Dissertations / Theses: 'Decoherence'

1

ASPREA, LORENZO. "Gravitational Decoherence." Doctoral thesis, Università degli Studi di Trieste, 2021. http://hdl.handle.net/11368/2981626.

Full text

Abstract:

The recent exciting first detections of gravitational waves, which marked a new era in astrophysics and cosmology, have pushed the scientific community towards the construction of ever more sophisticated ground and space based detectors to observe waves in a variety of ranges, possibly down to the cosmic background gravitational radiation. Detecting the latter would open the possibility to gain crucial information about the universe at its very primordial stage, at about 10^(-22) s after the Big Bang, where we expect our description of gravity to fail, especially because of its unclear relation with quantum matter. Most gravitational waves (which can be thought of as small perturbations of the metric propagating through spacetime at the speed of light) that arrive on the Earth are produced by different unresolved mechanisms and sources, and thus result in a stochastic perturbation of the flat spacetime background. Within the framework of quantum theory, this altered background affects the dynamics of matter propagation and, when the quantum state is in a superposition, it leads to decoherence effects, as it's typical of any noisy environment. In this scenario, the extreme sensitivity of matter waves to gravity gradients makes matter-wave interferometers a perfect candidate for exploring the gravitational wave background and, at the same time, for possibly answering some fundamental questions regarding the nature of gravity, and its coupling to quantum matter. Besides the technological challenge of building sensitive (therefore large) enough matter-wave interferometers, which realistically would have to operate in outer space, even from the theoretical point of view it is not clear how they would respond to a gravitational background produced by random sources, as no comprehensive dynamical description of the gravity induced decoherence process has been so far proposed. The decoherence effect of a stochastic (or quantum) perturbation of the metric has in fact been studied by several authors, each of whom has produced a different model for the evolution of off-diagonal elements of the density matrix of a quantum state or, more generally, the loss of interference in the system. However, that of giving a universal and meaningful description of the phenomenon is still an open problem, as the different models so far proposed refer to particular regimes of approximation and thus seem to lead to different and apparently incompatible conclusions. The goal of our work is to formulate a more general description of gravity induced deocherence, in the form of a master equation, which is able to encompass the existing literature and explain the apparent discrepancies, as well as extend the so far know results. With a more general and unambiguous dynamics, we aim at assessing whether and to what extent matter-wave interferometers constitute a viable platform for probing of the cosmic gravitational background.

APA, Harvard, Vancouver, ISO, and other styles

2

Abyaneh, Varqa. "Gravitationally induced decoherence." Thesis, University of York, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.428052.

Full text

APA, Harvard, Vancouver, ISO, and other styles

3

Dodd, Peter James. "Decoherence and emergent classicality." Thesis, Imperial College London, 2004. http://hdl.handle.net/10044/1/11529.

Full text

APA, Harvard, Vancouver, ISO, and other styles

4

Helm, Julius. "Classical vs. Quantum Decoherence." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-84542.

Full text

Abstract:

Based on the superposition principle, any two states of a quantum system may be coherently superposed to yield a novel state. Such a simple construction is at the heart of genuinely quantum phenomena such as interference of massive particles or quantum entanglement. Yet, these superpositions are susceptible to environmental influences, eventually leading to a complete disappearance of the system's quantum character. In principle, two distinct mechanisms responsible for this process of decoherence may be identified. In a classical decoherence setting, on the one hand, stochastic fluctuations of classical, ambient fields are the relevant source. This approach leads to a formulation in terms of stochastic Hamiltonians; the dynamics is unitary, yet stochastic. In a quantum decoherence scenario, on the other hand, the system is described in the language of open quantum systems. Here, the environmental degrees of freedom are to be treated quantum mechanically, too. The loss of coherence is then a direct consequence of growing correlations between system and environment. The purpose of the present thesis is to clarify the distinction between classical and quantum decoherence. It is known that there exist decoherence processes that are not reconcilable with the classical approach. We deem it desirable to have a simple, feasible model at hand of which it is known that it cannot be understood in terms of fluctuating fields. Indeed, we find such an example of true quantum decoherence. The calculation of the norm distance to the convex set of classical dynamics allows for a quantitative assessment of the results. In order to incorporate genuine irreversibility, we extend the original toy model by an additional bath. Here, the fragility of the true quantum nature of the dynamics under increasing coupling strength is evident. The geometric character of our findings offers remarkable insights into the geometry of the set of non-classical decoherence maps. We give a very intuitive geometrical measure---a volume---for the quantumness of dynamics. This enables us to identify the decoherence process of maximum quantumness, that is, having maximal distance to the convex set of dynamics consistent with the stochastic, classical approach. In addition, we observe a distinct correlation between the decoherence potential of a given dynamics and its achievable quantumness. In a last step, we study the notion of quantum decoherence in the context of a bipartite system which couples locally to the subsystems' respective environments. A simple argument shows that in the case of a separable environment the resulting dynamics is of classical nature. Based on a realistic experiment, we analyze the impact of entanglement between the local environments on the nature of the dynamics. Interestingly, despite the variety of entangled environmental states scrutinized, no single instance of true quantum decoherence is encountered. In part, the identification of the classical nature relies on numerical schemes. However, for a large class of dynamics, we are able to exclude analytically the true quantum nature.

APA, Harvard, Vancouver, ISO, and other styles

5

Branderhorst, Matthijs Pieter Arie. "Coherent control of decoherence." Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.670035.

Full text

APA, Harvard, Vancouver, ISO, and other styles

6

FAROOQ, UMER. "Decoherence in Quantum Networks." Doctoral thesis, Università degli Studi di Camerino, 2015. http://hdl.handle.net/11581/401743.

Full text

Abstract:

The title of the present dissertation Decoherence in Quantum Network sounds very general and all-inclusive. Indeed it embraces two topics (decoherence and quantum network) from the area of Quantum Mechanics each of which is described in all respects by a huge literature developed in the last three decades [...]. Quantum decoherence, as the name lets it mean, is the mechanism that makes a quantum system loose its coherence properties, and with them the capability of giving rise to interference phenomena or to other interesting quantum effects [...]. The key idea promoted by decoherence is the insight that realistic quantum systems are never isolated, but are immersed in the surrounding environment and interact continuously with it [...]. As an example one may consider a two-level quantum system (i.e. a quantum bit, usually shortened with a terminology from information science to \qubit" ) in contact with a wide environment. Hence, quantum systems are open systems, and continuously interact or exchange information with an external environment whose degrees of freedom are too numerous to be monitored. The resulting correlation between the system and the environment spoils quantum coherence and brings about the transition from a pure quantum state to a mixture of quantum state resulting a classical state. To describe decoherence different kind of approaches can be used (for example Master equation, random matrix, etc). A quantum network typically consists of a number of quantum objects (e.g., atoms, ions, quantum dots, cavities, etc.), to be referred to hereafter as the sites or the nodes of the network. They can interact and the interactions (or their correlations) will be usually described by the edges of a graph. Quantum networks can address different information processing tasks. For instance a quantum state can be transferred from qubit to qubit down a chain solely due to the interactions, that is according to the laws of quantum physics [...]. Quantum networks offer us new opportunities and phenomena as compared to classical networks. An extension to large scale of the idea of a quantum network could lead to a futurible quantum internet [...]. The study of networks has traditionally been the territory of graph theory [...], also with the advent of their quantum versions. Within simple quantum network model information processing is usually described by assuming perfect control of the underlying graph. However, this is not much realistic since randomness is often present and it leads to decoherence effects [...]. In contrast, the conservation of coherence is essential for any quantum information process [...], hence there is a persistent interest in decoherence effects in quantum networks, which motivate us to study models for describing such noisy effects. We consider a simple model of quantum network, employing qubits (spin-1/2 particles) attached to the nodes of an underlying graph and we study the simplest task, namely information storage (on a single and two qubits), when the graph randomly changes in time. Actually we randomly add edges to an initially disconnected graph according to the Gilbert model characterized by a weighting parameter ex [...] and in an identically and independent way at each time step. We find that by increasing ex the dynamics of relevant quantities like fidelity, entropy or concurrence, gradually transforms from damped to damped oscillatory and finally to purely oscillatory. That leads to the paper [see, Information dissipation in random quantum networks, by U. Farooq and S. Mancini, OSID 21(3), 1450004, 2014]. We also study a system composed by pairs of qubits attached to each node of a linear chain, a model that stems from quantum dot arrays. Here we use the approach of evolution with a stochastic Hamiltonian to describe the noisy effects. We then evaluate the effect of two most common disorders, namely exchange coupling and hyperfine interaction fluctuations, in adiabatic preparation of ground state in such model. We show that the adiabatic ground state preparation is highly robust against these disorders making the chain a good analog simulator. Moreover, we also study the adiabatic information transfer, using singlet-triplet states, across the chain. In contrast to ground state preparation the transfer mechanism is highly affected by disorders. This suggested that for communication tasks across such chains adiabatic evolution is not as effective and quantum quenches would be preferable. That leads to the paper [see, Adiabatic many-body state preparation and information transfer in quantum dot arrays, by U. Farooq, A. Bayat, S. Mancini and S. Bose Phys. Rev. B 91, 134303, 2015]. The present work is organized as follows. In chapter 1, we shall give a survey of the various types of approach which can be employed to analyse the dynamics of open quantum systems that leads to decoherence effects. In chapter 2, we shall give a general description about quantum network and its possible applications. In chapter 3, we shall discuss the problem of quantum state transfer in qubit network and shall give a brief overview of some scheme that enable nearly prefect state transfer. In chapter 4, we shall discuss singlet-triplet networks, that is networks having on each site a pair of (generally entangled) qubit. Then within this framework we propose a model stemming from quantum dot array. There we shall address the problem of ground state preparation and state transfer. Finally we shall describe the inherent entanglement of the ground state of strongly correlated systems can be exploited for both classical and quantum communications. In chapter 5, we shall propose a decoherence model for qubit networks based on edges representing XY interactions randomly added to a disconnected graph accordingly to a suitable probability distribution. In this way we shall describe dissipation of information initially localize in single or two qubits all over the network. In chapter 6 we shall model the noisy effects in the quantum dot array introduced in chapter 4 and investigate their consequences on the preparation of ground state and quantum state transfer mechanism. Finally we shall draw conclusions.

APA, Harvard, Vancouver, ISO, and other styles

7

Löfgren, Viktor. "Dissipative Quantum Dynamics and Decoherence." Thesis, Umeå universitet, Institutionen för fysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-44341.

Full text

Abstract:

Abstract This thesis has two parts, in the first, the Caldeira-Leggett model is introduced; its derivation and general consequences are explored following a paper by Caldeira and Leggett[1]. An operator-formalism shortcut through some of the more mathematically cumbersome parts of the derivation of the model is also developed. The correlation of the force resulting from reservoir-interaction is examined in the high- and low-temperature limits, and the Langevin equation is shown to emerge in the classical limit.Abstract The second part introduces decoherence through a thought experiment that demonstrates the destructive effect of random phase shifts on interference terms, and then follows another paper by Caldeira and Leggett[2] in applying their model further to study the phenomenon of dissipative decoherence. The time-evolution of the interference terms in a superposition of Gaussian wave packets in a harmonic oscillator potential is studied when interacting with a heat bath, and they are shown to vanish at a rate much faster than the relaxation of the system.

APA, Harvard, Vancouver, ISO, and other styles

8

Teklemariam, Grum 1965. "Explorations of quantum decoherence phenomena." Thesis, Massachusetts Institute of Technology, 2002. http://hdl.handle.net/1721.1/8484.

Full text

Abstract:

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002.
Includes bibliographical references (p. 77-80).
This thesis describes the experimental exploration of quantum decoherence using discrete and continuous-time decoherence maps. The experimental methodology uses liquid-state nuclear magnetic resonance spectroscopy techniques. Initially, a brief discussion of coherent control methods is given. Then, a detailed discussion of the decoherent control methods is presented. These methods describe how strong measurements can be emulated in an ensemble system by using pulsed magnetic field gradients, and how NMR decoupling techniques can be used to implement partial trace operations. Next, using quantum erasers we explore the stability of three-particle systems under different entangling interactions. With a two-spin system we illustrate the essential features of quantum erasers. The extension to three-spins allows us to use the pair of orthogonal decoherent operations used in quantum erasers to probe the two classes of entanglement in three-particle systems: the GHZ state and the W state. Finally, we develop a decoherence model of a decohering two-level system coupled to an environment with a few degrees of freedom. The couplings are of the [sigma]z [sigma]z type and only induce coherence damping. By introducing a stochastic evolution on the environment, the resulting randomization of the environment phases causes loss of information over the environment degrees of freedom and decohers the system. Control parameters in the stochastic driving of the environment were used to vary the rates of decoherence on the system, thereby allowing the establishment of a scaling law that related control parameters to decay rates.
by Grum Teklemariam.
Ph.D.

APA, Harvard, Vancouver, ISO, and other styles

9

Trubko, Raisa, and Alexander Cronin. "Decoherence Spectroscopy for Atom Interferometry." MDPI AG, 2016. http://hdl.handle.net/10150/621409.

Full text

Abstract:

Decoherence due to photon scattering in an atom interferometer was studied as a function of laser frequency near an atomic resonance. The resulting decoherence (contrast-loss) spectra will be used to calibrate measurements of tune-out wavelengths that are made with the same apparatus. To support this goal, a theoretical model of decoherence spectroscopy is presented here along with experimental tests of this model.

APA, Harvard, Vancouver, ISO, and other styles

10

Oniga, Teodora. "Theory of quantum gravitational decoherence." Thesis, University of Aberdeen, 2016. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=231085.

Full text

Abstract:

As quantum systems can never be isolated from their environment entirely, it is expected that the spacetime fluctuations will influence their evolution. In particular, the environmental interaction may cause the loss of quantum superpositions, or decoherence. In this thesis, we examine the effects of the quantised environmental background on a range of bosonic fields in the formalism of open quantum systems. We first quantise linearised gravity in a gauge invariant way, using Dirac's constraint quantisation. We then use the influence functional technique to obtain an exact master equation for general bosonic matter interacting with weak gravity. As an application of this, we investigate the decoherence of free scalar, electromagnetic and gravitational fields. For long-time decoherence, under the Markov approximation, the dissipative terms in the master equation vanish, leading to no decay of quantum interferences. As a short-time effect, we study the master equation for a many particle state of a free scalar field, massive or massless and relativistic or non-relativistic. We find that in this case, the particles exhibit a counterintuitive behaviour of bundling towards the same quantum state that is not shared by the single particle master equation. Such collective effects, as well as possible long-time decoherence for fields in an external potential may have important implications in setting limits for precision measurements and astronomical observations.

APA, Harvard, Vancouver, ISO, and other styles

11

Plato, Alexander Douglas Kerr. "Measurement, decoherence and master equations." Thesis, Imperial College London, 2011. http://hdl.handle.net/10044/1/7022.

Full text

Abstract:

In the first part of this thesis we concern ourselves with the problem of generating pseudo-random circuits. These are a series of quantum gates chosen at random, with the overall effect of implementing unitary operations with statistical properties close to that of unitaries drawn at random with respect to the Haar measure. Such circuits have a growing number of applications in quantum-information processing, but all known algorithms require an external input of classical randomness. We suggest a scheme to implement random circuits in a weighted graph state. The input state is entangled with the weighted graph state and a random circuit is implemented by performing local measurements in one fixed basis only. A central idea in the analysis of this proposal is the average bipartite entanglement generated by the repeated application of such circuits on a large number of randomly chosen input product states. For a truly random circuit, this should agree with that obtained by applying unitaries at random chosen uniformly with respect to the Haar measure, values which can be calculated using Pages Conjecture. Part II is largely concerned with continuous variables (CV) systems. In particular, we are interested in two descriptions. That of the class of Gaussian states, and that of systems which can be adequately described through the use of Markovian master equations. In the case of the latter, there are a number of approaches one may take in order to derive a suitable equation, all of which require some sort of approximation. These approximations can be made based on a mixture of mathematical and physical grounds. However, unfortunately it is not always clear how justified we are in making a particular choice, especially when the test system we wish to describe includes its own internal interactions. In an attempt to clarify this situation, we derive Markovian master equations for single and interacting harmonic systems under different scenarios, including strong internal coupling. By comparing the dynamics resulting from the corresponding master equations with numerical simulations of the global systems evolution, we assess the robustness of the assumptions usually made in the process of deriving the reduced Markovian dynamics. This serves to clarify the general properties of other open quantum system scenarios subject to treatment within a Markovian approximation. Finally, we extend the notions of the smooth min- and smooth max-entropies to the continuous variable setting. Specifically, we have provided expressions to evaluate these measures on arbitrary Gaussian states. These expressions rely only on the symplectic eigenvalues of the corresponding covariance matrix. As an application, we have considered their use as a suitable measure for detecting thermalisation.

APA, Harvard, Vancouver, ISO, and other styles

12

LUCAMARINI, Marco. "Quantum Decoherence and Quantum Cryptography." Doctoral thesis, La Sapienza, 2005. http://hdl.handle.net/11573/917148.

Full text

APA, Harvard, Vancouver, ISO, and other styles

13

Zoupas, Andreas. "Decoherence and localization in open systems." Thesis, Imperial College London, 1997. http://hdl.handle.net/10044/1/11530.

Full text

APA, Harvard, Vancouver, ISO, and other styles

14

Gooding, David William Francisco. "Self-gravitating interferometry and intrinsic decoherence." Thesis, University of British Columbia, 2015. http://hdl.handle.net/2429/53015.

Full text

Abstract:

To investigate the possibility that an intrinsic form of gravitational decoherence can be theoretically demonstrated within canonical quantum gravity, we develop a model of a self-gravitating interferometer, and analyze the WKB regime of its reduced phase space quantization. We search for evidence in the resulting interference pattern that general relativity necessarily places limits on coherence, due to the inherent ambiguity associated with forming superpositions of geometries. We construct the "beam" of the interferometer out of WKB states for an infinitesimally thin shell of matter, and work in spherical symmetry to eliminate the occurrence of gravitational waves. For internal consistency, we encode information about the beam optics within the dynamics of the shell itself, by arranging an ideal fluid on the surface of the shell with an equation of state that enforces beam-splitting and reflections. The interferometric analysis is performed for single-mode inputs, and coherence is shown to be fully present regardless of gravitational self-interaction. Next we explore the role coordinate choices play in our description of the interferometer, by considering a family of generalized coordinate systems and their corresponding quantizations. Included in this family are the Painleve-Gullstrand coordinates, which are related to a network of infalling observers that are asymptotically at rest, and the Eddington-Finkelstein coordinates, which are related to a network of infalling observers that travel at the speed of light. We then introduce another model, obtained by adding to the shell a harmonic oscillator as an internal degree of freedom. The internal oscillator evolves with respect to the local proper time of the shell, and therefore serves as a clock that ticks differently depending on the shell's position and momentum. If we focus only on the external dynamics, we must trace out the clock degree of freedom, and this results in a form of intrinsic decoherence that shares some features with a recently-proposed "universal" decoherence mechanism attributed to gravitational time dilation. We discuss some variations of this proposal, and point out a way to bootstrap the gravitational contribution to the time dilation decoherence with self-gravitation. We interpret this as a fundamentally gravitational intrinsic decoherence effect.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

APA, Harvard, Vancouver, ISO, and other styles

15

Doll, Roland. "Decoherence of spatially separated quantum bits." kostenfrei, 2008. http://d-nb.info/988615940/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

16

Schriefl, Josef [Verfasser]. "Decoherence in Josephson Qubits / Josef Schriefl." Aachen : Shaker, 2005. http://d-nb.info/1186579161/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

17

Magnan, Eric. "Spontaneous decoherence in large Rydberg systems." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLO008/document.

Full text

Abstract:

La simulation quantique consiste à réaliser expérimentalement des systèmes artificiels équivalent à des modèles proposés par les théoriciens. Pour réaliser ces systèmes, il est possible d'utiliser des atomes dont les états individuels et les interactions sont contrôlés par la lumière. En particulier, une fois excités dans un état de haute énergie (appelé état de Rydberg), les atomes peuvent être contrôlés individuellement et leurs interactions façonnées arbitrairement par des faisceaux laser. Cette thèse s'intéresse à deux types de simulateurs quantiques à base d'atomes de Rydberg, et en particulier à leurs potentielles limitations.Dans l'expérience du Joint Quantum Institute (USA), nous observons la décohérence dans une structure cubique contenant jusqu'à 40000 atomes. A partir d'atomes préparés dans un état de Rydberg bien défini, nous constatons l'apparition spontanée d'états de Rydberg voisins et le déclenchement d'un phénomène d'avalanche. Nous montrons que ce mécanisme émane de l'émission stimulée produite par le rayonnement du corps noir. Ce phénomène s'accompagne d'une diffusion induite par des interactions de type dipole-dipole résonant. Nous complétons ces observations avec un modèle de champ moyen en état stationnaire. Dans un second temps, l'étude de la dynamique du problème nous permet de mesurer les échelles de temps caractéristiques. La décohérence étant globalement néfaste pour la simulation quantique, nous proposons plusieurs solutions pour en atténuer les effets. Nous évaluons notamment la possibilité de travailler dans un environnement cryogénique, lequel permettrait de réduire le rayonnement du corps noir.Dans l'expérience du Laboratoire Charles Fabry à l'Institut d'Optique (France), nous analysons les limites d'un simulateur quantique générant des structures bi- et tridimensionnelles allant jusqu'à 70 atomes de Rydberg piégés individuellement dans des pinces optiques. Le système actuel étant limité par le temps de vie des structures, nous montrons que l'utilisation d'un cryostat permettrait d'atteindre des tailles de structures jusqu'à 300 atomes. Nous présentons les premiers pas d'une nouvelle expérience utilisant un cryostat à 4K, et en particulier les études amont pour le développement de composants optomécaniques placés sous vide et à froid
Quantum simulation consists in engineering well-controlled artificial systems that are ruled by the idealized models proposed by the theorists. Such toy models can be produced with individual atoms, where laser beams control individual atomic states and interatomic interactions. In particular, exciting atoms into a highly excited state (called a Rydberg state) allows to control individual atoms and taylor interatomic interactions with light. In this thesis, we investigate experimentally two different types of Rydberg-based quantum simulators and identify some possible limitations.At the Joint Quantum Institute, we observe the decoherence of an ensemble of up to 40000 Rydberg atoms arranged in a cubic geometry. Starting from the atoms prepared in a well-defined Rydberg state, we show that the spontaneous apparition of population in nearby Rydberg states leads to an avalanche process. We identify the origin of the mechanism as stimulated emission induced by black-body radiation followed by a diffusion induced by the resonant dipole-dipole interaction. We describe our observations with a steady-state mean-field analysis. We then study the dynamics of the phenomenon and measure its typical timescales. Since decoherence is overall negative for quantum simulation, we propose several solutions to mitigate the effect. Among them, we discuss the possibility to work at cryogenic temperatures, thus suppressing the black-body induced avalanche.In the experiment at Laboratoire Charles Fabry (Institut d'Optique), we analyze the limitation of a quantum simulator based on 2 and 3 dimensional arrays of up to 70 atoms trapped in optical tweezers and excited to Rydberg states. The current system is limited by the lifetime of the atomic structure. We show that working at cryogenic temperatures could allow to increase the size of the system up to N=300 atoms. In this context, we start a new experiment based on a 4K cryostat. We present the early stage of the new apparatus and some study concerning the optomechanical components to be placed inside the cryostat

APA, Harvard, Vancouver, ISO, and other styles

18

Dumlu, Cesim Kadri. "Quantum Decoherence And Quantum State Diffusion Formalism." Master's thesis, METU, 2007. http://etd.lib.metu.edu.tr/upload/12608563/index.pdf.

Full text

Abstract:

Foundational problems of quantum theory, regarding the appearance of classicality and the measurement problem are stated and their link to studies of open quantum systems is discussed. This study'
s main aim is to analyze the main approaches that are employed in the context of open quantum systems. The general form of Markovian master equations are derived by a constructive approach. The Quantum State Diffusion (QSD) formalism is stressed upon as an alternative method to the master equations. Using the Caldeira-Leggett model in the context of QSD, stationary solutions of a charged particle exposed to a uniform magnetic field are found. The important points are summarized and the results are discussed.

APA, Harvard, Vancouver, ISO, and other styles

19

Fischer, Timo [Verfasser]. "Decoherence of the Orientation State / Timo Fischer." München : Verlag Dr. Hut, 2015. http://d-nb.info/106770812X/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

20

Feller, Alexandre. "Entanglement and Decoherence in Loop Quantum Gravity." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSEN058/document.

Full text

Abstract:

Une théorie de gravitation quantique propose de décrire l'interaction gravitationnelle à toutes les échelles de distance et d'énergie. Cependant, comprendre l'émergence de notre espace-temps classique reste un problème toujours ouvert. Cette thèse s'y attaque en gravité quantique à boucles à partir d'outils de l'information quantique.Ceci est fait en plusieurs étapes. La gravité quantique à boucles étant toujours une théorie en cours de développement, un point de vue pragmatique est adopté en étudiant une classe d'état physique du champ gravitationnel, motivée à la fois par des intuitions simples et les résultats de la physique à N corps. Une analyse de la reconstruction de la géométrie à partir des corrélations peut être faite et des leçons peuvent être tirées sur la forme de la dynamique fondamentale. Dans un second temps, la physique des sous-systèmes est analysée en commençant d'abord par évaluer l'entropie d'intrication entre l'intérieur et l'extérieur de la région, permettant ainsi de retrouver la loi holographique de l'entropie des trous noirs et donnant une forme possible des états holographiques de la théorie. Plusieurs dynamiques de la frontière, vu comme un système isolé ou ouvert, sont ensuite analysées, éclairant de nouveau la forme de la dynamique fondamentale. Enfin, la dernière étape de ces recherches étudie la dynamique de la frontière en interaction avec un environnement formé des degrés de liberté (de matière ou gravitationnels) formant le reste de l'Univers et la décohérence sur la frontière qu'il induit. Ceci permet de discuter la transition quantique/classique et de mettre en lumière, dans un modèle donné, les états pointeurs de la géométrie
A quantum theory of gravitation aims at describing the gravitational interaction at every scales of energy and distance. However, understanding the emergence of our classical spacetime is still an open issue in many proposals. This thesis analyzes this problem in loop quantum gravity with tools borrowed from quantum information theory.This is done in several steps. Since loop quantum gravity is still under construction, a pragmatic point of view is advocated and an ansazt for physical states of the gravitational field is studied at first, motivated from condensed matter physics and simple intuitions. We analyze the proposal of reconstructing geometry from correlations. Lessons on the quantum dynamics and the Hamiltonian constraint are extracted. The second aspect of this work focuses on the physics of sub-systems and especially the physics of their boundary. We begin by calculating the entanglement entropy between the interior and the exterior of the region, recovering the holographic law known from classical black hole physics. Then different boundary dynamics are studied, both in the isolated and open cases, which shed lights again on the fundamental dynamics. Finally, the last aspect of this research studies the dynamics of the boundary interacting with an environment whose degrees of freedom (gravitational or matter) forming the rest of the Universe and especially the decoherence it induces. This allows to discuss the quantum to classical transition and understand, in a given model, the pointer states of geometry

APA, Harvard, Vancouver, ISO, and other styles

21

Yuan, Shengjun. "Relaxation and decoherence in quantum spin system." [S.l. : Groningen : s.n. ; University Library of Groningen] [Host], 2008. http://irs.ub.rug.nl/ppn/306075334.

Full text

APA, Harvard, Vancouver, ISO, and other styles

22

Nguyen, Hieu Duy. "It and Bit| Decoherence and Information Storage." Thesis, University of California, Santa Barbara, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3612010.

Full text

Abstract:

We studied two topics: i) how much physical resources are needed to store information and ii) decoherent histories theory applied to Grover search. Given a system consisting of d degrees of freedom each of mass m to store an amount S of information, we find that its average energy, ⟨H⟩, or size, ⟨r²⟩, can be made arbitrarily small individually, but its product ⟨P⟩ = ⟨H⟩⟨ r²⟩ is bounded below by (exp{S/d} − 1)²d²/m. This result is obtained in a nonrelativistic, quantum mechanical setting, and it is independent of earlier thermodynamical results such as the Bekenstein bound on the entropy of black holes.

The second topic is decoherent histories applied to the Grover search problem. The theory of decoherent histories is an attempt to derive classical physics from positing only quantum laws at the fundamental level without notions of a classical apparatus or collapse of the wave-function. Searching for a marked target in a list of N items requires Ω( N) oracle queries when using a classical computer, while a quantum computer can accomplish the same task in O([special characters omitted]) queries using Grover's quantum algorithm. We study a closed quantum system executing Grover algorithm in the framework of decoherent histories and find it to be an exactly solvable model, thus yielding an alternate derivation of Grover's famous result. We also subject the Grover-executing computer to a generic external influence without needing to know the specifics of the Hamiltonian insofar as the histories decohere. Depending on the amount of decoherence, which is captured in our model by a single parameter related to the amount of information obtained by the environment, the search time can range from quantum to classical. Thus, we identify a key effect induced by the environment that can adversely affect a quantum computer's performance and demonstrate exactly how classical computing can emerge from quantum laws.

APA, Harvard, Vancouver, ISO, and other styles

23

Valente, Diego. "DECOHERENCE IN SEMICONDUCTOR SOLID-STATE QUANTUM COMPUTERS." Doctoral diss., University of Central Florida, 2009. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/2797.

Full text

Abstract:

In this dissertation we discuss decoherence in charge qubits formed by multiple lateral quantum dots in the framework of the spin-boson model and the Born-Markov approximation. We consider the intrinsic decoherence caused by the coupling to bulk phonon modes and electromagnetic environmental fluctuations. In the case of decoherence caused by phonon coupling, two distinct quantum dot configurations are studied and proposed as setups that mitigate its nocive effects : (i) Three quantum dots in a ring geometry with one excess electron in total and (ii) arrays of quantum dots where the computational basis states form multipole charge configurations. For the three-dot qubit, we demonstrate the possibility of performing one- and two-qubit operations by solely tuning gate voltages. Compared to a previous proposal involving a linear three-dot spin qubit, the three-dot charge qubit allows for less overhead on two-qubit operations. For small interdot tunnel amplitudes, the three-dot qubits have Q factors much higher than those obtained for double-dot systems. The high-multipole dot configurations also show a substantial decrease in decoherence at low operation frequencies when compared to the double-dot qubit. We also discuss decoherence due to electromagnetic fluctuations in charge qubits formed by two lateral quantum dots. We use effective circuit models to evaluate correlations of voltage fluctuations in the qubit setup. These correlations allows us to estimate energy (T1) and phase (T2) relaxation times of the the qubit system. We also discuss the dependence the quality factor Q shows with respect to parameters of the setup, such as temperature and capacitive coupling between the electrodes.
Ph.D.
Department of Physics
Sciences
Physics PhD

APA, Harvard, Vancouver, ISO, and other styles

24

Schram, Matthew Christopher. "Semiclassical studies of decoherence produced by scattering." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/104533.

Full text

Abstract:

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 146-152).
The conventional notion of coherent atom-surface scattering originates from the existence of Bragg peaks in elastic scattering. The helium atom acts as a quantum mechanical matter wave that is coherent with itself; the well-defined phase relationship of the particle beam at the different spatial positions at surface impact implies the possibility of different non-specular outgoing beams thanks to the constructive interference of the emitted waves from each surface atom. Moreover, we still observe diffraction peaks when scattering off a lattice at finite temperature, although the peaks are here diminished by the Debye-Waller factor. However, in the case of inelastic scattering, the surface particles are displaced by the scattering atom itself and may then emit or absorb one or more phonons to the scatterer. Acoustic phonons produced by this process are gapless excitations; hence, extremely long-wavelength phonons will contribute vanishingly small shifts in energy and momentum. The difficulty in observing this is exacerbated due to the roughly 1eV resolution of high energy helium scattering experiments. So through phonon excitation the surface has "measured" the particle's presence which acts to destroy quantum coherence, though we still observe diffraction spots which imply coherent scattering. How do we reconcile these disparate viewpoints? We propose a new way of looking at the question of coherence in atom-surface scattering. Instead of considering a single beam of helium particles, we instead use semiclassical techniques to simulate an initially coherent superposition of helium particles with equal probabilities of interacting with the surface or not interacting with the surface. We then evolve the classical mechanical trajectories, and recombine the atoms after scattering to observe the resulting interference pattern. The degree to which phonons are excited in the lattice by the scattering process dictates the fringe contrast of the interference pattern of the resulting beams. We show that for a wide range of conditions, despite the massive change in the momentum perpendicular to the surface, we can still expect to have coherent (in the superposition sense) scattering.
by Matthew Christopher Schram.
Ph. D.

APA, Harvard, Vancouver, ISO, and other styles

25

Petersen, Evan Scot. "Decoherence of 31P Donor Spins in Silicon." Thesis, Princeton University, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10841795.

Full text

Abstract:

Spin coherence is important for the fields of electron spin resonance (ESR), nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), and quantum devices. For donor spins in silicon, coherence both quantifies their potential as qubits and measures environmental processes. By understanding those processes, we can construct experiments which remove them to obtain longer coherence times. Silicon crystals are uniquely suited to this task, benefiting from decades of advancements in purification. The two most well-known decoherence mechanisms for donors in silicon are 29Si atoms and the donor spins themselves. Although well studied for electron spins, these mechanisms are less understood for nuclear spins. Using crystals with controlled concentrations of 29Si and 31P donors, I evaluate the limitations imposed on 31P nuclear spins. I find that nuclear spin echo decay times vary linearly with 29Si concentration. The non-exponential decays shown here establish a range of 29Si flip-flop rates, with some being fast compared to experiment timescales and others being slow. Furthermore, when compared to measurements of ionized nuclear spins, the echo decays here imply a "frozen core" picture where the donor electron spin protects the nuclear spin by detuning neighboring 29Si atoms. In studying spin coherence relative to 31P concentration, I find that nuclear spin echo experiments can measure donor electron spin flip-flop rates. A stochastic model reproduces the experiments by fitting a local Zeeman frequency linewidth. However, experiments in more lightly doped crystals (<10

15 P/cm

3) suggest that coherence is not limited by flip-flops. The source of decoherence in these crystals is unknown, but the experiments serve as an upper bound on electric field noise. Magnetic field fluctuations are known to obstruct spin coherence measurements. One popular method for removing that noise is dynamical decoupling via repeated pi rotations. However, these sequences also elongate echo decays for ensemble spins known to be decohered instead by instantaneous diffusion. This result suggested that cumulative rotation errors might artificially inflate decay times. After demonstrating that such effects were insignificant in practice, I find instead that pi rotation errors reduce the dipole-dipole coupling between spins.

APA, Harvard, Vancouver, ISO, and other styles

26

Marzolino, Ugo. "Entanglement and decoherence in many-body physics." Doctoral thesis, Università degli studi di Trieste, 2011. http://hdl.handle.net/10077/5827.

Full text

Abstract:

2009/2010
The thesis deals with several features of quantum many-body systems. They are described both in terms of reversible unitary transformations and as an environment interacting with other systems. An introductory part introduces the main ideas of quantum noise and dissipative dynamics. A chapter is also dedicated to some useful aspects of entanglement. The second part of the thesis concerns the orginal results. A chapter describes the dynamics of two qubits interacting with a common environment. This chapter is focused on the derivation of a new Markovian approximation, finer than the standard weak coupling limit, and its application on the dynamical generation of the entanglement. The second topic concerns the developping of some procedures to reconstruct the parameters governing a large class of Markovian and non-Markovian dissipative dynamics of a quantum particle. These procedures are based on the symplectic tomography of the evolved state. The third topic concerns the physics of many identical bosons, with a special focus on Bose-Einstein condensates. The relevance of entanglement and spin squeezing for quantum metrology with high accuracy is discussed in connection with the quantum Fisher information and collective and squeezing inequalities. A third part summerizes the results. Some useful tools are described in the appendices.
XXIII Ciclo
1983

APA, Harvard, Vancouver, ISO, and other styles

27

Schlosshauer-Selbach, Maximilian. "The quantum-to-classical transition : decoherence and beyond /." Thesis, Connect to this title online; UW restricted, 2005. http://hdl.handle.net/1773/9673.

Full text

APA, Harvard, Vancouver, ISO, and other styles

28

Senozan, Selma. "A Continuum Model For Decoherence In 1d Transport." Master's thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12606703/index.pdf.

Full text

Abstract:

In this thesis we study the conductance of a one dimensional conductor in the presence of dephasing. Dephasing effects are modelled after generalizing Bü
ttiker&rsquo
s dephasing model (Phys. Rev. B 33, 3020 (1986)) to a continuous one. Infinitely many electron reservoirs are coupled to the conductor as phase breakers and the method for calculating the conductance is presented. We investigate how this continuum decoherence effect the conductance of a wire, with single and double rectangular barriers.

APA, Harvard, Vancouver, ISO, and other styles

29

Koks, Don. "Decoherence, entropy and thermal radiance using influence functionals /." Title page, contents and abstract only, 1996. http://web4.library.adelaide.edu.au/theses/09PH/09phk798.pdf.

Full text

APA, Harvard, Vancouver, ISO, and other styles

30

Barrett, S. "Decoherence and measurement in solid state quantum computers." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596410.

Full text

Abstract:

The discovery of quantum algorithms that can outperform the best known classical algorithms for certain problems has, in recent years, instigated a rapid growth in the study of quantum computation. A number of schemes for the practical implementation of quantum logic in a semiconductor device have been proposed. These schemes, being an extension of existing semiconductor technology, have the potential to be scaled to a large number of qubits. There is now a substantial experimental effort, worldwide, to build quantum logid devices in semiconductor systems. However, a number of important issues must be tackled if such a device is to be realized. In this work, I concentrate on theoretical aspects of two central problems in the implementation of a solid state quantum computer: decoherence and readout. After introducing some concepts in quantum information and quantum computation, I review a number of recent proposals for semiconductor based quantum logic: I then consider the problem of decoherence of a single charge qubit and propose an experimentally viable scheme to measure the decoherence rate! Subsequently, I consider a potentially serious decoherence mechanism in an electron spin based quantum computer. I show that the coupling of the charge degrees of freedom of the electrons to environmental electromagnetic fluctuations can lead to errors in the operation of a quantum logic gate. Finally, I consider the problem of quantum measurement (readout) of the spin state of electrons in a semiconductor based quantum computer I describe a method whereby the spin degree of freedom of a pair of electrons in a two-site system can be measured by observing spin-dependent charge fluctuations with a nearby electrometer. I argue that the measurement could be performed with existing detector technology.

APA, Harvard, Vancouver, ISO, and other styles

31

Bririd, Adel. "Study of decoherence and architecture for quantum computers." Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.614246.

Full text

APA, Harvard, Vancouver, ISO, and other styles

32

Jasiak, Rafal. "Ultrafast electron dynamics and decoherence in metallic nanostructures." Strasbourg, 2011. http://www.theses.fr/2011STRA6040.

Full text

Abstract:

La dynamique électronique ultrarapide dans des films métalliques minces a été étudiée numériquement en utilisant à la fois un modèle semi-classique (de Vlasov-Poisson) et un modèle quantique basé sur l'équation de Wigner. Pour de grandes énergies d'excitation, la dynamique quantique et la dynamique classique sont pratiquement identiques. En revanche, pour des plus basses énergies les cas classique et quantique divergent à partir d'un certain seuil, qui est de l’ordre de l'énergie du plasmon. Cet effet marque une transition classique – quantique, qui pourrait être observée dans les expériences de type pompe-sonde sur des films métalliques minces. A des échelles de temps plus longues, les électrons interagissent de façon incohérente avec les ions du réseau. Un temps de relaxation classique et un temps de décohérence quantique ont été mis en évidence à l’issue des résultats des simulations. Ces échelles de temps sont en bon accord avec les estimations phénoménologiques basées sur le modèle à deux températures, et reproduisent correctement les principales caractéristiques observées lors d'expériences sur des petits agrégats de sodium
The ultrafast electron dynamics in thin metal films was studied numerically using both a semiclassical model (Vlasov-Poisson) and a fully quantum approach based on the Wigner equation. For large excitation energies, the quantum and classical dynamics are virtually identical, whereas they diverge below a certain threshold, roughly equal to the plasmon energy. This is a clear signature of a quantum-mechanical effect, which should be observable in standard pump-probe experiments on thin metal films. For longer timescales, the electron dynamics becomes dissipative, as the electrons exchange energy incoherently with the ion lattice. A classical relaxation time and a quantum decoherence time were shown to emerge naturally from the simulations. These time scales are in good agreement with phenomenological estimates based on the two-temperature model, and correctly reproduce the main features observed in experiments on small sodium clusters

APA, Harvard, Vancouver, ISO, and other styles

33

Bishop, C. Allen. "Preparation and manipulation of qutrit decoherence-free subsystems /." Available to subscribers only, 2008. http://proquest.umi.com/pqdweb?did=1594490201&sid=1&Fmt=2&clientId=1509&RQT=309&VName=PQD.

Full text

APA, Harvard, Vancouver, ISO, and other styles

34

Marek, Petr. "Non-classicality of quantum states : decoherence and purification." Thesis, Queen's University Belfast, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.517357.

Full text

APA, Harvard, Vancouver, ISO, and other styles

35

Genkin, Michail Romanovič. "Resonances, dissipation and decoherence in exotic and artificial atoms." Stockholm : Department of Physics, Stockholm University, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-38153.

Full text

APA, Harvard, Vancouver, ISO, and other styles

36

Gutmann, Henryk. "Description and control of decoherence in quantum bit systems." Diss., lmu, 2005. http://nbn-resolving.de/urn:nbn:de:bvb:19-40861.

Full text

APA, Harvard, Vancouver, ISO, and other styles

37

Abel, Benjamin Simon. "Macroscopic superposition states and decoherence by quantum telegraph noise." Diss., lmu, 2009. http://nbn-resolving.de/urn:nbn:de:bvb:19-98286.

Full text

APA, Harvard, Vancouver, ISO, and other styles

38

Genkin, Mikhail. "Resonances, dissipation and decoherence in exotic and artificial atoms." Doctoral thesis, Stockholms universitet, Fysikum, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-38153.

Full text

Abstract:

There are several reasons why exotic and artificial atoms attract the interest of different scientific communities.In exotic atoms, matter and antimatter can coexist for surprisingly long times. Thus, they present a unique natural laboratory for high precision antimatter studies. In artificial atoms, electrons can be confined in an externally controlled way. This aspect is crucial, as it opens new possibilities for high precision measurements and also makes artificial atoms promising potential candidates for qubits, i.e. the essential bricks for quantum computation.The first part of the thesis presents theoretical studies of resonant states in antiprotonic atoms and spherical two-electron quantum dots, where well established techniques, frequently used for conventional atomic systems, can be applied after moderate modifications. In the framework of Markovian master equations, it is then demonstrated that systems containing resonant states can be approached as open systems in which the resonance width determines the environmental coupling. The second part of the thesis focuses on possible quantum computational aspects of two kinds of artificial atoms, quantum dots and Penning traps. Environmentally induced decoherence, the main obstacle for a practical realization of a quantum computer based on these devices, is studied within a simple phenomenological model. As a result, the dependence of the decoherence timescales on the temperature of the heat bath and environmental scattering rates is obtained.

APA, Harvard, Vancouver, ISO, and other styles

39

Gong, Jiangbin. "Coherent control, quantum chaos, and decoherence in molecular dynamics." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/NQ63601.pdf.

Full text

APA, Harvard, Vancouver, ISO, and other styles

40

Witzel, Wayne Martin. "Decoherence and dynamical decoupling in solid-state spin qubits." College Park, Md. : University of Maryland, 2007. http://hdl.handle.net/1903/6889.

Full text

Abstract:

Thesis (PhD) -- University of Maryland, College Park, 2007.
Thesis research directed by: Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

APA, Harvard, Vancouver, ISO, and other styles

41

Waldron, Alison. "Quantum gravity induced decoherence and phenomenology of discrete symmetries." Thesis, King's College London (University of London), 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.435195.

Full text

APA, Harvard, Vancouver, ISO, and other styles

42

Sarkar, Saubhik. "Many-body decoherence with cold atoms in optical lattices." Thesis, University of Strathclyde, 2017. http://digitool.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=28879.

Full text

Abstract:

Ultracold atoms in optical lattices have proven to be an ideal testbed for simulating strongly correlated condensed matter physics. The microscopic understanding of the underlying Hamiltonian and precise control over the Hamiltonian parameters via external fields allow faithful realisation of interesting many-body systems that are otherwise hard to study theoretically or experimentally. This thesis addresses the issue of many-body decoherence, using analytical and numerical techniques, in these optical lattice experiments that arises due to coupling to the environment. We specifically study fermionic systems to investigate the effects of incoherent light scattering on the dynamics. Starting from the atomic structure of fermionic species that are experimentally relevant we provide a framework to derive a microscopic master equation and look at the regimes of strong interactions. The interplay between the atomic physics and many-body physics is found to give rise to interesting observations like suppression of the decoherence effect for magnetically ordered insulators that occur for strong repulsive interactions and an enhancement of the decoherence effect for the case of superfluid pairs that occur for strong attractive interactions. The master equation framework is then applied to a recent experiment looking at the effect of controlled decoherence on a many-body localised system of ultracold fermions in an optical lattice. We determine the dissipative processes in the system looking at the atomic structure of the fermionic species. Lastly we study a system of two species bosons to investigate the effect of interspecies interaction in terms of bipartite entanglement between the species, and how this impacts upon the visibility of the momentum distribution. This study proposes a solution to a recent experimental observation of effects on the momentum distribution of impurity atoms in a Bose-Einstein condensate that would not be explained by polaronic behaviour alone.

APA, Harvard, Vancouver, ISO, and other styles

43

Carrasco, Martínez Juan Carlos. "Probing CPT breaking induced by quantum decoherence at DUNE." Master's thesis, Pontificia Universidad Católica del Perú, 2019. http://hdl.handle.net/20.500.12404/14341.

Full text

Abstract:

We consider a beyond standard physics scenario, where neutrino is considered as an open quantum system. In order to have that consideration we use the Lindblad master equation, which introduce the quantum phenomena called decoherence. In that context we explore one of the most exotic of its consecuences, the CPT violation. We work in three avor generation case where making the SU(3) decomposition of the operators we encounter that there exist fteen parameters in the decoherence matrix that explicitly violate CPT. Regarding four of those parameteres we choose one case of decoherence which will be tested at DUNE.
Trabajo de investigación

APA, Harvard, Vancouver, ISO, and other styles

44

Balian, S. J. "Quantum-bath decoherence of hybrid electron-nuclear spin qubits." Thesis, University College London (University of London), 2015. http://discovery.ucl.ac.uk/1470543/.

Full text

Abstract:

A major problem facing the realisation of scalable solid-state quantum computing is that of overcoming decoherence - the process whereby phase information encoded in a quantum bit ('qubit') is lost as the qubit interacts with its environment. Due to the vast number of environmental degrees of freedom, it is challenging to accurately calculate decoherence times T2, especially when the qubit and environment are highly correlated. Hybrid or mixed electron-nuclear spin qubits, such as donors in silicon, are amenable to fast quantum control with pulsed magnetic resonance. They also possess 'optimal working points' (OWPs) which are sweet-spots for reduced decoherence in magnetic fields. Analysis of sharp variations of T2 near OWPs was previously based on insensitivity to classical noise, even though hybrid qubits are situated in highly correlated quantum environments, such as the nuclear spin bath environment of 29Si impurities. This presented limited understanding of the underlying decoherence mechanism and gave unreliable predictions for T2. In this thesis, I present quantum many-body calculations of the qubit-bath dynamics, which (i) yield T2 for hybrid qubits in excellent agreement with experiments in multiple regimes, (ii) elucidate the many-body nature of the nuclear spin bath and (iii) expose significant differences between quantum-bath and classical-field decoherence. To achieve these results, the cluster correlation expansion was adapted to include electron-nuclear state mixing. In addition, an analysis supported by experiment was carried out to characterise the nuclear spin bath for a bismuth donor as the hybrid qubit, a simple analytical formula for T2 was derived with predictions in agreement with experiment, and the established method of dynamical decoupling was combined with operating near OWPs in order to maximise T2. Finally, the decoherence of a 29Si spin in proximity to the hybrid qubit was studied, in order to establish the feasibility for its use as a quantum register.

APA, Harvard, Vancouver, ISO, and other styles

45

Kielpinski, David. "Entanglement and decoherence in a trapped-ion quantum register." [Boulder, Colo. : University of Colorado], 2001. http://www.boulder.nist.gov/timefreq/ion/qucomp/papers/dkthesis/dkthesis.pdf.

Full text

APA, Harvard, Vancouver, ISO, and other styles

46

Basei, Gianpietro <1987&gt. "Structural Signatures through Continuous-Time Quantum Walks with Decoherence." Master's Degree Thesis, Università Ca' Foscari Venezia, 2015. http://hdl.handle.net/10579/5907.

Full text

Abstract:

Quantum walks are the analogue of classical random walks, and have been recently used to study and develop quantum algorithms: unlike the classical case, where the evolution of the walk is governed by a stochastic matrix, in the quantum case the evolution of the walk is governed by a complex unitary matrix. This implies that Quantum walks are non-ergodic and do not posses a limiting distribution. Quantum walks can be divided in two classes, the ones which have a discrete-time parameter and the ones with a continuous-time parameter. We focused our attention on the Continuous-time quantum random walks. In quantum mechanics, Decoherence describes the transition of quantum density matrices to classical probability distributions, in other words it describes the emergence of classical properties due to the interaction of the quantum system with the surrounding environment. In this work we studied the effects of Decoherence on continuous-time quantum walks in order to build a novel structural signature that is used to characterize nodes of a graph.

APA, Harvard, Vancouver, ISO, and other styles

47

Weick, Guillaume. "Quantum dissipation and decoherence of collective excitations in metallic nanoparticles." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=981745679.

Full text

APA, Harvard, Vancouver, ISO, and other styles

48

Schlesinger, Martin. "Quantum Dissipative Dynamics and Decoherence of Dimers on Helium Droplets." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-82729.

Full text

Abstract:

In this thesis, quantum dynamical simulations are performed in order to describe the vibrational motion of diatomic molecules in a highly quantum environment, so-called helium droplets. We aim to reproduce and explain experimental findings which were obtained from dimers on helium droplets. Nanometer-sized helium droplets contain several thousands of 4-He atoms. They serve as a host for embedded atoms or molecules and provide an ultracold “refrigerator” for them. Spectroscopy of molecules in or on these droplets reveals information on both the molecule and the helium environment. The droplets are known to be in the superfluid He II phase. Superfluidity in nanoscale systems is a steadily growing field of research. Spectra obtained from full quantum simulations for the unperturbed dimer show deviations from measurements with dimers on helium droplets. These deviations result from the influence of the helium environment on the dimer dynamics. In this work, a well-established quantum optical master equation is used in order to describe the dimer dynamics effectively. The master equation allows to describe damping fully quantum mechanically. By employing that equation in the quantum dynamical simulation, one can study the role of dissipation and decoherence in dimers on helium droplets. The effective description allows to explain experiments with Rb-2 dimers on helium droplets. Here, we identify vibrational damping and associated decoherence as the main explanation for the experimental results. The relation between decoherence and dissipation in Morse-like systems at zero temperature is studied in more detail. The dissipative model is also used to investigate experiments with K-2 dimers on helium droplets. However, by comparing numerical simulations with experimental data, one finds that further mechanisms are active. Here, a good agreement is obtained through accounting for rapid desorption of dimers. We find that decoherence occurs in the electronic manifold of the molecule. Finally, we are able to examine whether superfluidity of the host does play a role in these experiments
In dieser Dissertation werden quantendynamische Simulationen durchgeführt, um die Schwingungsbewegung zweiatomiger Moleküle in einer hochgradig quantenmechanischen Umgebung, sogenannten Heliumtröpfchen, zu beschreiben. Unser Ziel ist es, experimentelle Befunde zu reproduzieren und zu erklären, die von Dimeren auf Heliumtröpfchen erhalten wurden. Nanometergroße Heliumtröpfchen enthalten einige tausend 4-He Atome. Sie dienen als Wirt für eingebettete Atome oder Moleküle und stellen für dieseeinen ultrakalten „Kühlschrank“ bereit. Durch Spektroskopie mit Molekülen in oder auf diesen Tröpfchen erhält man Informationen sowohl über das Molekül selbst als auch über die Heliumumgebung. Man weiß, dass sich die Tröpfchen in der suprafluiden He II Phase befinden. Suprafluidität in Nanosystemen ist ein stetig wachsendes Forschungsgebiet. Spektren, die für das ungestörte Dimer durch voll quantenmechanische Simulationen erhalten werden, weichen von Messungen mit Dimeren auf Heliumtröpfchen ab. Diese Abweichungen lassen sich auf den Einfluss der Heliumumgebung auf die Dynamik des Dimers zurückführen. In dieser Arbeit wird eine etablierte quantenoptische Mastergleichung verwendet, um die Dynamik des Dimers effektiv zu beschreiben. Die Mastergleichung erlaubt es, Dämpfung voll quantenmechanisch zu beschreiben. Durch Verwendung dieser Gleichung in der Quantendynamik-Simulation lässt sich die Rolle von Dissipation und Dekohärenz in Dimeren auf Heliumtröpfchen untersuchen. Die effektive Beschreibung erlaubt es, Experimente mit Rb-2 Dimeren zu erklären. In diesen Untersuchungen wird Dissipation und die damit verbundene Dekohärenz im Schwingungsfreiheitsgrad als maßgebliche Erklärung für die experimentellen Resultate identifiziert. Die Beziehung zwischen Dekohärenz und Dissipation in Morse-artigen Systemen bei Temperatur Null wird genauer untersucht. Das Dissipationsmodell wird auch verwendet, um Experimente mit K-2 Dimeren auf Heliumtröpfchen zu untersuchen. Wie sich beim Vergleich von numerischen Simulationen mit experimentellen Daten allerdings herausstellt, treten weitere Mechanismen auf. Eine gute Übereinstimmung wird erzielt, wenn man eine schnelle Desorption der Dimere berücksichtigt. Wir stellen fest, dass ein Dekohärenzprozess im elektronischen Freiheitsgrad des Moleküls auftritt. Schlussendlich sind wir in der Lage herauszufinden, ob Suprafluidität des Wirts in diesen Experimenten eine Rolle spielt

APA, Harvard, Vancouver, ISO, and other styles

49

Hartmann, Udo. "Decoherence and Measurement of Charge Qubits in Double Quantum Dots." Diss., lmu, 2005. http://nbn-resolving.de/urn:nbn:de:bvb:19-44155.

Full text

APA, Harvard, Vancouver, ISO, and other styles

50

Storcz, Markus J. "Decoherence, control, and encoding of coupled solid-state quantum bits." Diss., lmu, 2005. http://nbn-resolving.de/urn:nbn:de:bvb:19-46047.

Full text

APA, Harvard, Vancouver, ISO, and other styles

Dissertations / Theses on the topic 'Decoherence'

Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles