Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 03/05

Major efforts in quantum information science are devoted to the development of methods that are superior to the one of classical information processing, for example the quantum computer or quantum simulations. For these purposes, superposition and entangled states are considered a decisive resource. Furthermore, since the early days of quantum mechanics, entanglement has revealed the discrepancy between the quantum mechanical and the everyday life perception of the physical world. This combination of fundamental science and application-oriented research makes the realization, characterization, and application of entanglement a challenge pursued by many researchers.

In this work, the observation of entangled states of polarization encoded photonic qubits is pushed forward in two directions: flexibility in state observation and increase in photon rate. To achieve flexibility two different schemes are developed: setup-based and entanglement-based observation of inequivalent multi-photon states. The setup-based scheme relies on multi-photon interference at a polarizing beam splitter with prior polarization manipulations. It allows the observation of a family of important four-photon entangled states. The entanglement-based scheme exploits the rich properties of Dicke states under particle projections or loss in order to obtain inequivalent multi-photon entangled states. The observed states are characterized using the fidelity and entanglement witnesses.

An increase in photon rate is crucial to achieve entanglement of higher photon numbers. This holds especially, when photon sources are utilized that emit photons spontaneously. To this end, a new photon source is presented based on a femtosecond ultraviolet enhancement cavity and applied to the observation of the six-photon Dicke state with three excitations.

The implemented schemes not only allow the observation of inequivalent types of entanglement, but also the realization of various quantum information tasks. In this work, the four-photon GHZ state has been used in a quantum simulation of a minimal instance of the toric code. This code enables the demonstration of basic properties of anyons, which are quasiparticles distinct from bosons and fermions. Further, the six-photon Dicke state has been applied for quantum metrology: It allows one to estimate a phase shift with a higher precision than by using only classical resources.

Altogether, a whole series of experiments for observing inequivalent multi-photon entangled states can now be substituted by a single experimental setup based on the designs developed in this work. In addition to this new approach of photon processing, a novel photon source has been implemented, paving the way to realizations of applications requiring higher photon numbers.

Black carbon (BC) and mineral dust particles are among the most

important atmospheric aerosol types forming ice crystals by

heterogeneous nucleation, the so called potential ice nuclei (PIN).

When emitted, most BC and dust particles are externally mixed with

other aerosol compounds. Through coagulation with other particles

and condensation of gases, externally

mixed particles gain a liquid coating and are, therefore, transferred

to an internal mixture.

This ageing process is essential for the direct and indirect effect

of BC and dust particles on the climate, since the coating changes

their radiative and hygroscopic properties and consequently

their cloud activation ability and lifetime.

Moreover, laboratory studies have shown that a liquid coating

influences the freezing properties of the particles and hence

their behavior as ice nuclei.

Due to large computational resources required, global climate

models mostly parameterize the particle ageing by using estimated

turnover times rather than simulating the ageing processes explicitly.

In the present study the population of PIN in the global upper

troposphere and lowermost stratosphere (UTLS) is characterized.

To reach this goal the new aerosol model ECHAM5/MESSy-MADEsoot

is developed.

The aerosol module MADEsoot is able to simulate separately

BC and dust particles in their different states of mixing

(internally or externally mixed) and BC and dust free aerosols,

as well as the relevant ageing processes of externally mixed particles.

MADEsoot is implemented in the global climate model ECHAM5/MESSY.

The resulting system is evaluated with aircraft and

surface measurements and performs well both in the boundary layer and

in the UTLS.

ECHAM5/MESSy-MADEsoot (E5/M-MADEsoot)

is the only existing model able to resolve the mixing state of BC

and dust particles while giving

a reliable representation of the UTLS.

E5/M-MADEsoot is applied to characterize the PIN through

their number and mass concentration, composition and

mixing state.

The results of this study show that PIN contribute

only up to 0.7% to the total aerosol number concentration

in the UTLS. At surface level PIN contribute between 10% and

50% to the total aerosol number concentration, where the highest values

are reached over the major emission areas.

Nearly all PIN in the UTLS are internally mixed with soluble material,

while only up to the 3% of PIN is externally mixed.

E5/M-MADEsoot allows also for the investigation of the

ageing process of BC and dust particles and for the determination

of its timescale, showing that the ageing process is mainly

driven by the condensation of vapor. The timescale of the ageing process

is calculated with E5/M-MADEsoot to be around some hours at

surface level and some days in the UTLS, but shows a

high geographical variability, especially in the boundary layer

and in the lower troposphere. The timescale of the ageing process shows

also a less pronounced seasonality, with higher values in winter and

lower in summer.

In den letzten Jahren haben sich atomare Quantengase in optischen Gittern zu einem faszinierenden und interdisziplinär bedeutsamen Forschungsfeld entwickelt. Die in den periodischen Potentialen gefangenen ultrakalten Atome stellen ein ideales Modellsystem dar, anhand dessen

sich grundlegende Fragestellungen der modernen Festkörper- und Vielteilchenphysik untersuchen lassen. In der vorliegenden Arbeit werden neue Methoden zur Manipulation und Analyse von Quantenzuständen in optischen Gittern demonstriert. Insbesondere wird mittels der sogenannten Rauschkorrelationsanalyse die Ordnung der Atome im Gitter bestimmt und erstmals fermionisches Antibunching an freien neutralen Atomen nachgewiesen.

Grundlage für die vorgestellten Experimente ist eine im Rahmen dieser Arbeit neu entwickelte Apparatur, mit der sich simultan entartete bosonische und fermionische Quantengase aus 87-Rubidium und 40-Kalium präparieren und in einem dreidimensionalen optischen Gitter untersuchen lassen. Die Apparatur zeichnet sich durch eine Serie technischer Innovationen aus: Eine neuartige Spulen- und Fallenkonfiguration eröffnet einen hervorragenden optischen Zugang zu den präparierten Ensemblen und ermöglicht es, starke homogene Magnetfelder bei einer geringen dissipierten Leistung zu erzeugen. Dies sind wichtige Voraussetzungen, um definierte Gitterpotentiale verwirklichen und die interatomaren Wechselwirkungen mittels Feshbach-Resonanzen beeinflussen zu können. Das optische Potential geht aus der Überlagerung einer gekreuzten Dipolfalle und eines blauverstimmten dreidimensionalen Gitters hervor. Eine solche Kombination erlaubt es, sehr tiefe und relativ homogene Gitterpotentiale zu erzeugen sowie den externen Einschluss unabhängig von der Gittertiefe zu variieren. Des Weiteren lassen sich über eine frei einstellbare Wellenlänge speziesabhängige Gitter realisieren. Die Vereinigung der hier aufgeführten Technologien liefert uns eine außergewöhnlich flexible Plattform für das Studium maßgeschneiderter Quantenzustände in periodischen Potentialen.

Durch den unabhängigen externen Einschluss kann erstmals ein Fermigas allein über dessen Kompression zwischen einem metallischen und einem isolierenden Zustand hin- und hergeschaltet und – in ersten Ansätzen – die entsprechende Dynamik beobachtet werden. Die Ergebnisse werden mit numerischen Simulationen verglichen. Neben der Durchführung von Transportmessungen lässt sich hieraus ein neues Diagnoseverfahren ableiten, das es ermöglicht, Quantenphasen, wie den bosonischen oder fermionischen Mott-Isolator, anhand der charakteristischen Kompressibilität zu identifizieren.

Als weiteres Diagnoseverfahren wird die Korrelationsanalyse von Flugzeitaufnahmen vorgestellt. Durch die Auswertung von Hanbury Brown und Twiss (HBT)-Korrelationen im Quantenrauschen der expandierenden Atomwolken lässt sich die mikroskopische Ordnung der Atome im Gitter nachweisen. Ausgangspunkt für die Messungen sind jeweils vollständig spinpolarisierte bosonische Mott-Isolatoren und fermionische Bandisolatoren. Trotz identischer Dichteverteilungen innerhalb des Gitters, weisen die Korrelationen von Bosonen und Fermionen entgegengesetzte Vorzeichen auf. Mit diesen Messungen gelingt es erstmals, fermionisches Antibunching an freien neutralen Atomen zu beobachten und innerhalb einer selben Apparatur mit dem bosonischen Bunching zu vergleichen. Neben dem Nachweis dieses fundamentalen Quanteneffektes lässt sich die Ordnung und die Temperatur der Fermionen im Gitter bis hinauf zur Fermi-Temperatur bestimmen. Damit erweist sich die Korrelationsanalyse als ein robustes Verfahren, mit dem sich in Zukunft noch weitaus komplexere Quantenphasen in optischen Gittern untersuchen lassen.

Complex plasmas consist of neutrals, ions, electrons and additional micrometer-sized grains. Fluxes of plasma particles onto the grain surface charge it, and the microparticles interact with each other. When the particles are illuminated, e.g., with a laser, recording the scattered light allows to perform fully resolved kinetic studies at all relevant frequencies, e.g., the Einstein frequency, Debye frequency, and plasma frequency.

In this thesis, dynamical effects in fluid complex plasmas are investigated using the PK-3 Plus setup, consisting of a radio-frequency plasma chamber, on the International Space Station (ISS), during a parabolic flight, and on the ground. The objective is the study of dynamical phenomena on the particle level, e.g., to determine when "corpuscular" effects become important and when fluid dynamics provides an accurate description. "Periodgrams" are used to examine the global dynamic structures. Velocities and forces acting on individual particles were analyzed by studying the particle motion.

In the ground laboratory, self-excited highly resolved wave structures were analyzed. Thermophoresis was used to vary the extent to which the particles were pushed into the sheath regions of the chamber. The self-excitation of the waves is due to the free energy in plasma ions, which stream fast relative to the microparticles, especially in the sheath region. This excitation mechanism also explains the observed pressure dependence of the excitation threshold. Using a simple model of the microparticle dynamics, the grain charge is estimated, which agrees well with that calculated using known theories. Self-excited waves were neither observed when the particles were completely suspended in the center of the chamber by the thermophoretic force nor during dedicated experiments during the microgravity phase of the parabolic flight.

Microparticle waves were externally excited in a complex plasma under microgravity conditions on board the ISS by a variable, low frequency modulation voltage. In addition to the vertical slashing at the externally imposed frequency f_mod, waves propagated both in the vertical and horizontal

direction. The horizontal oscillation did not depend on f_mod. When f_mod was resonant with the frequency of the horizontal oscillation, the excitation region of the vertically propagating waves started spreading obliquely towards the left part of the cloud. The dispersion relation of the oblique waves was quasi-sound like. Around f_mod = 9 Hz, the wave activity spread over the whole cloud. At higher frequencies, the oblique waves disappeared. The particle dynamics resembled those accompanying the self-excited waves observed in the ground setup.

Under similar conditions as for the self-excited waves, a new phenomenon occurring near the surface of the particle cloud was investigated - the formation of microparticle bubbles, blobs, and surface cusps. The behavior of microparticles was similar to that observed in fluid drops and with sedimenting particles, including the breakup mechanism and vortex motion of particles inside the blobs. The forces acting on the microparticles are analyzed and the velocity scaling with pressure is shown to be compatible with a possible flow induced by thermal creep inside the plasma chamber. Various effects indicate the presence of surface tension, such as the formation of cusps with angles like in Taylor cones, the self-confinement of blobs inside the void, and the break-up of bubble lids. Qualitative agreement with the Rayleigh-Taylor instability is demonstrated.

The Large Hadron Collider at CERN is the most modern proton-proton collider and data taking will start in 2009, with a centre-of-mass energy of 7 TeV. The ATLAS detector, which is one of two multi-purpose detectors at the Large Hadron Collider, is able to detect a Standard Model Higgs boson if it exists. This is one of the main tasks of the ATLAS experiment. This thesis deals with a Standard Model Higgs boson produced in association with a W boson. The Monte Carlo study is based on physics events generated at the nominal centre-of-mass energy of the Large Hadron Collider of 14 TeV.

Large parts of this analysis have been done using the global Grid infrastructure of the Large Hadron Collider experiments. A mass range of the Higgs boson of mH = 130 - 190 GeV has been taken into account. In this mass range,

the Higgs boson dominantly decays into a pair of W bosons, leading to initially three W bosons: WH -> WWW. Two orthogonal analysis channels have been investigated in detailed studies of the background properties.

The first channel considers the leptonic decay of two W bosons, such that the leptons are of opposite charge. The third W boson then decays hadronically. The analysis is based on one-dimensional cuts, where the best cuts found are strict cuts on the transverse momenta of the leptons, a cut on the invariant mass of the jets, as well as a cut on the transverse jet momenta and the missing transverse

energy.

The second decay channel studied is dedicated to the leptonic decay of all three W bosons. Again, cuts on the transverse momenta of the leptons and the

jets have been proven to be efficient, as well as the use of the spatial correlation of the decay products of the Higgs boson. The invariant mass of the leptons with opposite sign has been emerged as a very efficient cut to reject dominant diboson background contributions.

The discovery reach of both channels separately as well as the combination has been calculated using Bayesian methods. The discovery reach is at maximum

for a mass range of mH = 150 - 170 GeV, with a peak for mH = 170 GeV at

5 sigma. All results are scaled to an integrated luminosity of L = 30 fb^-1, which corresponds to approximately three years of data-taking at the design luminosity of 10^33 cm^-2 s^-1. The associated WH production improves the discovery reach for a Standard Model Higgs boson at the ATLAS detector and would also be useful for precision measurements of the couplings of the Higgs boson.

This thesis is devoted to theoretical studies of the interaction of intense laser pulses with solid-state targets. This area of laser physics is very active and fast growing as it might possess a number of useful applications in material science, physics, biology and medicine.

The main part of the thesis is devoted to the generation of high-order harmonics on the vacuum-plasma interface due to the longitudinal oscillatory motion of the reflecting surface. This has a prospect of generation of trains or even single attosecond pulses that have much more intensity than those generated in atomic media.

Before making this source an instrument for studying electron dynamics in condensed matter or for laser-vacuum interactions, one has to know how to control the important properties of the harmonic beam, namely its temporal and spatial structure. To pursue the answering of the question of control, analytical and numerical studies were performed. Most of the ideas are based on the shaping of the laser pulse (both in temporal and spatial domains) and of the target.

The second part of the thesis is devoted to the studies of the generation of energetic ion beams. These beams can be used, for example, in cancer therapy, plasma radiography and isotope production. The studies of the influence of laser pulse ellipticity and target thickness on ion beam monoenergetic features and energy allows one to use the results of this thesis for optimization of future experiments.

Die vorliegende Dissertation beschäftigt sich mit der Herstellung

und der wissenschaftlichen Untersuchung von auf GaAs/AlGaAs lateral definierten Quantenpunktkontakten und gekoppelten Quantenpunkten. Die im Rahmen dieser Arbeit eigenständig hergestellten Proben werden bei tiefen Temperaturen (T < 100mK) mittels niederfrequenten Messungen im Bereich sehr niedriger Ströme (I < 1pA) sowie Mikrowellenmessungen (f > 300MHz) charakterisiert. Die experimentelle Entwicklung und Realisierung einer Anpassschaltung zum Auslesen der Reflexion eines kapazitiv an einen Doppelquantenpunkt (DQD) gekoppelten Quantenpunktkontakts (QPC) wird präsentiert. Die physikalischen sowie technischen Grenzen der eingesetzten Lösung werden aufgezeigt.

Der wichtigste Teil dieser Arbeit umfasst Experimente an einem DQD mit einem integrierten Nanomagneten. Die vorgestellten Messungen zeigen, dass ein einfaches Model der sequentiellen Auffüllung des DQDs angesichts der Anwesenheit des lokalen Magnetfelds des Nanomagneten nicht anwendbar ist. Dank der sehr freundlichen Unterstützung vom Dr. Geza Giedke aus Max-Planck-Institut war es möglich, anhand vorhandener Literatur ein theoretisches Model dieses DQDs aufzustellen.

Die systematischen Untersuchungen im Bereich der Pauli-Spinblockade zusammen mit der theoretischen Interpretation lassen das Verhalten des DQDs als dynamische Wechselwirkung zwischen elektronischem Spin im DQD und den Spins der Kerne des Probematerials identifizieren. Darüber hinaus erlaubt die theoretische Beschreibung eine Bestimmung der an der Kernpolarisation beteiligten Zustände. Eine gezielte Initialisierung des DQDs erlaubt dabei die Untersuchung der Kerndynamik für verschiedene Übergänge zwischen den Eigenzuständen der Spinblockade. Diese Übergänge und die demzufolge erzeugte Kernpolarisation können dabei mittels rf-Anregung beeinflusst werden. Die vorliegende Arbeit zeigt die Möglichkeiten in Bezug auf die Untersuchung und Nutzung der dynamischen Wechselwirkung zwischen Elektronen und den Kernspins des Probematerials.