integrated fluorescence signal due to overlaps among the superposed wave packets provide amplitude-level information on the nuclear and electronic dynamics. We test both the control strategy and its spectroscopic investigation by calculating pump-probe difference signals for various combinations of pulse polarizations. That signal is the limiting case of the control-influenced md-WPI signal in which the two pulses in the pump pulse-pair coincide, as do the two pulses in the probe pulse-pair. We present calculated pump-probe difference signals for a variety of systems including a simplified model of the covalent dimer dithia-anthracenophane (DTA) in which we treat only the weakly Franck-Condon active ν 12 anthracene vibration at 385 cm -1 . We further present calculated nl-WPI difference signals for an oriented DTA complex, which reveal amplitude-level dynamical information about the interaction of nuclear motion and electronic energy transfer. We also present pump-probe difference signals from a model system in which a CF 3 group, whose torsional angle is strongly Franck-Condon active, has been added to the anthracene monomers which make up DTA. We make use of electronic structure calculations to find the torsional potential of the monomer, from which we calculate the spectroscopic signals of the dimer. We show that a significant measure of control over short-time EET is achievable in this system. This dissertation includes previously published coauthored material.
Commitee in charge: Dr. Michael E. Kellman, Chair; Dr. Jeffrey A. Cina, Advisor; Dr. David R. Herrick; Dr. Andrew H. Marcus; Dr. Daniel A. Steck
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Thiele, Stefan. "Read-out and coherent manipulation of an isolated nuclear spin using a single-molecule magnet spin-transistor." Phd thesis, Université de Grenoble, 2014. http://tel.archives-ouvertes.fr/tel-00984973.
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La réalisation d'un ordinateur quantique fonctionnel est l'un des objectifs tech- nologiques les plus ambitieux pour les scientifiques d'aujourd'hui. Sa brique de base est composée d'un système quantique à deux niveaux, appelé bit quantique (ou qubit). Parmi les différents concepts existants, les dispositifs à base de spin sont très attractifs car ils bénéficient de la progression constante des techniques de nanofabrication et permettent la lecture électrique de l'état du qubit. Dans ce contexte, les dispositifs à base de spins nucléaires offrent un temps de cohérence supérieur à celui des dispositifs à base de spin electronique en raison de leur meilleure isolation à l'environnement. Mais ce couplage faible a un prix: la détection et la manipulation des spins nucléaires individuels restent des tâches difficiles. De très bonnes conditions expérimentales étaient donc essentielles pour la réussite de ce projet. Outre des systèmes de filtrage des radiofréquences à très basses températures et des amplificateurs à très faible bruit, j'ai développé de nouveaux supports d'échantillons et des bobines de champ magnétique trois axes compacts avec l'appui des services techniques de l'Institut Néel. Chaque partie a été optimisée afin d'améliorer la qualité de l'installation et évaluée de manière quantitative. Le dispositif lui-même, un qubit réalisé grâce à un transistor de spin nucléaire, est composé d'un aimant à molécule unique couplé à des électrodes source, drain et grille. Il nous a permis de réaliser la lecture électrique de l'état d'un spin nucléaire unique, par un processus de mesure non destructif de son état quantique. Par conséquent, en sondant les états quantique de spin plus rapidement que le temps de relaxation caractéristique de celui-ci, nous avons réalisé la mesure de la trajectoire quantique d'un qubit nucléaire isolé. Cette expérience a mis en lumière le temps de relaxation T1 du spin nucléaire ainsi que son mécanisme de relaxation dominant. La manipulation cohérente du spin nucléaire a été réalisée en utilisant des champs électriques externes au lieu d'un champ magnétique. Cette idée originale a plusieurs avantages. Outre une réduction considérable du chauffage par effet Joule, les champs électriques permettent de contrôler et de manipuler le spin unique de façon très rapide. Cependant, pour coupler le spin à un champ électrique, un processus intermédiaire est nécessaire. Un tel procédé est l'interaction hyperfine, qui, si elle est modifiée par un champ électrique, est également désigné sous le nom d'effet Stark hyperfin. En utilisant cet effet, nous avons mis en évidence la manipulation cohérente d'un spin nucléaire unique et déterminé le temps de cohérence T2 . En outre, l'exploitation de l'effet Stark hyperfin statique nous avons permis de régler le qubit de spin nucléaire à et hors résonance par l'intermédiaire de la tension de grille. Cela pourrait être utilisé pour établir le contrôle de l'intrication entre les différents qubits nucléaires. En résumé, nous avons démontré pour la première fois la possibilité de réaliser et de manipuler un bit quantique basé sur un aimant à molécule unique, étendant ainsi le potentiel de la spintronique moléculaire au delà du stockage de données classique. De plus, la grande polyvalence des molécules aimants est très prometteuse pour une variété d'applications futures qui, peut-être un jour, parviendront à la réalisation d'un ordinateur quantique moléculaire.
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Liang, Dong. "Semiconductor Nanowires: Synthesis and Quantum Transport." Case Western Reserve University School of Graduate Studies / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=case1327641946.
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Pramanik, Sandipan. "Spin Polarized Transport in Nanoscale Devices." VCU Scholars Compass, 2006. http://scholarscompass.vcu.edu/etd/1092.
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The ultimate goal in the rapidly burgeoning field of spintronics is to realize semiconductor-based devices that utilize the spin degree of freedom of a single charge carrier (electron or hole) or an ensemble of such carriers to achieve novel and/or enhanced device functionalities such as spin based light emitting devices, spin transistors and femto-Tesla magnetic field sensors. These devices share a common feature: they all rely on controlled transport of spins in semiconductors. A prototypical spintronic device has a transistor-like configuration in which a semiconducting channel is sandwiched between two contacts (source and drain) with a gate electrode sitting on top of the channel. Unlike conventional charge-based transistors, the source electrode of a spin transistor is a ferromagnetic (or half-metallic) material which injects spin polarized electrons in the channel. During transit, the spin polarizations of the electrons are controllably rotated by a gate electric field mediated spin-orbit coupling effect. The drain contact is ferromagnetic (or half-metallic) as well and the transmission probability of an electron through this drain electrode depends on the relative orientation of electron spin polarization and the (fixed) magnetization of the drain. When the spins of the electrons are parallel to the drain magnetization, they are transmitted by the drain resulting in a large device current (ON state of spin FET). However, these electrons will be completely blocked if their spins are antiparallel to the drain magnetization, and ideally, in this situation device current will be zero (OFF state of spinFET). Thus, if we vary the gate voltage, we can modulate the channel current by controlling the spin orientations of the electrons with respect to the drain magnetization. This is how transistor action is realized (Datta-Das model). However, during transport, electrons' velocities change randomly with time due to scattering and hence different electrons experience different spin-orbit magnetic fields. As a result, even though all electrons start their journey with identical spin orientations, soon after injection spins of different electrons point along different directions in space. This randomization of initial spin polarization is referred to as spin relaxation and this is detrimental to the spintronic devices. In particular, for Datta-Das transistor, this will lead to inefficient gate control and large leakage current in the OFF state of the spinFET. The aim of this work is to understand various spin relaxation processes that are operative in semiconductor nanostructures and to indicate possible ways of minimizing them. The theoretical aspect of this work (Chapters 2-5) focuses on the D'yakonov-Perel' process of spin relaxation in a semiconductor quantum wire. This process of spin relaxation occurs because during transport electron spin precesses like a spinning top about the spin-orbit magnetic field. We show that the conventional drift-diffusion model of spin transport, which has been used extensively in literature, completely breaks down in case of a quantum confined system (e.g. a quantum wire). Our approach employs a semi-classical model which couples the spin density matrix evolution with the Boltzmann transport equation. Using this model we have thoroughly studied spin relaxation in a semiconductor quantum wire and identified several inconsistencies of the drift-diffusion formalism.The experimental side of this work (Chapters 6-8) deals with two different issues: (a) performing spin transport experiments in order to extract spin relaxation length and time in various materials (e.g. Cu, Alq3) under one-dimensional confinement, and (b) measurement of the ensemble spin dephasing time in self-assembled cadmium sulfide quantum dots using electron spin resonance technique. The spin transport experiment, as described in Chapter 7 of this dissertation, shows that the spin relaxation time in organic semiconductor (Alq3) is extremely long, approaching a few seconds at low temperatures. Alq3 is the chemical formula of tris- 8 hydroxy-quinoline aluminum, which is a small molecular weight organic semiconductor. This material is extensively used in organic display industry as the electron transport and emission layer in green organic light emitting diodes. The long spin relaxation time in Alq3 makes it an ideal platform for spintronics. This also indicates that it may be possible to realize spin based organic light emitting diodes which will have much higher internal quantum efficiency than their conventional non-spin counterparts. From spin transport experiments mentioned above we have also identified Elliott-Yafet mode as the dominant spin relaxation mechanism operative in organic semiconductors. Electron spin resonance experiment performed on self-assembled quantum dots (Chapter 8) allows us to determine the ensemble spin dephasing time (or transverse spin relaxation time) of electrons confined in these systems. In quantum dots electrons are strongly localized in space. Surprisingly, the ensemble spin dephasing time shows an increasing trend as we increase temperature. The most likely explanation for this phenomenon is that spin dephasing in quantum dots (unlike quantum wells and wires) is dominated by nuclear hyperfine interaction, which weakens progressively with temperature. We hope that our work, which elaborates on all of the above mentioned topics in great detail, will be a significant contribution towards the current state of knowledge of subtle spin-based issues operative in nanoscale device structures, and will ultimately lead to realization of novel nano-spintronic devices.
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García, Arellano Guadalupe. "Influence of the concentration and temperature on the spin relaxation time of donor-bound electrons immersed in a CdTe quantum well." Thesis, Sorbonne université, 2019. http://www.theses.fr/2019SORUS109.
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Ce travail présente une étude de l'influence de la concentration de dopage, de la température et du champ magnétique longitudinal sur le temps de relaxation de spin des électrons liés aux donneurs immergés au milieu d'un puits quantique (PQ) de CdTe. En insérant les donneurs dans un PQ, les règles de sélection optique de la lumière polarisée circulairement sont purifiées, ce qui permet un meilleur degré d'orientation optique des spins des électrons que dans les cristaux 3D. En utilisant une technique de rotation Faraday photo-induite, nous mesurons d’abord le temps de relaxation de spin des électrons liés aux donneurs pour différentes concentrations de dopage à basse température dans le régime isolant. Ensuite, pour évaluer les mécanismes de relaxation de spin dans notre système, nous calculons l'énergie d'échange d'une paire d'électrons liés aux donneurs immergés au milieu d'un PQ infini, pour toute distance inter-donneur et différentes épaisseurs. En utilisant ce calcul, nous expliquons le comportement expérimental comme une interaction de deux mécanismes : l’interaction hyperfine et l’anisotrope d’échange. De plus, nous déterminons la constante spin-orbite dans CdTe αso = 0.079. Ensuite, nous présentons le développement d’une expérience pompe-sonde étendue permettant de mesurer les temps de relaxation de spin à l’échelle microseconde. Nous discutons brièvement des premiers résultats expérimentaux pour le temps de relaxation de spin longitudinal d'électrons liés aux donneurs immergés dans un PQ de CdTe avec différentes concentrations de dopage. Enfin, nous étudions l'évolution en température de la relaxation de spin de 10 à 80 K. On explique le comportement expérimental en invoquant l'échange de spin entre les électrons localisés et le spin d'électrons promus en états de conduction. Le spin des électrons localisés subit l’effet des interactions hyperfine et anisotrope, le mécanisme de D’yakonov-Perel’ régit la relaxation de spin des électrons de conductionThis work presents a study of the influence of doping concentration, temperature and longitudinal magnetic field on the spin relaxation time of donor-bound electrons immersed in the middle of a CdTe quantum well (QW). By inserting the donors in a QW, the optical selection rules for circularly polarized light are purified, allowing a higher degree of optical orientation of the electron spins than in 3D crystals. By using a photo-induced Faraday rotation technique, we first measure the spin relaxation time of donor-bound electrons for different doping concentrations at low temperature in the insulating regime. Then, in order to evaluate the spin relaxation mechanisms in our system, we calculate the exchange energy of a pair of donor-bound electrons immersed in the middle of an infinite QW, for any inter-donor distance and for different thicknesses. By using this calculation, we explain the experimental behavior as an interplay of two mechanisms: hyperfine and anisotropic exchange interactions. Moreover we determine the CdTe spin-orbit constant: αso = 0.079. Afterwards we present the development of an extended pump-probe experiment allowing to measure spin relaxation times at the microsecond scale. We briefly discuss the first experimental results for the longitudinal spin relaxation time of donor-bound electrons immersed in a CdTe QW with different doping concentrations. Finally, we investigate the temperature evolution of the spin relaxation in the range 10-80 K. The experimental behavior is explained by invoking spin exchange between electron spins localized on donors and the spin of electrons promoted to conduction states. The spin of localized electrons undergoes the effect of hyperfine and anisotropic exchange interactions, the D’yakonov-Perel’ mechanism governs the spin relaxation of the conduction electrons
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Bertrand, Benoit. "Long-range transfer of spin information using individual electrons." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAY020/document.
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L'usage du spin des électrons pour le traitement de l'information est devenu un vaste sujet de recherche aujourd'hui, notamment grâce aux nombreuses possibilités qui en découlent. Les recherches actuelles s'étendent de la génération de courants polarisés en spin à la manipulation cohérente de spin d'électrons uniques dans des boîtes quantiques, avec des applications en électronique de spin ou en information quantique. L'objectif de cette thèse est d'étendre le développement de l'électronique de spin à l'échelle de l'électron unique. Pour cela, nous cherchons à accomplir le transport cohérent d'un spin d'électron entre deux boites quantiques. Cela constituerait un moyen prometteur d'interconnecter les différents nœuds d'un nanoprocesseur quantique. Le principe utilisé repose sur l'emploi d'ondes acoustiques de surface qui, grâce aux propriétés piézoélectriques du matériau, permettent la génération de boites quantiques en mouvement. Tout d'abord, une étude de l'injection d'un électron dans une de ces boites quantiques en mouvement a été effectuée. Le contrôle à la nanoseconde de ce processus a été démontré grâce à l'application de pulses de tension modifiant pendant un bref instant le potentiel qui confine l'électron. Dans un deuxième temps, la préparation d'une superposition cohérente d'états de spin a été réalisée à l'aide d'une double boite quantique isolée, dans une position compatible avec le transport par onde acoustique de surface. Enfin, le transport d'information de spin, codée sur un unique ou sur deux électrons, a été accompli avec une fidélité atteignant 30%Recently a growing interest emerged towards the use of electron spins for information processing. The current developments range from the generation of spin polarized currents to the coherent manipulation of single electron spins in quantum dots, with applications in spintronics and quantum information processing respectively. The main objective of this thesis was to develop the equivalent of spintronics at the single electron level. For that purpose, we try to achieve the coherent transport of a single electron spin between distant quantum dots. This could be a promising means of interconnecting different nodes of a quantum nanoprocessor. The electron transfer is ensured by a surface acoustic wave (SAW) that induces dynamical quantum dots thanks to the material piezoelectricity. First, the injection of a single electron from a static to a dynamical quantum dot has been studied. It enables the control of single electron transfer with unity probability down to the nanosecond timescale, thanks to a fast engineering of the static confining potential. Next, we demonstrate the possibility to prepare a coherent spin superposition, using an isolated double quantum dot in a metastable position that is compatible with SAW-assisted electron transfer. This type of isolated dot systems offers more liberty in terms of control. Taking advantage of this feature, a new scheme for coherent spin manipulations has been implemented and proved to have reduced noise sensitivity. Finally, transfer of spin information encoded in one or two electrons has been achieved, with fidelities reaching 30%
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Brasseur, Paul. "Mach Zehnder interferometry and coherent manipulation of the valley in a graphene PN junction." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASP012.
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L’optique quantique électronique, i.e. la réalisation de l’analogue électronique d’expériences d’optique quantique, constitue un champ de recherche récent, en plein développement, et offrant des perspectives intéressantes pour l’informatique quantique. Dans ce cadre, l’un des enjeux est la réalisation de bits quantiques en utilisant des états électroniques, ainsi que la formation d’états électroniques intriqués, éléments de bases pour réaliser des calculs quantiques plus élaborés. Les expériences menées jusqu’à présent dans des hétérostructures semi-conductrices de GaAs/AlGaAs ont mis en évidence la possibilité d’encoder l’information dans la charge ou le spin d’un électron, mais la décohérence importante de ces systèmes induit une grande fragilité de ces états quantiques, qui ne peuvent exister qu’en-dessous de 100mK et pour des tensions résiduelles inférieures à 40μV. Cette fragilité rend difficile la fabrication d’états intriqués, et est limitante pour le développement de calculs quantiques complexes. En 2005, la découverte d’un matériau novateur, le graphène, a ouvert de nouvelles perspectives avec la prédiction d’une cohérence de phase plus grande, et, d’autre part, l’existence en plus du spin d’un nouveau degré de liberté, la vallée, donnant accès à de nouvelles possibilités pour encoder l’information. Dans un premier temps, ce travail de thèse porte sur la manipulation cohérente de la vallée, nécessaire à la réalisation d’un bit quantique de vallée dans le graphène. Pour cela est utilisée, en régime Hall quantique, une jonction pn, formée à l’aide de grilles déposées sur un échantillon de graphène encapsulé dans du nitrure de Bohr. Afin d’obtenir un contrôle électrostatique sur la polarisation en vallée des électrons incidents, des grilles locales ont été déposées, à l’intersection de la jonction pn avec le bord physique du graphène. En alliant ce contrôle électrostatique à celui de la phase Aharanov-Bohm, il nous est possible de manipuler de manière cohérente la vallée d’un électron sur l’ensemble de la sphère de Bloch représentant la polarisation en vallée. Dans la suite, la cohérence des états quantiques formés est étudiée grâce à un interféromètre de Mach Zehnder, via l’observation de la dépendance des interférences en fonction de la tension appliquée sur les électrons incidents, et de la température du système. Les états quantiques obtenus sont exceptionnellement résistants, ils persistent au-delà de 1.5K et de 1mV, soit à des énergies près de 20 fois supérieures à celles observées dans le GaAs/AlGaAs.Puis, ce manuscrit décrit l’étude de la longueur de cohérence, correspondant à la distance sur laquelle un électron peut se propager en gardant sa cohérence de phase, ce qui n’avait encore jamais été mesuré dans le graphène. Pour ce faire, la dépendance des interférences vis-à-vis de la température a été mesurée sur trois jonctions pn de longueurs différentes. Une longueur de cohérence a ainsi été extraite pour les deux régimes de décohérence observés ; dont une record, pour le régime à basses températures, de plus de 374μm à 20mK. Pour finir, est investigué un des mécanismes causant la décohérence dans le système : les ondes de spin, se propageant lorsque le cœur du graphène est magnétique. Ainsi, au cours de ce projet, nous avons mis en évidence la possibilité d’encoder de l’information dans la vallée, ouvrant la voie vers un nouveau domaine : la vallée-tronique. D’autre part, la cohérence du système est exceptionnelle, permettant d’envisager la réalisation d’états intriqués grâce à une géométrie de double Mach Zehnder. Cela offre des perspectives prometteuses du point de vue de l’informatique quantique, mais aussi d’un point de vue fondamental avec la possibilité de démontrer pour la première fois, avec des fermions, la validité des prédictions de l’interprétation de Copenhague de la physique quantique dans le cadre du paradoxe EPRElectron quantum optics, i.e. the realization of the electronic analogue of quantum optics experiments, represents a developing and recent research field, offering interesting perspectives for quantum computing. In this context, one of the main stakes is the achievement of quantum bits using electronic states, as well as the creation of entangled electronic states, which are the building blocks to achieve complex quantum computations. Up to now, the experiments carried out in semi-conducting GaAs/AlGaAs heterostructures exhibited the possibility to encode information in the charge or the spin of an electron, but strong decoherence in these systems implies a great weakness of these quantum states, which survives only below temperatures of 100mK and electrical biases of 40μV. This fragility makes it difficult to achieve entangled states and limits the development of complex quantum computations. In 2005, the discovery of a novel material, graphene, opened new prospects with on one hand the prediction of a larger phase coherence, and on the other hand the existence, in addition to the spin, of a new degree of freedom, named the valley, giving access to new possibilities to encode information. In a first part, this PhD work deals with the coherent manipulation of the valley, which is necessary to achieve a valley quantum bit in graphene. For this aim, we used, in the quantum Hall regime, a graphene pn junction, formed thanks to gates deposited on top of a stack composed of a graphene sheet encapsulated in Boron nitride crystals. In order to obtain an electrostatic control of the valley polarization of incoming electrons, we deposited local gates at the intersections between the pn junction and the graphene physical edge. Associating this electrostatic control to a tuning of the Aharanov-Bohm phase, we can coherently manipulate the valley of an electron over the whole states described by a valley Bloch sphere. In what follows, the coherence of the quantum states is investigated thanks to Mach Zehnder interferometry, by measuring the interferences dependence on the chemical potential of incoming electrons and on the temperature of the system. The quantum states formed are exceptionally steady, they persist up to 1.5K and 1mV, in other words at energies 20 times higher than what was observed in GaAs/AlGaAs.Then, the manuscript describes the study of the coherence length, i.e. the distance on which an electron can propagate while keeping its phase coherence, which has never been measured in the quantum Hall regime in graphene. To that end, the interferences dependence on the temperature was measured in three pn junctions of different lengths. By doing so, two coherence lengths, corresponding to two different regimes of decoherence, were extracted; in the regime occurring at low temperature, a record value of 374μm at 20mK was obtained.Finally, we investigated one of the mechanisms of decoherence in our system: spin waves, propagating in the graphene bulk when it is magnetized. During this project, we have shown the possibility to encode information in the valley and to manipulate coherently this degree of freedom, paving the way towards a new domain: the valleytronics. Furthermore, the coherence of the system is exceptional, enabling to envision the achievement of entangled electronic states by using a double Mach Zehnder interferometer geometry. This opens promising prospects for quantum computing, but also for fundamental purposes, with the possibility to demonstrate, for the first time with fermions, the validity of the Copenhagen interpretation of quantum physics within the EPR paradox framework
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Kruglyak, Yu A. "Non-Equilibrium Green’s Function Method in Matrix Representation and Model Transport Problems of Nanoelectronics." Thesis, Sumy State University, 2013. http://essuir.sumdu.edu.ua/handle/123456789/35352.
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Non-equilibrium Green’s functions method in matrix representation is presented and applied to model transport problems for 1D and 2D conductors using a nearest neighbor orthogonal tight-binding model in the frame of the «bottom-up» approach of modern nanoelectronics. Simple methods to account for electric contacts in Schrödinger equation to solve quantum electron transport problems are given.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35352
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Ruess, Frank Joachim Physics Faculty of Science UNSW. "Atomically controlled device fabrication using STM." Awarded by:University of New South Wales. Physics, 2006. http://handle.unsw.edu.au/1959.4/24855.
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We present the development of a novel, UHV-compatible device fabrication strategy for the realisation of nano- and atomic-scale devices in silicon by harnessing the atomic-resolution capability of a scanning tunnelling microscope (STM). We develop etched registration markers in the silicon substrate in combination with a custom-designed STM/ molecular beam epitaxy system (MBE) to solve one of the key problems in STM device fabrication ??? connecting devices, fabricated in UHV, to the outside world. Using hydrogen-based STM lithography in combination with phosphine, as a dopant source, and silicon MBE, we then go on to fabricate several planar Si:P devices on one chip, including control devices that demonstrate the efficiency of each stage of the fabrication process. We demonstrate that we can perform four terminal magnetoconductance measurements at cryogenic temperatures after ex-situ alignment of metal contacts to the buried device. Using this process, we demonstrate the lateral confinement of P dopants in a delta-doped plane to a line of width 90nm; and observe the cross-over from 2D to 1D magnetotransport. These measurements enable us to extract the wire width which is in excellent agreement with STM images of the patterned wire. We then create STM-patterned Si:P wires with widths from 90nm to 8nm that show ohmic conduction and low resistivities of 1 to 20 micro Ohm-cm respectively ??? some of the highest conductivity wires reported in silicon. We study the dominant scattering mechanisms in the wires and find that temperature-dependent magnetoconductance can be described by a combination of both 1D weak localisation and 1D electron-electron interaction theories with a potential crossover to strong localisation at lower temperatures. We present results from STM-patterned tunnel junctions with gap sizes of 50nm and 17nm exhibiting clean, non-linear characteristics. We also present preliminary conductance results from a 70nm long and 90nm wide dot between source-drain leads which show evidence of Coulomb blockade behaviour. The thesis demonstrates the viability of using STM lithography to make devices in silicon down to atomic-scale dimensions. In particular, we show the enormous potential of this technology to directly correlate images of the doped regions with ex-situ electrical device characteristics.
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Xu, Qing. "Détection Optique Homodyne: application à la cryptographie quantique." Phd thesis, Télécom ParisTech, 2009. http://pastel.archives-ouvertes.fr/pastel-00005580.
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Les réseaux et systèmes de télécommunications mondiaux fondent aujourd'hui leur confidentialité sur la cryptographie classique, qui repose sur des hypothèses mathématiques fragiles. La distribution quantique de clef (QKD) est aujourd'hui la seule façon connue pour distribuer des clefs avec une sécurité inconditionnelle. Ce travail de thèse contribue à combler de manière pluridisciplinaire et polyvalente le gap entre les limites physiques fondamentales et l'implémentation expérimentale, en termes de vitesse, fiabilité et robustesse. Dans un premier temps, nous avons donc proposé une implémentation du protocole BB84 utilisant les états de phase cohérents. Le récepteur homodyne a été conçu de manière à compenser les fluctuations de phase et de polarisation dans les interféromètres, ainsi que dans le reste du canal de propagation. Ensuite, nous avons mis en place un dispositif expérimental de système QKD à la longueur d'onde 1550 nm, avec une modulation QPSK fonctionnant avec un trajet et un sens de parcours uniques, dans une fibre optique mono-mode. Les deux schémas de détection: le comptage de photons (PC) et la détection homodyne équilibrée (BHD) ont été mis en œuvre. Enfin, nous avons effectué des comparaisons théoriques et expérimentales de ces deux récepteurs. Le récepteur BHD a été élaboré avec une décision à double seuil. La mise en œuvre d'un tel processus accepte des mesures non-conclusives, et réduit l'efficacité de génération des clés, mais reste encore bien meilleur que celle des PCs à 1550 nm. Nous avons également prouvé que ce système est robust sous la plupart des attaques potentielles.
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Xu, Qing. "Détection optique homodyne : application à la cryptographie quantique." Phd thesis, Paris, ENST, 2009. https://pastel.hal.science/pastel-00005580.
Full textAbstract:
Les réseaux et systèmes de télécommunications mondiaux fondent aujourd'hui leur confidentialité sur la cryptographie classique, qui repose sur des hypothèses mathématiques fragiles. La distribution quantique de clef (QKD) est aujourd'hui la seule façon connue pour distribuer des clefs avec une sécurité inconditionnelle. Ce travail de thèse contribue à combler de manière pluridisciplinaire et polyvalente le gap entre les limites physiques fondamentales et l'implémentation expérimentale, en termes de vitesse, fiabilité et robustesse. Dans un premier temps, nous avons donc proposé une implémentation du protocole BB84 utilisant les états de phase cohérents. Le récepteur homodyne a été conçu de manière à compenser les fluctuations de phase et de polarisation dans les interféromètres, ainsi que dans le reste du canal de propagation. Ensuite, nous avons mis en place un dispositif expérimental de système QKD à la longueur d'onde 1550 nm, avec une modulation QPSK fonctionnant avec un trajet et un sens de parcours uniques, dans une fibre optique mono-mode. Les deux schémas de détection: le comptage de photons (PC) et la détection homodyne équilibrée (BHD) ont été mis en œuvre. Enfin, nous avons effectué des comparaisons théoriques et expérimentales de ces deux récepteurs. Le récepteur BHD a été élaboré avec une décision à double seuil. La mise en œuvre d'un tel processus accepte des mesures non-conclusives, et réduit l'efficacité de génération des clés, mais reste encore bien meilleur que celle des PCs à 1550 nm. Nous avons également prouvé que ce système est robust sous la plupart des attaques potentiellesNowadays the information security and privacy of the telecommunications Networks are based on the classical cryptography, which relies on the fragile mathematical assumptions. The quantum key distribution (QKD) is presently the only known way to distribute keys with unconditional security. This thesis aims to apply a multidisciplinary versatile approach to fill the gap between the fundamental physical limits and the experimental system implementations, in terms of speed, reliability and robustness. First of all, we proposed a BB84 protocol implementation using coherent phase states. The homodyne receiver was designed to compensate the phase and polarization fluctuations in the interferometers, as well as in the rest of the propagation channel. Then we established an experimental one-way QKD system operating at 1550 nm Telecom wavelength in a single mode fiber link, with QPSK modulation. Both the photon counting detection (PC) and the balanced homodyne detection (BHD) schemes have been implemented. Finally, we conducted theoretical and experimental comparisons of these two receivers. The BHD receiver has been improved with a dual-threshold post-decision. The implementation of such a process accepts non-conclusive measurements, and reduced key generation efficiency, but its permanence remains still better than the PC receiver at 1550 nm. We also proved that this system is robust under most common potential attacks