Podcasts about ddc:500

Diese Dissertation berichtet über ein neuartiges Quantengasmikroskop, mit dem Vielteilchensysteme von fermionischen Atomen in optischen Gittern untersucht werden. Die einzelplatzaufgelöste Abbildung ultrakalter Gase im Gitter hat mächtige Experimente an bosonischen Vielteilchensystemen ermöglicht. Die Erweiterung dieser Fähigkeit auf Fermigase bietet neue Aussichten, komplexe Phänomene stark korrelierter Systeme zu erforschen, für die numerische Simulationen oft nicht möglich sind. Mit Standardtechniken der Laserkühlung, optischen Fallen und Verdampfungskühlung werden ultrakalte Fermigase von 6Li präpariert und in ein 2D optisches Gitter mit flexibler Geometrie geladen. Die Atomverteilung wird mithilfe eines zweiten, kurzskaligen Gitters eingefroren. Durch Raman-Seitenbandkühlung wird an jedem Atom Fluoreszenz induziert, während seine Position festgehalten wird. Zusammen mit hochauflösender Abbildung erlaubt die Fluoreszenz die Rekonstruktion der ursprünglichen Verteilung mit Einzelplatzauflösung und hoher Genauigkeit. Mithilfe von magnetisch angetriebener Verdampfungskühlung produzieren wir entartete Fermigase mit fast einheitlicher Füllung im ersten Gitter. Dies ermöglicht die ersten mikroskopischen Untersuchungen an einem ultrakalten Gas mit klaren Anzeichen von Fermi-Statistik. Durch die Präparation eines Ensembles spinpolarisierter Fermigase detektieren wir eine Abflachung im Dichteprofil im Zentrum der Wolke, ein Charakteristikum bandisolierender Zustände. In einem Satz von Experimenten weisen wir nach, dass Verluste von Atompaaren an einem Gitterplatz, bedingt durch lichtinduzierte Stöße, umgangen werden. Die Überabtastung des zweiten Gitters erlaubt eine deterministische Trennung der Atompaare in unterschiedliche Gitterplätze. Die Kompression einer dichten Wolke in der Falle vor dem Laden ins Gitter führt zu vielen Doppelbesetzungen von Atomen in unterschiedlichen Bändern, die wir ohne Anzeichen von paarweisen Verlusten abbilden können. Somit erhalten wir die wahre Besetzungsstatistik an jedem Gitterplatz. Mithilfe dieser Besonderheit werten wir die lokale Besetzungsstatistik an einem Ensemble bandisolierenderWolken aus. Im Zentrum bei hoher Füllung sind die Atomzahlfluktuationen um eine Größenordnung unterdrückt, verglichen mit klassischen Gasen, eine Manifestation des Pauliverbots. Die Besetzungswahrscheinlichkeiten werden verwendet, um die lokale Entropie an jedem Gitterplatz zu messen. Eine niedrige Entropie pro Atom bis 0.34kB wird im Zentrum des Bandisolators gefunden. Die Erweiterung der Quantengasmikroskopie auf entartete Fermigase eröffnet neue Möglichkeiten der Quantensimulation stark korrelierter Vielteilchensysteme und kann einzigartige Erkenntnisse über fermionische Systeme im und außerhalb vom Gleichgewicht, Quantenmagnetismus und verschiedene Phasen des Fermi-Hubbard-Modells ergeben.

The observed accelerated expansion of the universe is successfully parameterized by a cosmological constant. However, since this parameter in Einstein's equations is not protected against quantum corrections, the observed and theoretically expected value vastly differ, thus giving rise to the cosmological constant problem. In this thesis, the issue is addressed by embedding our universe--represented by a brane--in a six-dimensional bulk spacetime, where the cosmological constant plays the role of a brane tension, which then no longer needs to imply an expansion of the three apparent spatial dimensions; rather, it curves the extra space and hence stays hidden from a brane observer. In this context, the crucial question is whether this so-called degravitation mechanism may be implemented in a phenomenologically viable and 't Hooft natural way. Corresponding answers will be given in the case of four different models. The main part of this thesis has its focus on the 6D brane induced gravity model--a higher-dimensional generalization of the Dvali-Gabadadze-Porrati model--according to which a brane with sub-critical tension curves the bulk into a cone of infinite spatial extent. First, it is shown that the model is free of ghost instabilities only if the tension is not unnaturally small. This in turn opens a window of opportunity to study theoretically consistent modified cosmologies. In this context, it is shown that a homogeneous and isotropic brane acts as an antenna that emits and absorbs cylindrically symmetric Einstein-Rosen waves. We encounter two interesting types of solutions--sub-critical ones, which feature dynamical degravitation but are incompatible with observations, as well as compact super-critical ones, which still might be phenomenologically viable but certainly not technically natural. While this clearly shows that the cosmological constant problem cannot be solved in a 6D version of the model, our results point towards higher-dimensional constructions as the remaining playground for future research. Next, we introduce a new two-brane model where a thick super-critical brane curves the extra space into a cigar that closes in a microscopically thin sub-critical brane, representing our universe. In the case both branes only host a tension, we derive fully analytic solutions, which correspond to a de Sitter phase on our brane and are hence phenomenologically promising. Unfortunately, as a fine-tuning of the brane tension is required, they are not technically natural. The failure is attributed to the compactness of the extra space. To further exemplify the virtue of infinite volume extra dimensions, we devise a hybrid model where the brane is wrapped around an infinitely long cylinder of microscopic width. This construction turns out to be the minimal setup that features bulk waves as a dynamical ingredient of a modified cosmology. We find that, due to the existence of an infinitely large dimension, the system admits a degravitating solution. While being conceptually interesting, a supernova fit shows that the corresponding 4D cosmology cannot describe our universe. Finally, we turn to the model of supersymmetric large extra dimensions that had been claimed to successfully address the cosmological constant problem. Here, a Maxwell flux stabilizes the extra space that has the shape of a rugby ball. We critically review the corresponding mechanism, and find that a vanishing brane curvature--as required by the degravitation idea--is only ensured by a scale invariant brane sector, which however leads to an unavoidable parameter constraint due to a flux quantization condition. In a second step, we generalize our analysis to solutions that admit a de Sitter phase on the brane. Provided the model parameters are not tuned, we find that either the brane curvature or the volume of the extra space exceeds its phenomenological bound by many orders of magnitude. Our results significantly narrow down the search for solutions of the cosmological constant problem in the realm of extra-dimensional scenarios. In particular, models with infinite volume extra dimensions are found to offer a working mechanism, which yet requires refinement to comply with the observational bounds.

Eine aussagekräftige theoretische Beschreibung des Infrarot (IR)-Schwingungsspektrums eines Biomoleküls in seiner nativen Umgebung durch Molekulardynamik (MD)-Simulationen benötigt hinreichend genaue Modelle sowohl für das Biomolekül, als auch für das umgebende Lösungsmittel. Die quantenmechanische Dichtefunktionaltheorie (DFT) stellt solche genauen Modelle zur Verfügung, zieht aber hohen Rechenaufwand nach sich. Daher ist dieser Ansatz nicht zur Simulation der MD ausgedehnter Biomolekül-Lösungsmittel-Komplexe einsetzbar. Solche Systeme können effizient mit polarisierbaren molekülmechanischen (PMM) Kraftfeldern behandelt werden, die jedoch nicht die zur Berechnung von IR-Spektren nötige Genauigkeit liefern. Einen Ausweg aus dem skizzierten Dilemma bieten Hybridverfahren, die einen relevanten Teil eines Simulationssystems mit DFT, und die ausgedehnte Lösungsmittelumgebung mit einem (P)MM-Kraftfeld beschreiben. Im Rahmen dieser Arbeit wird, ausgehend von einer DFT/MM-Hybridmethode [Eichinger et al., J. Chem. Phys. 110, 10452-10467 (1999)], ein genaues und hocheffizientes DFT/PMM-Rechenverfahren entwickelt, das der wissenschaftlichen Ọ̈ffentlichkeit nun in Form des auf Großrechnern einsetzbaren Programmpakets IPHIGENIE/CPMD zur Verfügung steht. Die neue DFT/PMM-Methode fußt auf der optimalen Integration des DFT-Fragments in die "schnelle strukturadaptierte Multipolmethode" (SAMM) zur effizienten approximativen Berechnung der Wechselwirkungen zwischen den mit gitterbasierter DFT bzw. mit PMM beschriebenen Subsystemen. Dies erlaubt stabile Hamilton'sche MD-Simulationen sowie die Steigerung der Performanz (d.h. dem Produkt aus Genauigkeit und Recheneffizienz) um mehr als eine Größenordnung. Die eingeführte explizite Modellierung der elektronischen Polarisierbarkeit im PMM-Subsystem durch induzierbare Gauß'sche Dipole ermöglicht die Verwendung wesentlich genauerer PMM-Lösungsmittelmodelle. Ein effizientes Abtastens von Peptidkonformationen mit DFT/ PMM-MD kann mit einer generalisierten Ensemblemethode erfolgen. Durch die Entwicklung eines Gauß'schen polarisierbaren Sechspunktmodells (GP6P) für Wasser und die Parametrisierung der Modellpotentiale für van der Waals-Wechselwirkungen zwischen GP6P-Molekülen und der Amidgruppe (AG) von N-Methyl-Acetamid (NMA) wird ein DFT/PMM-Modell für (Poly-)Peptide und Proteine in wässriger Lösung konstruiert. Das neue GP6P-Modell kann die Eigenschaften von flüssigem Wasser mit guter Qualität beschreiben. Ferner können die mit DFT/PMM-MD berechneten IR-Spektren eines in GP6P gelösten DFT-Modells von NMA die experimentelle Evidenz mit hervorragender Genauigkeit reproduzieren. Somit ist nun ein hocheffizientes und ausgereiftes DFT/PMM-MD-Verfahren zur genauen Berechnung der Konformationslandschaften und IR-Schwingungsspektren von in Wasser gelösten Proteinen verfügbar.

A detailed study of alpha interactions on the passivated surface of a germanium detector is presented. Germanium detectors can be used to search for both neutrinoless double beta decay of 76Ge and direct interaction of dark matter. In order to increase the sensitivity to both neutrinoless double beta decay and dark matter beyond the current state of the art, the next generation of germanium-based experiments has to have a mass of about one ton and has to reduce the background by a factor of ten. The choices of detector technology facilitating both searches and the background reduction are one of the biggest challenges for such an experiment. Surface contaminations on the material close to the detectors or on the detectors themselves, can generate a background due to alpha particles, which was found to be limiting in some experiments. The characterization of events induced by alpha particles will help to identify such events and thus eliminate them as sources of background. An especially designed segmented true-coaxial detector was probed with alpha particles from an 241Am source inside the test-stand GALATEA, located at the MPI f¨ur Physik in Munich. Pulse shape analysis was performed to identify the characteristics of alpha events. The properties of the detector directly underneath the passivation layer on the end-plate were also studied. As part of the detector characterization, the thickness of the effective dead layer was determined. The studies presented here suggest improvements on detector design, which would allow an effective reduction of alpha background in next generation of germanium-based experiments.

Thu, 28 Apr 2016 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19419/ https://edoc.ub.uni-muenchen.de/19419/1/Engelbrecht_Stefan_Gerhard.pdf Engelbrecht, Stefan Gerhard

In this thesis, we investigate the gravitational consequences of theories in which the four spacetime dimensions of our universe are augmented by two spatial extra dimensions. More specifically, the focus is on braneworld scenarios, where our world is confined on a hypersurface in the higher-dimensional bulk, allowing the extra dimensions to be large or even infinite. Our main motivation for studying such models is that they could in principle be able to solve the cosmological constant (CC) problem via degravitation: the CC only curves the extra space, leaving the brane geometry flat. A major difference to the simpler case of a codimension-one brane is that here, gravitational waves can be emitted into the bulk, even at the 3D homogeneous and isotropic level, as is relevant for cosmology. Therefore, we first analyze the question how an outgoing wave boundary condition can be implemented, which is necessary in order to obtain a closed set of modified Friedmann equations predicting the cosmological on-brane evolution. We find that a potential tool from the literature, provided by a certain decomposition of the Weyl tensor - while being applicable to plane gravitational waves - fails for cylindrical waves. This failure is related to the fact that it is already impossible to locally separate incoming from outgoing linear cylindrical waves (on flat spacetime), as we demonstrate by explicitly deriving the corresponding nonreflecting boundary condition, which is nonlocal in time. We then consider a generalization of the Dvali-Gabadadze-Porrati (DGP) model, containing an additional compact on-brane dimension on top of the one infinite codimension. Since here the 3D maximally symmetric brane emits plane waves, the Weyl tensor criterion can be used to exclude incoming bulk waves, and we derive the resulting Friedmann equations. If the compact dimension is stabilized, DGP cosmology is recovered, but we find indications that the stabilization should break down when the CC starts to dominate, which would lead to additional, potentially interesting late time modifications. If, on the other hand, the compact direction is allowed to expand freely, there are dynamically degravitating solutions - which, however, lack a 4D regime and are thus ruled out, as we demonstrate by fitting to supernova data. Next, we turn to the codimension-two version of the DGP model. By numerically solving the full nonlinear coupled bulk-brane system for cosmological symmetries on the (regularized) brane, we show that in some region of parameter space, a CC - but also any other fluid component - gets degravitated dynamically, and a static geometry is approached via the emission of Einstein-Rosen waves. For other model parameters, pathological super-accelerating solutions are encountered. The origin of this unstable behavior is traced back to a tachyonic ghost mode which is identified in this parameter region by studying linear metric perturbations around a nontrivial pure tension background. While confirming the ghost result on Minkowski from the literature, we gain the important insight that the ghost disappears if the brane tension is large enough, thereby reconciling the model with the physical expectation of a healthy low energy effective theory. Unfortunately, the healthy region is again incompatible with an appropriate 4D gravity regime, and therefore ruled out phenomenologically. The preceding analysis only covered sub-critical brane tensions, meaning that the deficit angle of the exterior conical geometry is less than 2π. In the following chapter, we investigate super-critical tensions (first in 4D), and find that the (regularized) static solution is no longer stable. Instead, the axial direction expands at an asymptotically constant rate, and the exterior geometry (which is necessarily compact) takes the form of a growing cigar. We are able to derive an analytic relation between the expansion rate and the tension, which - when adapted to the 6D setup - only yields a (small) constant shift in the CC, and can therefore not help with the CC problem. Finally, the case of two finite codimensions is analyzed within the model of supersymmetric large extra dimensions (SLED). First, we show that - contrary to recent claims in the literature - a brane-localized flux cannot help avoiding the fine-tuning (which is here imposed by flux quantization) in order to obtain 4D flat solutions, basically because only scale invariant brane couplings ensure a flat brane. Next, we ask if a more realistic model with a finite brane width and scale invariance breaking couplings could still be successful by predicting a small enough (albeit nonzero) 4D curvature, but find a negative answer: If the extra-dimensional volume is within its currently allowed range, both effects give way too large contributions to the curvature, unless the brane width were many orders of magnitude below the bulk Planck length, and again some sort of fine-tuning were invoked.

We formulate a quantum theory of classical solutions in gravity and field theory in terms of a large number of constituent degrees of freedom. The description is realized in two different ways. In the first part we introduce the so-called auxiliary current description. The basic idea is to represent the true quantum state of the solution one considers in terms of a multi- local composite operator of the fields of the microscopic theory. Although the approach is completely general, we will be mostly interested in representing black holes as bound states of a large number of gravitons. We show how the mass of the black hole arises microscopically as a collective effect of N gravitons composing the bound state. For that purpose we compute observables associated to the black hole interior such as the constituent density of gravitons and their energy density, respectively. As a next step, it is shown how these observables can be embedded within S-matrix processes. In particular, it is demonstrated that an outside observer has access to the black hole interior doing scattering experiments. Measuring the cross section for the scattering of particles on black holes, the outside observer is sensitive to the distribution of gravitons in the black hole. Possible implications concerning the information paradox are discussed. Finally, we show how geometric concepts, and in particular the Schwarzschild solution emerge as an effective description derived from our construction. In the second part, an alternative approach based on coherent states in presented. First, we apply our reasoning to solitons in field theory. In particular, we explicitly show how well-known properties of solitons such as interactions, false vacuum decay or conservation of topological charge follow easily from the basic properties of coherent states. Secondly, we develop in detail a similar quantum picture of instantons. Since instantons can be understood in terms of solitons in one more spatial dimension evolving in Euclidean time, a coherent state description of the latter implies a similar description of the former. Using the coherent state picture we develop a novel quantum mechanical understanding of the physics of instanton-induced transitions and the concept of resurgence. Finally, we consider solitons in supersymmetric theories. It is shown that the corpuscular effects lead to a novel mechanism of supersymmetry breaking which can never be accounted for in the semi- classical approach. In the last part of the thesis we resolve anti-de Sitter (AdS) space-time as a coherent state. On the one hand, we explain how well-known holographic and geometric properties can easily be understood in terms of the occupation number of gravitons in the state. On the other hand, we explicitly compute corpuscular corrections to the scalar propagator in AdS. Furthermore, it is shown that corpuscular effects lead to deviations from thermality an Unruh observer in AdS measures.

Wed, 13 Apr 2016 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19470/ https://edoc.ub.uni-muenchen.de/19470/2/Salazar_Albornoz_Salvador.pdf Salazar Albornoz, Salvador ddc:530, ddc:50

In this thesis we introduce a novel approach viewing spacetime geometry as an emergent phenomenon based on the condensation of a large number of quanta on a distinguished flat background. We advertise this idea with regard to investigations of spacetime singularities within a quantum field theoretical framework and semiclassical considerations of black holes. Given that in any physical theory apart from General Relativity the metric background is determined in advance, singularities are only associated with observables and can either be removed by renormalization techniques or are otherwise regarded as unphysical. The appearance of singularities in the spacetime structure itself, however, is pathological. The prediction of said singularities in the sense of geodesic incompleteness culminated in the famous singularity theorems established by Hawking and Penrose. Though these theorems are based on rather general assumptions we argue their physical relevance. Using the example of a black hole we show that any classical detector theory breaks down far before geodesic incompleteness can set in. Apart from that, we point out that the employment of point particles as diagnostic tools for spacetime anomalies is an oversimplification that is no longer valid in high curvature regimes. In view of these results the question arises to what extent quantum objects are affected by spacetime singularities. Based on the definition of geodesic incompleteness customized for quantum mechanical test particles we collect ideas for completeness concepts in dynamical spacetimes. As it turns out, a further development of these ideas has shown that Schwarzschild black holes, in particular, allow for a evolution of quantum probes that is well-defined all over. This fact, however, must not distract from such semiclassical considerations being accompanied by many so far unresolved paradoxes. We are therefore compelled to take steps towards a full quantum resolution of geometrical backgrounds. First steps towards such a microscopic description are made by means of a non-relativistic scalar toy model mimicking properties of General Relativity. In particular, we model black holes as quantum bound states of a large number N of soft quanta subject to a strong collective potential. Operating at the verge of a quantum phase transition perturbation theory naturally breaks down and a numerical analysis of the model becomes inevitable. Though indicating 1/N corrections as advertised in the underlying so-called Quantum-N portrait relevant for a possible purification of Hawking radiation and henceforth a resolution of the long-standing information paradox we recognize that such a non-relativistic model is simply not capable of capturing all relevant requirements of a proper black hole treatment. We therefore seek a relativistic framework mapping spacetime geometry to large-N quantum bound states. Given a non-trivial vacuum structure supporting graviton condensation this is achieved via in-medium modifications that can be linked to a collective binding potential. Viewing Minkowski spacetime as fundamental, the classical notion of any other spacetime geometry is recovered in the limit of an infinite constituent number of the corresponding bound state living on Minkowski. This construction works in analogy to the description of hadrons in quantum chromodynamics and, in particular, also uses non-perturbative methods like the auxiliary current description and the operator product expansion. Concentrating on black holes we develop a bound state description in accordance with the isometries of Schwarzschild spacetime. Subsequently, expressions for the constituent number density and the energy density are reviewed. With their help, it can be concluded that the mass of a black hole at parton level is proportional to its constituent number. Going beyond this level we then consider the scattering of a massless scalar particle off a black hole. Using previous results we can explicitly show that the constituent distribution represents an observable and therefore might ultimately be measured in experiments to confirm our approach. We furthermore suggest how the formation of black holes or Hawking radiation can be understood within this framework. After all, the gained insights, capable of depriving their mysteries, highlights the dubiety of treating black holes by means of classical tools. Since our setup allows to view other, non-black-hole geometries, as bound states as well, we point out that our formalism could also shed light on the cosmological constant problem by computing the vacuum energy in a de Sitter state. In addition, we point our that even quantum chromodynamics, and, in fact, any theory comprising bound states, can profit from our formalism. Apart from this, we discuss an alternative proposal describing classical solutions in terms of coherent states in the limit of an infinite occupation number of so-called corpuscles. Here, we will focus on the coherent state description of Anti-de Sitter spacetime. Since most parts of this thesis are directed to find a constituent description of black holes we will exclude this corpuscular description from the main part and introduce it in the appendix.

This thesis reports on experimental demonstrations of a novel direct frequency-comb spectroscopic technique for the measurement of one- and two-photon excitation spectra. An optical-frequency-comb generator emits a multitude of highly coherent laser modes whose oscillation frequencies are evenly spaced and uniquely determined by only two measurable and adjustable radio-frequency parameters and the integer-valued mode number. Direct frequency-comb spectroscopy can traditionally be performed by scanning the comb lines of the frequency comb across the transitions of interest and measuring a signal that is proportional to the excitation by all comb lines in concert. Since the modes that contribute to the excitation cannot be singled out, transition frequencies can only be measured modulo the comb-line spacing with this scheme. The so arising limitations are overcome by the technique presented here, where the first frequency comb is spatially overlapped with a second frequency comb. Both combs of this so-called dual-comb setup are ideally identical except for having different carrier-envelope frequencies and slightly different repetition rates. The interference between the two combs leads to beat notes between adjacent comb lines, forming pairs (with one line from each comb) with an effectively modulated excitation amplitudes. Consequently the probability of excitation by any given comb-line pair is also modulated at the respective beat-note frequency. These beat-note frequencies are spaced by the repetition-rate difference and uniquely encode for individual comb-line pairs, thus enabling the identification of the comb lines causing an observed excitation. In a first demonstration, Doppler-limited one-photon excitation spectra of the transitions 5S_{1/2}-5P_{3/2} (at 384 Thz/780 nm), 5P_{3/2}-5D_{3/2}, and 5P_{3/2}-5D_{5/2} (both at 386 Thz/776 nm), and two-photon spectra of the 5S_{1/2}-5D_{5/2} (at 2x385 Thz/2x778 nm) transition, agreeing well with simulated spectra, are simultaneously measured for both stable Rb isotopes. Within an 18-s measurement time, a spectral range of more than 10 THz (20 nm) is covered at a signal-to-noise ratio (SNR) of up to 550. To my knowledge, this is the first demonstration of both dual-comb-based two-photon spectroscopy and fluorescence-based dual-comb spectroscopy. In a follow-up experiment probing the same sample and two-photon transitions, the Doppler-resolution limit is overcome by implementation of an anti-resonant ring configuration. Cancellation of the first-order Doppler effect makes it possible to resolve 33 hyperfine two-photon transitions. The highly resolved (1 MHz point spacing), narrow transition-linewidth (5 MHz), accurate (systematic uncertainty of ~340 kHz), high-SNR (10^4) spectra are shown to be consistent with basic simulation-based predictions. As the spectral span is, in principle, only limited by the bandwidths of the excitation sources, the acquisition of Doppler-free two-photon spectra spanning 10s of THz appears to be in reach. To my knowledge, this is the first demonstration of Doppler-free Fourier-transform spectroscopy. Lastly, the possibility of extending the technique's scope to applications in the field of biochemistry, such as two-photon microscopy, are explored. To that end, first high-speed, low-resolution (>>1 GHz) experiments are carried out identifying comb-stabilization requirements and measurement constraints due to the limited dynamic range of the presented highly multiplexed spectroscopic technique.

Moderne Rotverschiebungs-Galaxiendurchmusterungen können mittels Mehrfach-Faser-Spektroskopie große Bereiche des Himmels abdecken. Dank der immer größer werdenden Datensätze hat sich die Analyse der großskaligen Galaxienverteilung im Universum zu einer unschätzbaren Wissensquelle für die Kosmologie entwickelt. Zusammen mit den Beobachtungen des kosmischen Mikrowellenhintergrunds (MWH) und Entfernungsbestimmungen anhand von großen Typ-Ia-Supernova-Datensätzen (SN) bilden die Galaxiendurchmusterungen ausschlaggebende Indikatoren für die Korrektheit der Paradigmen des kosmologischen Weltbilds, des ΛCDM-Modells. Die Auswertung der Galaxienverteilung erlaubt mit Hilfe des Standardlineals, das durch die Baryonisch-akustischen Oszillationen gegeben ist, Entfernungsmessungen von ungesehener Präzision. Dies gewährt Einblick in die zugrundeliegende physikalische Natur der Dunklen Energie (DE), welche für die Beschleunigung der Ausdehung unseres Universums verantwortlich gemacht wird, indem die zeitliche Entwicklung der DE-Zustandsgleichung einge- schränkt werden kann. Zudem kann aus dem Signal der Verzerrungen im Rotverschiebungsraum die Wachstumsrate von kosmologischer Struktur bestimmt werden. Dies stellt einen Test der Relativitätstheorie dar, weil mögliche erweiterte Gravitationstheorien abweichende Wachstumsraten vorhersagen können. Die abgeschlossenen Rotverschiebungsmessungen des ‘Baryon Acoustic Oscillation Survey’-Programms (kurz BOSS) brachten einen Galaxienkatalog hervor, der ein bisher unerreichtes Volumen abdeckt. In dieser Dissertation wird die kosmologische Information, die im räumlichen Leistungsdichtespektrum (LDS) der Rotverschiebungsraum-Galaxienverteilung des BOSS-Katalogs enthalten ist, genutzt, um den Parameterraum des ΛCDM-Modells und der wichtigsten möglichen Erweiterungen einzuschränken. Vorherige Analysen des anisotropen Galaxien-LDS waren auf die Messung der Multipolzerlegung beschränkt. Für die hier präsentierte Analyse wurde das Konzept der sogenannten ‘Clustering Wedges’ auf den Fourierraum übertragen, um einen komplementären Ansatz zur Vermessung des anisotropen LDS zu verfolgen. Dazu wird der varianzoptimierte Schätzer für LDS-Wedges definiert und an die Galaxiengewichtung, die unvermeidbare Beobachtungsfehler im BOSS-Katalog behebt, angepasst. Zudem wird auch der Formalismus zur Beschreibung der Fensterfunktion auf die Wedges erweitert. Das verwendete Modell für das anistrope Galaxien-LDS ist auf neuartigen Ansätzen zur Modellierung der nichtlinearen Gravitationsdynamik und der Verzerrungen im Rotverschiebungsraum aufgebaut, welche die Genauigkeit der Modellvorhersagen speziell im Übergang in den nichtlinearen Bereich signifikant verbessern. Daher kann das LDS bis zu kleineren Skalen als in vorherigen Analysen ausgewertet werden, wodurch engere Einschränkungen des kosmologischen Parameterraums erreicht werden. Die Modellierung wurde mit Hilfe von synthetischen Katalogen, die auf großvolumigen Mehrkörpersimulationen basieren, verifiziert. Dazu ist eine theoretische Vorhersage der Kovarianzmatrix der anisotropischen Vermessung der Galaxienverteilung nötig, wofür ein Gaußsches Vorhersagemodell entwickelt wurde. Dieses ist neben den Wedges auch für die komplementäre Multipolzerlegung sowohl des LDS als auch dessen Fouriertransformierten, der Zwei-Punkt-Korrelationsfunktion, anwendbar. Die LDS-Analyse anhand von Clustering Wedges, wie in dieser Arbeit präsentiert, ist Teil der kombinierten Analyse des finalen Galaxienkatalogs im Rahmen der BOSS-Kollaboration. Unter Verwendung von zwei sich nicht überschneidenden Rotverschiebungsbereichen wird die Winkeldurchmesserentfernung zu D_M(z_eff = 0.38) (rfid_d / r_d) = 1525 +-24 h^-1 Mpc und D_M(z_eff = 0.61) (rfid_d / r_d) = 2281 +42 -43 h^-1 Mpc bestimmt. Weiterhin wird der Hubbleparameter zu H(z_eff = 0.38) (r_d / rfid_d) = 81.2 +2.2 −2.3 km s^-1 Mpc^-1 und H(z_eff = 0.61) (r_d / rfid_d) = 94.9 +-2.5 km s^-1 Mpc^-1 vermessen (alle hier angegebenen Bereiche entsprechen einem Konfidenzintervall von 68%). Die Wachstumsrate wird eingeschränkt auf fσ_8 (z_eff = 0.38) = 0.498 +0.044 -0.045 und fσ_8 (z_eff = 0.61) = 0.409 +-0.040. Zusammen mit den Ergebnissen der komplementären Methoden, die innerhalb der BOSS-Kollaboration zur Clustering-Analyse des finalen Galaxienkatalogs eingesetzt werden, werden diese Resultate zu einem abschließenden Konsensergebnis zusammengefasst. Nur mit den Clustering-Weges-Messungen im Fourierraum, kombiniert mit MWH- und SN-Daten, kann der Materiedichteparameter auf Ω_M = 0.311 +0.009 -0.010 und die Hubble-Konstante auf H_0 = 67.6 +0.7 -0.6 km s^-1 Mpc^−1 unter Annahme des ΛCDM-Modells eingeschränken werden. Wird ein Nichtstandard-Modell für DE angenommen, so ergibt sich ein DE-Zustandsgleichungsparameter von w_DE = 1.019 +0.048 -0.039. Modifikationen der Wachstumsrate, parametrisiert durch f(z) = [Ω_M(z)]^γ, werden auf γ = 0.52 +- 0.10 eingeschränkt. Diese beiden Messungen sind in perfekter Übereinstimmung mit den Vorhersagen des ΛCDM-Modells, ebenso wie weitere Ergebnisse, die sich unter der Annahme eines noch großzügigeren DE-Modells (welches eine zeitliche Entwicklung von w_DE erlaubt) ergeben. Daher wird das ΛCDM-Modell durch die hier beschriebene Analyse weiter gefestigt. Die Summe der Neutrinomassen wird zu sum(m_ν) < 0.143 eV bestimmt. Dieses obere Limit befindet sich nicht weit entfernt von der unteren Schranke, die sich aus Teilchenphysik-Experimenten ergibt. Somit ist zu erwarten, dass die kosmologische Signatur, die massebehaftete Neutrinos in der großskaligen Struktur des Universums hinterlassen, in naher Zukunft detektiert werden kann.

Sogenannte wechselwirkungsfreie Messungen sind ein aus der Quantenmechanik bekanntes Interferenzphänomen, mit dessen Hilfe die Anwesenheit eines Objekts detektiert werden kann, ohne das Objekt in irgendeiner Weise zu stören. Der erste Teil dieser Arbeit befasst sich mit wechselwirkungsfreien Messungen mit Elektronen. Integriert in ein Mikroskop könnte diese Technik es ermöglichen, die bei Elektronenmikropskopie auftretenden Strahlenschäden erheblich zu reduzieren. Es werden verschiedene Ansätze zur Realisierung von wechselwirkungsfreien Messungen mit Elektronen und die dabei auftretenden Schwierigkeiten besprochen. Hauptthema hierbei ist der benötigte Elektronen-Strahlteiler. Wir stellen einen möglichen Ansatz vor, der auf der Kontrolle und dem Einschluss eines Elektronenstrahls durch Mikrowellenfelder beruht. Mit diesem Strahlteiler ist es gelungen, einen langsamen Elektronenstrahl mit kinetischer Energie von ungefähr 1 eV in zwei Strahlen zu spalten. Wir diskutieren in einem vereinfachten quantenmechanischen Modell, welche Eigenschaften ein solcher Strahlteiler aufweisen muss, um Elektronenwellen ohne Störung zu teilen und wechselwirkungsfreie Messungen zu ermöglichen. Außerdem beschäftigen wir uns mit der Anwendung von interaktionsfreien Messungen in der Bildgebung, insbesondere mit der Frage, inwiefern sie die Messung und Unterscheidung von Graustufen erlauben. Es stellt sich heraus, dass die Messung von Graustufen im typischen Interferenzaufbau einer wechselwirkungsfreien Messung zwar möglich ist, aber der dabei entstehende Schaden am Messobjekt nur in speziellen Fällen geringer ist als in einer herkömmlichen Transmissionsmessung. Wir untersuchen auch den Einfluss von Phasenverschiebungen. Bei Messobjekten, die Graustufen aufweisen und Phasenverschiebungen verursachen, können wechselwirkungsfreie Messungen für Objekte mit hoher Transparenz weniger Schaden verursachen als konventionelle Transmissionsmessungen und Messungen mit einem Mach-Zehnder-Interferometer. Ein weiteres Thema dieser Arbeit ist die optische Feldverstärkung an Nanospitzen. Wir untersuchen in numerischen Simulationen über einen großen Parameterbereich, wie die Höhe der Feldverstärkung von der Geometrie und dem Material der Spitze abhängt. Dabei stellen wir fest, dass neben dem Krümmungsradius der Spitze auch der Öffnungswinkel einen überraschend großen Einfluss auf die Feldverstärkung hat, welchen wir durch ein vereinfachtes Modell qualitativ erklären können. Anwendung findet die optische Feldverstärkung in der Photoemission von Elektronen aus scharfen Metallspitzen. Hierzu zeigen wir Experimente in verschiedenen Regimes der Photoemission: einerseits Multiphotonenemission mit einem Erbium-Faserlaser und andererseits Photoemission im Starkfeldregime mit einem Titan-Saphir-Oszillator. Letztere Messungen erlauben es, mit Hilfe einer neuen, auf Elektronen-Rückstreuung beruhenden Methode die optische Feldverstärkung in unmittelbarer Nähe der Spitzenoberfläche zu ermitteln. Die so erhaltenen Ergebnisse stimmen gut mit den Simulationen überein.

The conditions under which stars are formed and the reasons for triggering and quenching of starburst events in high-z galaxies, are still not well understood. Studying the interstellar medium (ISM) and the morphology of high-z galaxies are therefore key points in order to understand galaxy evolution. The cosmic star formation rate density peaks between 11, and low to moderate [CII] optical depth tau(CII)2, pave the road for future investigations of the star-forming ISM in high-z galaxies, by illustrating the importance of multi-wavelength, fine structure- and molecular line studies.

Mastering the generation, propagation and detection of electro-magnetic waves has enabled a technological breakthrough that has changed our entire society. World-wide communication through the telephone and the internet has become an integral part of our daily-life, which is expected to grow even further with the emergence of the internet of things. While secure communication was of concern mostly for governmental and financial institutions, digital security has now caught the attention of the general public. The weaknesses of cur- rent encryption protocols, such as the existence of back-doors or the predicted breakdown of popular algorithms such as RSA, reveal the need for alternative encryption schemes ensuring unconditional security on all types of devices. Quantum Key Distribution (QKD) has emerged as a powerful option to ensure a private communication between two users. Based on the laws of quantum mechanics, this class of protocols offers the possibility to detect the presence of a third party trying to intercept the key during its distribution, and even to quantify the amount of leaked information. While most research projects focus on long distance applications, little attention has been devoted to short distance schemes such as wireless payment, network access and authentication, which could highly benefit from QKD-enhanced security. This thesis focuses on the development of a miniature QKD sender add-on that could be embedded either in mobile devices or in existing optical communication platforms, thus allowing for a secure key exchange with a shared dedicated receiver over a free- space link. The proposed optics architecture (35 × 20 × 8 mm 3 ) is optimised for BB84-like protocols and uses an array of four Vertical-Cavity Surface-Emitting Lasers with highly similar properties to generate 40 ps long near-infrared faint coherent pulses at 100 MHz repetition rate. Under strong modulation, the polarisation of the pulses is not well defined and enables an external control of each diode’s emission by a wire-grid polariser. The four beams are spatially overlapped in a polarisation-insensitive femtosecond laser written waveguide array, and combined with a red beacon laser using an external beamsplitter to ensure a stable, synchronised optical link with the receiver. The complete module is compatible with current smartphone technology, allowing to run the classical post-processing over WLAN in the future. First tests with a free-space receiver indicate an average error ratio of 3.3 % and an asymptotic secure key rate of 54 kHz under static alignment. For the first time, a secure key exchange between a mobile platform held by a user and a receiver equipped with a dynamic alignment system could be demonstrated with an error ratio of 4.1 % and a secure key rate of 31 Hz. The further optimisation of the experimental parameters and the implementation of a decoy protocol will enhance the key generation rate as well as the general security of the system. The results of this thesis pave the way towards unprecedented security in wireless optical networks, as examplified for the communication between a mobile device and a dedicated receiver.

Fri, 11 Mar 2016 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19254/ https://edoc.ub.uni-muenchen.de/19254/2/Chou_Shao-Wei.pdf Chou, Shao-Wei ddc:530, ddc:500, Fakultät für Physik

Die Masse des Top Quarks ist ein fundamentaler Parameter des Standardmodells und ihre präzise Bestimmung ist von großer Bedeutung für die Teilchenphysik. In dieser Dissertation werden Messungen der Top Quark Masse im dileptonischen Zerfallskanal von Top Quark Paaren präsentiert und experimentelle und theoretische Aspekte der Präzisionsmessung untersucht. Neben technischen Maßnahmen zur Gewährleistung optimaler Nachweiskapazität für zukünftige Datennahme werden Messungen der Top Quark Masse mit den Daten der Jahre 2011 und 2012 des ATLAS Detektors durchgeführt, basierend auf Proton-Proton Kollisionen mit einer Schwerpunktsenergie von √s = 7 und 8 TeV. Verschiedene Techniken zur Reduzierung der statistischen und wichtigsten systematischen Unsicherheiten werden angewandt, was zur bisher genauesten Einzelmessung der Top Quark Masse im dileptonischen Top Quark Paar Zerfallskanal weltweit führt. Durch eine Kombination mit ATLAS Messungen unter Berücksichtigung der Korrelationen wird die Präzision weiter erhöht. Die in einer Blindstudie ermittelte Masse des Top Quarks ist mtop = 172.40 ± 0.31 (stat) ± 0.62 (syst) GeV/c2 = 172.40 ± 0.70 GeV/c2, wobei die Unsicherheit von der begrenzten Auflösung der Jetenergiemessungen dominiert wird. Außerdem werden mit einer Entfaltungsmethode die Daten von Detektoreffekten bereinigt und die ersten Schritte zu einer Messung der Top Quark Masse auf dem Niveau stabiler Teilchen durchgeführt. Anschließend werden die Auswirkungen von vollen QCD Rechnungen in zweiter Ordnung Störungstheorie auf Top Quark Massenmessungen untersucht.

Fri, 12 Feb 2016 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19205/ https://edoc.ub.uni-muenchen.de/19205/1/Forster_Florian.pdf Forster, Florian ddc:530, ddc:500, Fakultät für Physik

Fri, 12 Feb 2016 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19196/ https://edoc.ub.uni-muenchen.de/19196/1/Pause_Christian.pdf Pause, Christian ddc:530, ddc:500, Fakul

This thesis focuses on the development and application of Bayesian inference techniques for early-Universe signals and on the advancement of mathematical tools for information retrieval. A crucial quantity required to gain information from the early Universe is the primordial scalar potential and its statistics. We reconstruct this scalar potential from cosmic microwave background data. Technically, the inference is done by splitting the large inverse problem of such a reconstruction into many, each of them solved by an optimal linear filter. Once the primordial scalar potential and its correlation structure have been obtained the underlying physics can be directly inferred from it. Small deviations of the scalar potential from Gaussianity, for instance, can be used to study parameters of inflationary models. A method to infer such parameters from non-Gaussianity is presented. To avoid expensive numerical techniques the method is kept analytical as far as possible. This is achieved by introducing an approximation of the desired posterior probability including a Taylor expansion of a matrix determinant. The calculation of a determinant is also essential in many other Bayesian approaches, both apart from and within cosmology. In cases where a Taylor approximation fails, its evaluation is usually challenging. The evaluation is in particular difficult, when dealing with big data, where matrices are to huge to be accessible directly, but need to be represented indirectly by a computer routine implementing the action of the matrix. To solve this problem, we develop a method to calculate the determinant of a matrix by using well-known sampling techniques and an integral representation of the log-determinant. The prerequisite for the presented methods as well as for every data analysis of scientific experiments is a proper calibration of the measurement device. Therefore we advance the theory of self-calibration at the beginning of the thesis to infer signal and calibration simultaneously from data. This is achieved by successively absorbing more and more portions of calibration uncertainty into the signal inference equations. The result, the Calibration-Uncertainty Renormalized Estimator, follows from the solution of a coupled differential equation.

Der anthropogene Klimawandel wird vor allem durch die Emissionen von Treibhausgasen (GHG) verursacht, welche den Energiehaushalt der Erde ändern. Der Anstieg in GHG- Konzentrationen verstärkt nicht nur den strahlungsgetriebenen Treibhauseffekt, sondern beeinflusst auch die atmosphärische Zirkulation sowie biogeochemische Kreisläufe. Rückkopplungsprozesse von Biogeochemischen Kreisläufen können dabei die Klimaerwärmung verstärken oder abschwächen. Aktuelle Erdsystemmodelle (ESMs) aus der fünften Phase des Coupled Model Intercomparison Project (CMIP5), beinhalten solche biogeochemische Prozesse. Diese ermöglichen die Untersuchung von biogeochemischen und Klima Feedbacks des Erdsystems. Diese Feedbacks in Klimaprojektionen unterliegen jedoch großen Unsicherheiten, da das Verständnis der zugrundeliegenden Prozessen und deren Repräsentation in ESMs oft noch unzureichend ist. Das Ziel dieser Arbeit ist zu untersuchen wie beobachtbare Eigenschaften des aktuellen Klimas genutzt werden können, um Unsicherheiten in ausgesuchten Rückkopplungsprozessen zu reduzieren. Um den Zusammenhang zwischen der Klimasensitivität auf anthropogen verursachte Klimaänderungen und beobachtbare Eigenschaften des globalen Klimasystems besser zu verstehen, wurde die relativ neue Methode der so genannten Emergent Constraints verwendet. Emergent Constraints beschreiben dabei Zusammenhänge zwischen einem Aspekt der simulierten Erdsystemsensitivität und einem beobachtbaren Trend oder Variation des aktuellen Klimas. Diese Methode wurde in dieser Arbeit verwendet um Feedbacks im Kohlenstoffkreislauf sowie Änderungen in der Position des Südhemisphären (SH) Jets auf anthropogene Klimaänderungen genauer zu bestimmen. Dafür wurden neue Diagnostiken entwickelt und in das Earth System Model Evaluation Tool (ESMValTool) implementiert. Diese erste Studie nutzt Beobachtungsdaten, um den Kohlenstofkreislauf-Klima- Feedback genauer zu bestimmen und wurde in Journal of Geophysical Research 2014 publiziert. In den meisten Klimaprojektionen führt eine Erwärmung des Klimas zu einer geringeren Aufnahmefähigkeit von atmosphärischem Kohlenstoff Dioxid (CO2) durch die terrestrische Senke. Als Ergebnis bleibt mehr CO2 in der Atmosphäre zurück wo es als GHG klimawirksam ist. Dieser Effekt beschreibt einen positiven Rückkopplungsprozess des Kohlenstoffkreislaufes zur Klimaerwärmung (L) und wird durch den anteiligen Kohlenstoffverlust pro Kelvin Erwärmung quantifiziert, in Einheiten von GtC pro K. Dieser unterliegt jedoch starken Unsicherheiten in Klimaprojektionen des 21. Jahrhunderts. CMIP5 Modelle simulieren den Betrag der tropischen terrestrischen Kohlenstoffsenke, bei ausgeblendeten Klimaeinwirkungen auf den Kohlenstoffkreislauf, im Bereich von 252 ± 112 GtC für eine Verdopplung atmosphärischen CO2 Konzentrationen. Eine gute Korrelation zwischen dem Kohlenstoffkreislauf-Klima-Rückkopplungsfaktor und der beobachtbaren Sensitivität der interannualen CO2-Wachstumsrate auf Temperaturschwankungen ermöglicht es die Unsicherheiten in Klimaprojektionen mit Beobachtungen einzuschränken. Die beobachtete Sensitivität (-4.4 ± 0.9 GtC per year and K) reduziert dabei die Unsicherheiten zu -44 ± 14 GtC pro K um mehr als die Hälfte im Vergleich zum Multimodellmittelwert von 49 ± 40 GtC pro K. Die Ergebnisse der ersten Studie implizieren, dass mit einem Temperaturanstieg weniger Kohlenstoff in der terrestrischen Senke gespeichert wird. Dieser Effekt ist im Vergleich zum Multimodellmittel für den neu berechneten Wert geringer, was einen geringeren Anstieg der CO2 Konzentration durch Klimaerwärmung bedeutet. Die zweite Studie nutzt Beobachtungsdaten, um den Kohlestofkreislauf-CO2 Feedback genauer zu bestimmen und ist in der Begutachtung bei Nature. Unsicherheiten in der Sensitivität des Landökosystems auf erhöhte atmosphärische CO2 Konzentrationen tragen zusätzlich zu Unsicherheiten von Klimaprojektionen bei. CMIP5 Modelle mit interaktivem Kohlenstoffkreislauf simulieren für einen Anstieg der atmosphärischen CO2 Konzentration eine Erhöhung der terrestrischen Brutto Primärproduktion (GPP). Dieser Düngeeffekt wird jedoch von den CMIP5 Modellen unterschiedlich stark für eine aktuelle atmosphärische CO2 Konzentration (ca. 400 ppmv) simuliert und ist im Bereich von 7.5 ± 7 GtC relativ zu vorindustriellen Zeiten. In dieser Studie wurde eine starke Korrelation zwischen dem Düngeeffekt von CO2 auf GPP in höheren Breiten sowie den Extratropen und der beobachteten Änderung der CO2 Amplitude im Jahresgangs (0.05 ± 0.001 ppmv pro ppmv) festgestellt. Mithilfe der Beobachtungen konnte für eine Verdopplung der atmosphärischen CO2 Konzentrationen ein Düngeeffekt auf GPP in hohen Breiten von 0.14% pro ppmv und für GPP in den extratropischen Regionen von 0.12% pro ppmv ermittelt werden. Durch die Anwendung der beobachtungsbasierte Methode auf den Kohlestofkreislauf-CO2 Feedback konnte deutliche Verringerung der Unsicherheiten des Düngeeffekts erzielt werden. Die dritte Studie nutzt Beobachtungen um die Position des SH Jets in Klimaprojektionen genauer zu bestimmen und wurde im Journal of Climate 2016 publiziert. Die Zuname stratosphärischen Ozons und den Anstieg von GHG haben einen starken Einfluss auf die SH extratropische Zirkulation was eine Verlagerung der SH Jetposition zur Folge hat. Die mittlere SH Jetposition ist in CMIP Modellen in Bezug auf Beobachtungsdaten zum Äquartor verschoben und die Modelle simulieren eine Verteilung der Jetposition über 10 Grad in der historischen Klimatologie und in Klimaprojektionen. Die Multiple Diagnostik Ensemble Regression (MDER) Methode wurde verwendet um prozess-orientierte Diagnostiken des aktuellen Klimas mit Projektionen der SH Jetposition zu korrelieren. Die MDER Methode wurde auf den Zeitraum 2015 - 2034 angewendet, wo sie aus den 20 Diagnostiken die historische Jetposition als die wichtigste Größe aussucht. Die Methode detektiert den zum Äquator hin verschobenen Bias in der historischen Jetposition und berechnet eine Korrektur von 1.5 Grad südlich für die Vorhersage. Durch die Analyse konnte somit eine Verbesserung zum Ensemblemittelwert und dessen Unsicherheit erzielt werden. Emergent Constraints, wie sie in dieser Arbeit untersucht wurden, können helfen Modellentwicklungen und Beobachtungen auf Prozesse zu fokussieren, die zur Größenordnung und den Unsicherheiten zukünftiger Klimavorhersagen maßgeblich beitragen.

Fri, 5 Feb 2016 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19173/ https://edoc.ub.uni-muenchen.de/19173/1/Puhlfuerst_Georg.pdf Puhlfürst, Georg ddc:530, ddc:500, Fakultät für Physik

The ability to probe and manipulate electron dynamics and correlations on their characteristic time scales would open up many technological and scientific possibilities. While modern laser technology already allows to do that in principle, a lot of theoretical ground work is still missing. This thesis focuses on the elementary effect of laser strong field ionization of the two simplest systems: The Hydrogen and Helium atoms. To that end, the time-dependent Schroedinger equation is solved numerically, and photo-electron spectra are extracted using the highly efficient tSurff technique. We implemented both the one and two particle versions of tSurff together with several other numerical techniques in a new parallelizable C++ code. We provide details on the employed methods and algorithms, and study numerical efficiency properties of various approaches. We propose a description of the electric field interaction in a mixture of length and velocity gauge for the correct and most efficient implementation of a coupled channels approach, which can be used to compute accurate single ionization photo-electron spectra from true multi-electron systems, even molecules. We provide extensive numerical data for a detailed study of the Hydrogen atom in an Attoclock experimental setup, where it is found that the involved strong field tunnel ionization processes can be considered instantaneous. In particular, there appear no tunneling delays, which can be used as a calibration for experiments with more complicated targets. Similarly, it is investigated whether discrepancies between theory and experimental data for the longitudinal photo-electron momentum spread, resulting from photo-ionization of Helium in elliptically polarized laser pulses, can be explained by non-adiabatic effects, and a related consistency problem in current laser intensity calibration methods is pointed out. We further show that Fano resonance line shapes of doubly excited states in the Helium atom, prominently appearing in single ionization spectra generated by short wavelength laser pulses, can be controlled by an external long wavelength streaking field. The resulting line shapes are still characterized by the general Fano situation, but with a complex - rather than real - Fano parameter. We provide a theoretical description of this two color process and prove numerically that the entire doubly excited state series exhibits synchronized line shape modifications as the specifics of the involved states are unimportant. Finally, we compute fully differential double ionization spectra and suggest a measure of correlation that is directly applicable to experimental data. We confirm literature results at short wavelengths, and achieve to compute five-fold differential double ionization photo-electron spectra at infrared wavelengths from the Helium atom, thereby reproducing a characteristic several orders of magnitude enhancement of double emission due to correlation effects.

While quantum theory has been tested to an incredible degree on microscopic scales, quantum effects are seldom observed in our everyday macroscopic world. The curious results of applying quantum mechanics to macroscopic objects are perhaps best illustrated by Erwin Schrödinger's famous thought experiment, where a cat can be put into a superposition state of being both dead and alive. Obviously, these quantum predictions are in stark contradiction to our common experience. Even with plenty of theoretical explanations put forward to explain this discrepancy, a large number of questions about the frontier between the quantum and the classical world remain unanswered. To distinguish between classical and quantum behavior, two fundamental concepts inherent to classical physics have been established over the years: The world view of local realism limits the power of classical experiments to establish correlations over space, while the world view of macroscopic realism (or macrorealism) restricts temporal correlations. Necessary conditions for both world views have been formulated in the form of Bell and Leggett-Garg inequalities, and Bell inequalities have been shown to be violated by quantum mechanics through increasingly conclusive experiments. Furthermore, many challenging steps towards convincing violations of macrorealism have been taken in a number of recent experiments. In the first part of this thesis, conditions for macrorealism are analyzed in detail. Two necessary conditions for macrorealism, the original Leggett-Garg inequality and the recently proposed no-signaling in time condition, are presented. It is then shown that a combination of no-signaling in time conditions is not only necessary but also sufficient for the existence of a macrorealistic description. Finally, an operational formulation of no-signaling in time, in terms of positive-operator valued measurements and Hamiltonians, is derived. In the next part, we argue that these results lead to a suitable definition of classical behavior. In particular, we provide a formalism to judge the classicality of measurements and time evolutions. We then proceed to apply it to a number of exemplary measurement operators and Hamiltonians. Finally, we argue for the importance of spontaneously realized Hamiltonians in our intuition of classical behavior. Next, differences between local realism and macrorealism are analyzed. For this purpose, the probability polytopes for spatially and temporally separated experiments are compared, and a fundamental difference in the power of quantum mechanics to build both types of correlations is discovered. This result shows that Fine's theorem, which states that a set of Bell inequalities is necessary and sufficient for local realism, is not transferable to macrorealism. Thus, (Leggett-Garg) inequalities are in principle not well-suited for tests of macrorealism, as they can never form a necessary and sufficient condition, and unnecessarily restrict the violating parameter space. No-signaling in time is both better suited and more strongly motivated from the underlying physical theory. In the final part of this thesis, a concrete experimental setup for implementing quantum experiments with macroscopic objects is proposed. It consists of a superconducting micro-sphere in the Meißner state, which is levitated by magnetic fields. Through its expelled magnetic field, the sphere's center-of-mass motion couples to a superconducting quantum circuit. Properly tuned, ground state cooling can be realized, since the sphere's motion is extremely well isolated from the surrounding environment. This setup therefore is a promising candidate for the observation of quantum effects in macroscopic systems.

Mon, 25 Jan 2016 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19093/ https://edoc.ub.uni-muenchen.de/19093/1/Gaczkowski_Benjamin.pdf Gaczkowski, Benjamin ddc:530, ddc:500, Fakultät für Physik

This thesis aims to answer the question if 3D effects of thermal radiative transfer need to be considered in cloud resolving simulations and if an influence of 3D thermal heating and cooling rates exists in contrast to common 1D approximations. To study this question with the help of a cloud resolving model, an accurate, yet fast parameterization of 3D radiative transfer is needed. First, an accurate 3D Monte Carlo model was developed which was used as benchmark for developing the fast `Neighboring Column approximation' (NCA), which was then coupled to the UCLA-LES to study the effects of 3D thermal heating and cooling rates in comparison to common 1D radiative transfer approximations. First, differences between common 1D radiative transfer approximations and a correct 3D radiative transfer model were analyzed. For this, efficient Monte Carlo variance reduction methods have been developed and implemented in MYSTIC, a Monte Carlo radiative transfer model. The dependence of 1D and 3D heating and cooling rates on cloud geometry has been investigated by analyzing idealized clouds such as cubes or half spheres. Further more, 1D and 3D heating and cooling rates in realistic cloud fields were simulated and compared. It could be shown that cooling rates reach maximum values of several 100 K/d at cloud tops if the model resolution was between 50 m to 200 m. Additional cloud side cooling of several 10 to 100 K/d was found in 3D heating and cooling rate simulations. At the cloud bottom, modest warming of a few 10 K/d occurs. Heating and cooling rates depend on the vertical location of the cloud in the atmosphere, the liquid water content of the cloud, the shape of the cloud and the geometry of the cloud field (for example the distance between clouds). Based on the results of a detailed analysis of exact simulations of 3D thermal heating and cooling rates, a fast, but still accurate 3D parameterization for thermal heating and cooling rates has been developed. This parameterization, the `Neighboring Column Approximation' (NCA), is based on a 1D radiative transfer solution and uses the next neighboring columns of a column to estimate the 3D heating or cooling rate. The method can be used in parallelized models. With the NCA, it is possible to simulate 3D cloud side cooling and warming. It was shown that the NCA is a factor of 1.5 to 2 more expensive in terms of computational time when used in a cloud resolving model, compared to a 1D radiative transfer approximation. The NCA was implemented in UCLA-LES, a cloud resolving, large-eddy simulation model. With the UCLA-LES and the NCA it was possible for the first time to study the effects of 3D interactive thermal radiation on cloud development. Simulations without radiation, with 1D thermal radiation and 3D thermal (NCA) radiation have been performed and differences have been analyzed. First, single, isolated clouds were investigated. Depending on the cloud shape, 3D thermal radiation changes cloud development in comparison to 1D thermal radiation. Overall it could be shown that a thermal radiation effect on cloud development exists in general. Whether there is a differences between 1D and 3D thermal radiation on cloud development seems to depend on the specific situation. One of the main features of thermal radiation affecting a single cloud is a change in the cloud circulation. Stronger updrafts in the cloud core and stronger downdrafts at the cloud sides were found, causing an enhanced cloud development at first, but a faster decay of the cloud in the end. Second, large scale simulations of a shallow cumulus cloud field in a 25 x 25 km^2 domain with 100~m horizontal resolution were analyzed. To the authors knowledge, this is the first time that a cloud field of this size and resolution was simulated including 3D interactive thermal radiation. It was shown that on average, updrafts, downdrafts and liquid water increases if thermal radiation is accounted for. While most variables (for example liquid water mixing ratio or cloud cover) did not show significant systematic difference between no-radiation simulation and the simulations with 1D and 3D thermal radiation, the cloud size (or horizontal extent) was larger in the simulations with interactive 3D thermal radiation. Convective organization set in after a few hours already. This is a clear indication that 3D thermal radiation could trigger convective organization.

The interaction of photons with individual quantum systems is a very fundamental process in physics. Thereby, the emission rate as well as the angular emission pattern of a quantum emitter are not only a function of intrinsic properties of the emitter itself, but are also strongly modified by its surrounding. For instance, by restricting the optical modes which are allowed at the position of the dipole, the emission rate can be strongly modified and the emitted photons can be directed into specific optical modes. This effect can be demonstrated by the interaction of a single optically active quantum emitter with the strongly confined optical mode of a single-mode dielectric waveguide. Efficient coupling of the emitter to the dielectric structure can be achieved by placing the quantum emitter inside the evanescent field of the guided mode. This evanescent field coupling mechanism is discussed and demonstrated experimentally. A single nitrogen-vacancy center (NV-center), hosted in a nanodiamond is deterministically coupled to a tapered optical fiber (TOF) via the evanescent field of its guided mode (coupling efficiencies exceeding 30% are predicted). By employing an AFM-based nanomanipulation technique, the diamond nanocrystal is placed on the nanofiber waist of the TOF. Beforehand, the diamond nanocrystal has been characterized to guarantee that it hosts only one fluorescing NV-center. While the diamond nanocrystal is optically exited, single photon fluorescence of the NV-center is detected at both outputs of the tapered optical fiber. This verifies the evanescent coupling of the emitter to the guided mode. In order to quantify the coupling, the comparison of the emission rate into free space with the rate into the fiber yields that 10.0(5) of the emitted photons are coupled into the tapered optical fiber. In the determination of this value, the orientation of the emitting dipoles and the emission pattern, which are modified by the TOF, have been considered. The NV-center features a broad emission spectrum which can be used to investigate the wavelength-dependence of the coupling. Comparing the spectra of the emission into the fiber mode with the emission into free space modes roughly resembles the expected wavelength dependency of the coupling efficiency. The evanescent coupling and the deterministic positioning of preselected fluorescing diamond nanocrystals, which has been demonstrated with the TOF, can be applied to other waveguide structures as well. Dielectric single-mode waveguides made of Ta2O5 on a SiO2 substrate promise similar coupling efficiencies to tapered optical fibers (above 30%). With the design of the on-chip wave-guiding structure being flexible, the combination with other optical on-chip elements is feasible, rendering it a promising platform for on-chip photonic experiments. Test structures of this waveguide design are realized using lithographic processes and are characterized. These waveguides are equipped with inverted taper structures to allow efficient off-chip coupling with butt-coupling to standard single-mode fibers. The evanescent coupling of a single quantum emitter to a singe optical mode can be used to efficiently collect emission of the quantum emitter. This can help building a compact single photon source and is beneficial for the optical read-out of the quantum emitter's internal degree of freedom, which can be either used as probe (sensing) or as information-storage. Utilizing the high coupling efficiency, for instance, the non-linearities of the quantum system can be exploited to build a single photon transistor. The evanescent coupling is very broadband (about hundred nanometers), allowing to efficiently collect emission from broadband emitters like the NV-center, but it can also be used for multi-wavelength manipulation schemes.

Wed, 16 Dec 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19038/ https://edoc.ub.uni-muenchen.de/19038/1/Huber_Markus_B.pdf Huber, Markus B. dd

Active Galactic Nuclei (AGN) are the accreting super massive black hole (SMBH) at the center of massive galaxies. The tight M-σ and M_BH-M_bulge correlations reveal that the host galaxies are affected by the effects of the SMBHs. In addition, many works studying on the rest-frame color-magnitude relation have shown that AGN host galaxies have intermediate colors, which are considered as a transition from the blue cloud to red sequence in host-galaxy color evolution. Some works interpreted this result as an evidence for the AGN feedback, in the sense that the accretion process not only builds up the massive black holes, but also has a powerful influence on the surrounding environment, triggering or suppressing the star-forming activity in the host galaxy. These correlations make obvious the need to investigate AGN-host coevolution. One way to understand this coevolution is to study the AGN duty cycle (i.e., the time scale that the SMBH is active), which can be obtained by estimating AGN population among all the galaxies through cosmic times. Such demographic studies require a well-sampled census and accurate redshift information. In particular at high redshift, objects are extremely faint and sample numbers are very small. This could cause large statistical errors. For example, Aird et al. (2010) argued that luminosity-dependent density evolution with a flattening faint-end slope of the X-ray luminosity function (XLF) at z > 1.2 may result from catastrophic photo-z failures caused by observational limitations and improper templates used for photo-z computation. However, spectroscopic redshifts are time-consuming and difficult to be obtained for faint sources at high redshifts. Therefore we have to rely on photometric redshifts (photo-z) techniques which need to be tuned specifically to be reliable for AGNs (i.e., proper magnitude priors, appropriate AGN-galaxy hybrid template for SED-fitting, and correct multi-wavelength counterparts). In recent years, many deep and high-resolution observations become available in multiple wavebands, specially at near/mid-infrared. This allows us to reach higher redshift, and make more accurate analysis on the multi-wavelength properties of AGNs. In this thesis, we focus on the ECDFS area which comprise also the GOODS-S and CDFS regions. This is the portion of the sky with the deepest and most complete photometric information from X-ray to radio, including intermediate bands from the Subaru telescope, and optical/near-infrared data from the Hubble space telescope. To compute accurate photo-z using these data, first we combined multi-wavelength catalogs from UV to infrared after the astrometric calibration and correction for the different methods of flux extraction (e.g., total fluxes, flux apertures and PSF- fitted photometry). Second we identified the best multi-wavelength counterparts for X-ray sources from the 4Ms-CDFS and 250ks-ECDFS surveys, taking into account the positional errors and multiple magnitude distributions as priors simultaneously. We found that more then 96% of X-ray sources have multi-wavelength counterparts. Thirdly we built a new library of active galactic nuclei/galaxy hybrid templates appropriate for the faint X-ray population in the CDFS to simulate the AGN spectral energy distribution from low to high redshift. For X-ray-selected AGNs, we achieved a photo-z accuracy of 0.013 with an outliers fraction of 5.3%, while for non-X-ray galaxies, the photo-z accuracy is 0.010 with an outlier fraction of 4.6%. With the SED-fitting results of our well-trained AGN-galaxy hybrids, we further studied the galaxy and AGN host properties via the rest-frame color-magnitude diagram (CMD) which is an useful probe to trace the stellar populations. We made corrections for dust extinction and/or AGN contamination for the galaxy/AGN host colors in the CANDELS/GOODS-S region. We found that the AGN host colors also present bimodality in the CMD up to z~2.5 as found in normal galaxies, and the position of the blue peaks in the AGN samples are almost constant with cosmic time. This implies a weak connection between AGN activity and star formation in the host galaxy. For the X-ray sources in the 4Ms-CDFS survey, we found that for most of the sources, the correction for dust extinction is larger than the correction for the AGN contribution. This is because the AGN population in this field is dominated by low-luminosity AGNs which have host-dominated SEDs. However for few bright sources, their host colors are strongly effected by AGN contribution rather than by the dust extinction. For these sources, the correction for AGN contribution is about two times larger than the correction for dust extinction in general. Therefore AGN/galaxy decomposition becomes more important in a shallower and wider X-ray surveys, e.g., XMM-COSMOS and eROSITA, which contains a larger fraction of bright AGNs. Furthermore, with our accurate redshifts for galaxies and AGNs, we defined a high-redshift (high-z) sample using the redshift probability distribution function P(z) rather than relying on the best-fit value of photo-z. We integrated P(z) within a given redshift range to obtain the photo-z probability in that range and selected high-z sources above a given threshold. When computing the number of sources in a given redshift range, each source will not be counted as "1" but as the proportion of it. We compared this P(z) technique with traditional color techniques adopted for galaxy evolutionary stages, like the Lyman break galaxy and the BzK color-color selection via sample completeness and purity. We found that the P(z) technique is the most efficient and reliable method for selecting high-z sources. This is not surprising as it makes use of photometric information from the entire SED rather than using only three photometric points. Lastly, we built a high-z (z > 3) sources list for X-ray sources in ECDFS region, and compared our list with previous work. In our work, we made better X-ray-to-optical/NIR associations considering the positional errors and magnitude distribution. In addition, we obtained accurate photo-z using well-established AGN-galaxy hybrids for X-ray selected AGNs and applied P(z) for each source. These procedures help improving on our high-z sample selection.

The work presented in this thesis was aimed at developing a high-repetition rate source of coherent radiation in the extreme ultra-violet (XUV) spectral region, envisaging applications in attosecond physics or precision metrology in the XUV. Due to the lack of laser oscillators operating in the XUV, the method of choice was the frequency upconversion of a near-infrared laser via the nonlinear process of high-order harmonic generation. Obtaining sufficient XUV photon flux per pulse at repetition rates of several tens of MHz, despite the inherently low conversion efficiency, requires a powerful driving source. To date, passive enhancement of ultrashort pulses in an external resonator has been the most successful strategy in meeting this demand. In this thesis four main achievements towards extending this technique and understanding its limitations are presented. A first experiment was dedicated to obtaining shorter intracavity pulses without compromising the high average power available from Yb-based laser technology. To this end, we spectrally broadened and temporally compressed the pulses prior to the enhancement in a broadband resonator. Aside from being a prerequisite for time-domain applications, shorter intracavity pulses led to improved conditions for the harmonic generation process. Furthermore, we addressed the task of extracting the intracavity generated XUV light. We established two methods for geometrical XUV output coupling, one employing the fundamental mode of the cavity, and the other a tailored transverse mode, which offers additional degrees of freedom to shape the harmonic emission. Both techniques are particularly suited for the intracavity generation of attosecond pulses, because they afford an unparalleled flexibility for the resonator design, and exhibit a broadband output coupling efficiency approaching unity for short-wavelength radiation. This enabled a significant improvement of the crucial parameters, photon flux and photon energy. In a combined experimental and theoretical study, we investigated the ionization-related intensity limitations observed in state-of-the-art enhancement cavities. The quantitative modeling of the nonlinear interaction allows for an estimation of the achievable intracavity parameters and for a global optimization of the XUV photon flux. Based on this model, we proposed a strategy to mitigate this limitation by using the nonlinearity in combination with customized cavity optics for a further spectral broadening and temporal compression of the pulse in the resonator. More importantly, this work establishes enhancement cavities as a tool to investigate nonlinear light-matter interactions with the increased sensitivity provided by the resonator. The last study was dedicated to the technological challenge of building a resonator in which the electric field of the circulating pulse is reproduced at each round-trip. This is an essential prerequisite to generate identical XUV emission with each driving pulse. By tailoring the spectral phase of the cavity mirrors we succeeded in enhancing pulses of less than 30 fs (less than nine cycles of the driving field) to a few kilowatts of average power with zero pulse-to-pulse carrier-to-envelope phase slip. At similar pulse durations, the generation of isolated attosecond pulses has already been demonstrated in single-pass geometries. In conclusion, the results presented in this thesis are milestones on the way to a powerful, compact and coherent source of ultrashort XUV radiation. The unique property of the source, that is, its high repetition rate lays the foundation for advancing attosecond physics and precision spectroscopy in the XUV region

Thu, 3 Dec 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19001/ https://edoc.ub.uni-muenchen.de/19001/1/Guggenmos_Alexander.pdf Guggenmos, Alexander ddc:530, ddc:500, Fakultät für Ph

Wed, 2 Dec 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18988/ https://edoc.ub.uni-muenchen.de/18988/1/Sommer_Annkatrin.pdf Sommer, Annkatrin ddc:530, ddc:500, Fakultät für Physik

Spectroscopy of fundamental vibrational transitions offers a label-free alternative for high-chemical contrast measurements. These transitions can be interrogated either directly by using mid-infrared light or indirectly through Raman scattering. This thesis aims to advance dual-comb spectroscopy to improve the acquisition speed, resolution and spectral coverage of vibrational spectroscopy. Dual-comb spectroscopy is a time domain technique, which combines optical frequency combs -coherent light sources with a spectrum constituted of discrete evenly spaced lines - and Fourier transform spectroscopy. For linear spectroscopy, a mid-infrared optical parametric oscillator was developed and characterized. Its idler-pulse duration can be as short as a few cycles (~3 to 6 cycles), with a central wavelength tunable from 2180nm to 3732nm (2679cm-1 - 4587cm-1), allowing more than 2500nm (2861 cm-1) of total coverage while maintaining an average power of tens of milliwatts. The high peak power of this system was exploited for spectral broadening; generation of phase-coherent supercontinua was achieved in waveguides, made from either silicon or chalcogenide glass, producing octave spanning spectra ~1500nm to 3300nm (3030cm-1 - 6666cm-1) for silicon and from ~1600nm beyond 3860nm (2590 cm-1 - 6250 cm-1) for chalcogenide glass). Two optical parametric oscillator were constructed, advancing toward a dual-comb mid-infrared spectrometer. Since the optical parametric oscillators are not stabilized, an additional correction scheme was set up and characterized. Coherent Raman scattering was also investigated, as a means to access optically active and inactive fundamental vibrational transitions. Several spectroscopy setups were developed to measure the Raman blue or red shifted light in forward and backward scattered direction as well as a differential detection between blue and red shifted light. There is a dead time between consecutive interferograms existent, up to a factor of 1000 larger than the measurement time. This dead time could be reduced by an order of magnitude using a laser with ~1GHz and a laser with 100MHz repetition rate instead of two lasers with ~100MHz repetition rate. All implementations achieved excellent acquisition times (in the microsecond range), signal-to-noise ratios up to 1000 and spectral coverage of about ~1200 cm-1). These advantages enabled measuring spectrally resolved images, in a first rudimentary microscopy-setup.

Fusion plasmas contain impurities, either intrinsic originating from the wall, or injected willfully with the aim of reducing power loads on machine components by converting heat flux into radiation. The understanding and the prediction of the effects of these impurities and their radiation on plasma performances is crucial in order to retain good confinement. In addition, it is important to understand the impact of pellet injection on plasma performance since this technique allows higher core densities which are required to maximise the fusion power. This thesis contributes to these efforts through both experimental investigations and modelling. Experiments were conducted at ASDEX Upgrade which has a full-W wall. Impurity seeding was applied to H-modes by injecting nitrogen and also medium-Z impurities such as Kr and Ar to assess the impact of both edge and central radiation on confinement. A database of about 25 discharges has been collected and analysed. A wide range of plasma parameters was achieved up to ITER relevant values such as high Greenwald and high radiation fractions. Transport analyses taking into account the radiation distribution reveal that edge localised radiation losses do not significantly impact confinement as long as the H-mode pedestal is sustained. N seeding induces higher pedestal pressure which is propagated to the core via profile stiffness. Central radiation must be limited and controlled to avoid confinement degradation. This requires reliable control of the impurity concentration but also possibilities to act on the ELM frequency which must be kept high enough to avoid an irreversible impurity accumulation in the centre and the consequent radiation collapse. The key role of the ELM frequency is confirmed also by the analysis of N+He discharges. Non-coronal effects affect the radiation of low-Z impurities at the plasma edge. Due to the radial transport, the steep temperature gradients and the ELM flush out, a local equilibrium cannot be establish an the radiation increases in this region. To account for these effects, an empirical non-coronal model was developed which takes the impurity residence time at the pedestal into account. The validity of this assumption was verified by modelling the evolution of the impurities and radiation for ASDEX Upgrade H-modes with nitrogen seeding by coupling the ASTRA transport code with STRAHL. The time-dependent simulations include impurity radiation due to nitrogen and tungsten and the transport effects induced at the edge by the ELMs. The modelling results have been validated against the experimental data. The modelled radiation profiles show a very good agreement with the measured ones over both radius and time. In particular, the strong enhancement of the nitrogen radiation caused by non-coronal effects through the ELM-induced transport is well reproduced. The radiation properties of tungsten are very weakly influenced by non-coronal effects due to the faster equilibration. W radiation, which is highly dependent on the Elm frequency, strongly increases when this is decreased, due to the lack of sufficiently strong flush out of this impurity. This is in agreement with the experimental observations and indicates that maintaining high ELM frequency is essential for the stability and performance of the discharges. Analyses of the high density scenario with pellets indicate that several processes take place when pellets are injected into the plasma. In particular, due to their cooling effect, the temperature drops as soon as pellets are injected. This is compensated by an increase in density. These processes occur mainly at the edge and are propagated to the core via stiffness. This explains why the confinement stays approximately constant during the whole discharge. Both experiments and transport calculations reveal that the energy confinement time is independent of the density indicating that the currently used scaling is not valid in this regime. The results of this thesis will contribute towards an extension of the confinement scaling which is currently being undertaken.

In dieser Arbeit wird der Aufbau eines neuartigen, atomphysikalischen Experiments beschrieben, das zum Ziel hat, stark wechselwirkende Vielteilchensysteme bestehend aus polaren 23Na40K Molekülen zu erzeugen und zu studieren. Die anisotrope und langreichweitige Dipol-Dipol Wechselwirkung zwischen den Molekülen sollte es möglich machen, bisher nicht beobachtete Quanten-Vielteilchenzustände zu beobachten und prototypische Gittermodelle der Festkörperphysik zu simulieren, die zur Beschreibung von Quantenmagnetismus und Hochtemperatursupraleitern verwendet werden. Das 23Na40K Molekül ist für diesen Zweck besonders gut geeignet, da es in einer zwei-Körper Kollision chemisch stabil ist, fermionischer Quantenstatistik unterliegt und ein starkes Dipol Moment aufweist. Die experimentelle Prozedur zur Erzeugung eines ultrakalten Quantengases aus hetero-nuklearen Molekülen erfordert es, zuerst die elementaren Bestandteile des Moleküls durch Laser- und Verdampfungskühlen in den Zustand der simultanen Quantenentartung zu überführen. Die Wechselwirkung zwischen den bosonischen 23Na und den fermi-ionischen 40K Atomen lässt sich durch Ausnutzen einer Feshbach Streuresonanz mit einem externen Magnetfeld kontrollieren. In der Nähe einer solchen Feshbach Resonanz werden schwach gebundene 23Na40K Moleküle durch Radiofrequenzassoziation erzeugt. In einem weiteren Schritt sollen diese Feshbach Moleküle durch eine stimulierte Raman adiabatische Passage (STIRAP) in den rovibronischen und Hyperfein-Grundzustand des Moleküls überführt werden. Die Differenz der Bindungsenergie wird hierbei nicht spontan frei, was unweigerlich die Aufhebung der Quantenentartung des Molekülgases zur Folge hätte, sondern wird durch stimulierte Emission kontrolliert abgeführt. Die Kombination beider Methoden, der Feshbach Assoziation und der STIRAP, erlaubt es den Prozess der Molekülbindung auf fundamentaler, quantenmechanischer Ebene zu steuern. Um die STIRAP zu implementieren ist es notwendig, ein geeignetes molekulares Zwischenniveau in einem elektronisch angeregten Zustand zu identifizieren, über welches das Feshbach Molekül mit dem rovibronischen Grundzustand in einen zwei-Photonen Übergang gekoppelt wird. Ein solches Zwischenniveau konnte durch hochauflösende Molekülspektroskopie im elektronisch angeregten 3Pi Zustand identifiziert werden. Dieser Vibrationszustand ($vert^3Pi_{Omega=1}nu=5rangle$) ist durch molekulare Spin-Orbit Wechselwirkung an einen nah-resonanten Vibrationszustand im $D^1Pi$ Zustand gekoppelt. Erst durch die Beimischung dieses Spin-Singulett Zustands ist es möglich den rovibronischen Grundzustand (ebenfalls Spin-Singulett) zu adressieren. Die zugehörige Übergangsfrequenz konnte durch kohärente Zwei-Photonen Spektroskopie bestimmt werden. Durch elektromagnetisch induzierte Transparenz wurden die Rabifrequenzen beider STIRAP Übergänge bestimmt und die Kohärenzeigenschaften des Dunkelzustandes untersucht. Bis zum heutigen Zeitpunkt ist es nicht möglich den identifizierten Zwischenzustand zu benutzen um 23Na40K Moleküle in den rovibronischen Grundzustand zu überführen. Das Phasenrauschen der zum Einsatz kommenden Halbleiter-Laser konnte als limitierender Faktor identifiziert werden. Darüberhinaus führt die spektroskopisch nicht auflösbare molekulare Hyperfeinstruktur des $vert^3Pi_{Omega=1}nu=5rangle$ Zustands zu einer Konfiguration in der kein echter Dunkelzustand existiert, der für die STIRAP benutzt werden kann. Aus diesen Gründen erscheint es unwahrscheinlich, dass das gegenwärtige STIRAP Schema (Halbleiterlaser, $vert^3Pi_{Omega=1}nu=5rangle$ Zwischenniveau, resonante STIRAP) Grundzustandsmoleküle mit hoher Effizienz erzeugen wird. Dieses Schema kann jedoch durch ein anderes ersetzt werden, das erst kürzlich erfolgreich für den Grundzustands-Transfer von 23Na40K verwendet wurde. Die günstigen Eigenschaften des 23Na40K Moleküls in Kombination mit dem hier präsentierten Experimentaufbau sollten es daher in Zukunft möglich machen, dipolare Vielteilchensysteme zu erzeugen und zu studieren.

In der vorliegenden Dissertation werden elektrische Eigenschaften stark gekoppelter Systeme in Anwesenheit von Störungen untersucht. Dies erfolgt anhand der Dualität zwischen Eich- und Gravitationstheorien, die eine Beschreibung solcher Systeme mittels einer schwach gekoppelten Gravitationstheorie ermöglicht. Besondere Aufmerksamkeit wird hierbei der Berechnung von Ladungsdichten und Leitfähigkeiten gewidmet, sowie der Untersuchung der von den Störungen hervorgerufenen Auswirkungen auf diese. Unseren Rechnungen liegt die AdS/CFT-Korrespondenz zugrunde. Diese besagt, dass konforme Quantenfeldtheorien im flachen Minkowskiraum höherdimensionalen Stringtheorien im Anti-de-Sitter Raum gleichzusetzen sind. Einen besonders interessanten Grenzfall stellt der Limes dar, in dem die Quantenfeldtheorie einer sehr stark gekoppelten mit vielen internen Freiheitsgraden ausgestatteten Eichsymmetrie unterliegt. Die duale Stringtheorie kann in diesem Falle zu einer klassischen Gravitationstheorie im Anti-de-Sitter Raum vereinfacht werden. Ein relevantes Merkmal, aus dem der große praktische Wert der Dualität entspringt, liegt hierbei in der Tatsache, dass aus schwach gekoppelten Gravitationstheorien stammende Ergebnisse im Rahmen stark gekoppelter Quantenfeldtheorien interpretierbar sind. Angesichts des hohen technischen Schwierigkeitsgrades, den stark gekoppelte Theorien aufweisen, macht diese Eigenschaft die Dualität zu einem mächtigen mathematischen Werkzeug hinsichtlich eines besseren Verständnisses der Physik letzterer. Trotz fehlendem formellem Beweis ihrer allgemeinen Gültigkeit hat die AdS/CFT-Korrespondenz im Laufe der letzten Jahre wichtige Fortschritte in diesem Zusammenhang zuwege gebracht. Hervorzuheben sind Berechnungen von Transportkoeffizienten stark gekoppelter Theorien wie Viskositäten, Leitfähigkeiten und Diffusionskonstanten. Störungen treten in realen physikalischen Systemen immer auf. Jedoch ist wenig über deren Auswirkungen auf stark gekoppelte Materie bekannt. Die AdS/CFT-Korrespondenz ebnet den Weg zu einem besseren Verständnis hiervon. Um den Einfluss von Unreinheiten auf die oben genannten Transporteigenschaften stark gekoppelter Systeme mithilfe der AdS/CFT-Korrespondenz zu untersuchen muss die Abhängigkeit der Felder von mindestens zwei Koordinaten vorausgesetzt werden. Die zugehörigen Bewegungsgleichungen sind partielle Differentialgleichungen, deren analytische Handhabung technisch nicht durchführbar ist. Rechnergestützte numerische Methoden stellen die einzige Möglichkeit dar, diesem Problem beizukommen. Besonders geeignet hierfür erweisen sich die sogenannten Spektralmethoden, deren Anwendung auf Rechnungen im Rahmen der AdS/CFT-Korrespondenz in Detail erläutert wird. In der vorliegenden Arbeit bedienen wir uns der oben erwähnten Methoden, um numerische Lösungen von Gravitationstheorien zu ermitteln, die aufgrund der Dualität inhomogenen stark gekoppelten Systemen fundamentaler Teilchen entsprechen. Die Störungen, deren Auswirkungen auf die Transporteigenschaften des dualen Systems zu untersuchen sind, werden durch eine nichttriviale räumliche Struktur von physikalischen Größen der Gravitationstheorie eingeführt. Diese wird in einer ersten Ausführung von einem stufigen raumabhängigen Massenprofil dargestellt, das eine lokalisierte Störung in Form einer Grenzoberfläche bildet. Der Analyse der resultierenden Ladungsdichten und Leitfähigkeiten kann entnommen werden, dass die Präsenz der Grenzoberfläche eine Lokalisierung der Ladungsdichte in derer unmittelbaren Umgebung bewirkt. Des Weiteren wird eine lokale Erhöhung der Leitfähigkeit bei niedrigen Frequenzen in der zur Grenzoberfläche parallelen Richtung festgestellt. In der senkrechten Richtung nimmt die Leitfähigkeit bei niedrigen Frequenzen einen konstanten Wert an und wird in Vergleich zur parallelen Richtung abgeschwächt. Das Hochfrequenzverhalten der Leitfähigkeiten in beiden Richtungen wird nicht von der Inhomogenität gestört und weist keine Unterschiede auf. In einem zweiten Fall wird die nichttriviale räumliche Struktur in Form einer zufälligen Raumabhängigkeit des chemischen Potenzials entlang einer Richtung eingeführt, die die Störungen in der lokalen Energie der Ladungsträger nachbildet. Dabei wird festgestellt, dass diese Art von delokalisierten Störungen ein globales Anwachsen der Ladungsdichte des Systems herbeiführt. Die Leitfähigkeit wird von den Störungen abgeschwächt und ihr Verhalten weist qualitative Übereinstimmung mit Modellen der Transporteigenschaften von Graphen in der Physik der kondensierten Materie.

Wed, 28 Oct 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19269/ https://edoc.ub.uni-muenchen.de/19269/1/Hertrich_Samira.pdf Hertrich, Samira ddc:530, ddc:500, Fakultät für Physik

To survive, organisms must respond appropriately to a variety of challenges posed by a dynamic and uncertain environment. The mechanisms underlying such responses can in general be framed as input-output devices which map environment states (inputs) to associated responses (output. In this light, it is appealing to attempt to model these systems using information theory, a well developed mathematical framework to describe input-output systems. Under the information theoretical perspective, an organism’s behavior is fully characterized by the repertoire of its outputs under different environmental conditions. Due to natural selection, it is reasonable to assume this input-output mapping has been fine tuned in such a way as to maximize the organism’s fitness. If that is the case, it should be possible to abstract away the mechanistic implementation details and obtain the general principles that lead to fitness under a certain environment. These can then be used inferentially to both generate hypotheses about the underlying implementation as well as predict novel responses under external perturbations. In this work I use information theory to address the question of how biological systems generate complex outputs using relatively simple mechanisms in a robust manner. In particular, I will examine how communication and distributed processing can lead to emergent phenomena which allow collective systems to respond in a much richer way than a single organism could.

Thu, 22 Oct 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18818/ https://edoc.ub.uni-muenchen.de/18818/1/Kulkarni_Sandesh.pdf Kulkarni, Sandesh ddc:530, d

Wed, 21 Oct 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18808/ https://edoc.ub.uni-muenchen.de/18808/1/Schneider_Waldemar.pdf Schneider, Waldemar ddc:530, ddc:500, Fakultät für Physik

Tue, 20 Oct 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18825/ https://edoc.ub.uni-muenchen.de/18825/1/Nedev_Spas_Nedev.pdf Nedev, Spas Nedev ddc:530, ddc:500, Fakultät für Physik

This thesis advances and applies matrix product state (MPS) based algorithms to the solution of dynamical mean-field theory (DMFT) and its variants. The advances enable to solve quantum many-body problems in and out of equilibrium that were previously out of reach for any numerical treatment. In equilibrium, this concerns in particular the computation of the electronic --- such as insulating, metallic, spin-freezed and many other --- phases of highly complex realistic models for correlated materials. In non-equilibrium, this concerns in particular the understanding of the fundamental mechanisms of the relaxation behavior of quantum many-body systems on short and intermediate time scales.

Mon, 12 Oct 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18768/ https://edoc.ub.uni-muenchen.de/18768/1/Betz_Andre_W.pdf Betz, André Wolfgang ddc:530, ddc:500, Fakultät für Physik 0

Fri, 9 Oct 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19000/ https://edoc.ub.uni-muenchen.de/19000/1/Lorenzen_Konstantin.pdf Lorenzen, Konstantin

Wed, 7 Oct 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18802/ https://edoc.ub.uni-muenchen.de/18802/1/Kuehler_Paul.pdf Kühler, Paul ddc:530, ddc:500, Fakultät

Tue, 6 Oct 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18741/ https://edoc.ub.uni-muenchen.de/18741/1/Majety_Naga_Venkata_Vinay_Pramod.pdf Majety, Naga Venkata Vinay Pramod ddc:530, ddc:500, Fakultät für Physik

Electron spins confined in quantum dots (QDs) are among the leading contenders for implementing quantum information processing. In this Thesis we address two of the most significant technological challenges towards developing a scalable quantum information processor based on spins in quantum dots: (i) decoherence of the electronic spin qubit due to the surrounding nuclear spin bath, and (ii) long-range spin-spin coupling between remote qubits. To this end, we develop novel strategies that turn the unavoidable coupling to the solid-state environment (in particular, nuclear spins and phonons) into a valuable asset rather than a liability. In the first part of this Thesis, we investigate electron transport through single and double QDs, with the aim of harnessing the (dissipative) coupling to the electronic degrees of freedom for the creation of coherence in both the transient and steady-state behaviour of the ambient nuclear spins. First, we theoretically show that intriguing features of coherent many-body physics can be observed in electron transport through a single QD. To this end, we first develop a master-equation-based formalism for electron transport in the Coulomb-blockade regime assisted by hyperfine (HF) interaction with the nuclear spin ensemble in the QD. This general tool is then used to study the leakage current through a single QD in a transport setting. When starting from an initially uncorrelated, highly polarized state, the nuclear system experiences a strong correlation buildup, due to the collective nature of the coupling to the central electron spin. We demonstrate that this results in a sudden intensity burst in the electronic tunneling current emitted from the QD system, which exceeds the maximal current of a corresponding classical system by several orders of magnitude. This gives rise to the new paradigm of electronic superradiance. Second, building upon the insight that the nuclear spin dynamics are governed by collective interactions giving rise to coherent effects such as superradiance, we propose a scheme for the deterministic generation of steady-state entanglement between the two nuclear spin ensembles in an electrically defined double quantum dot. Because of quantum interference in the collective coupling to the electronic degrees of freedom, the nuclear system is actively driven into a two-mode squeezedlike target state. The entanglement buildup is accompanied by a self-polarization of the nuclear spins towards large Overhauser field gradients. Moreover, the feedback between the electronic and nuclear dynamics is shown to lead to intriguing effects such as multistability and criticality in the steady-state solutions. In the second part of this Thesis, our focus turns towards the realization of long-range spin-spin coupling between remote qubits. We propose a universal, on-chip quantum transducer based on surface acoustic waves in piezo-active materials. Because of the intrinsic piezoelectric (and/or magnetostrictive) properties of the material, our approach provides a universal platform capable of coherently linking a broad array of qubits, including quantum dots, trapped ions, nitrogen-vacancy centers or superconducting qubits. The quantized modes of surface acoustic waves lie in the gigahertz range, can be strongly confined close to the surface in phononic cavities and guided in acoustic waveguides. We show that this type of surface acoustic excitations can be utilized efficiently as a quantum bus, serving as an on-chip, mechanical cavity-QED equivalent of microwave photons and enabling long-range coupling of a wide range of qubits. In summary, this thesis provides contributions towards developing a scalable quantum information processor based on spins in quantum dots in two different aspects. The first part is dedicated to a deeper understanding of the nuclear spin dynamics in quantum dots. In the second part we put forward a novel sound-based strategy to realize long-range spin-spin coupling between remote qubits. This completes a broad picture of spin-based quantum information processing which integrates different perspectives, ranging from the single-qubit level to a broader quantum network level.

Wed, 30 Sep 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19113/ https://edoc.ub.uni-muenchen.de/19113/1/Schwemmer_Christian.pdf Schwemmer, Christian ddc:530, ddc:500, Fakultät für Physik

Ultrafast electron diffraction is a pump--probe technique that allows the visualisation of molecular dynamics with atomic scale resolution. However, the fastest electronic and atomic dynamics in light-driven matter transformations are, as yet, unmeasureable with this technique. This is because the temporal resolution in ultrafast electron diffraction is limited by difficulties in producing the shortest electron pulses, caused by the electron charge, via Coulomb repulsion (space charge), and rest mass, via vacuum dispersion of the electron wavefunction. Space charge effects and a finite energy bandwidth both lead to temporal broadening of electron pulses. Methods to compress such pulses in microwave fields have been developed, but these are fundamentally limited by the achievable temporal synchronisation of the employed microwave with the excitation laser pulses. This work is aimed at breaking this limitation and thereby advancing ultrafast electron diffraction towards the ultimate temporal resolution of any realistic light--matter interaction. Firstly, a high-resolution optical-microwave phase detector based on optical interferometry is designed for operation around the 800-nm wavelength of Ti:sapphire lasers best suited for sample excitation. The phase detector provides a resolution of 3 fs and the capability of functioning as an integral component in a phase-locked loop for synchronising a low-noise dielectric resonator oscillator with the Ti:sapphire laser. Furthermore, we demonstrate a separate, novel, passive synchronisation technique through direct microwave extraction of a harmonic of the laser repetition rate by photodetection. A record-low residual phase noise over nine frequency decades (mHz--MHz) is achieved through implementation of an optical-mode filter which circumvents thermal noise problems at low pulses energies to simultaneously reduce detrimental amplitude-to-phase noise conversion in the photodetection process. An amplification chain is designed to achieve a microwave power suitable for electron compression while preserving this excellent phase noise. Rigorous out-of-loop characterisation of the synchronisation with the optical-microwave phase detector shows a root-mean-square (rms) timing stability of 4.8 fs. This superior synchronisation has allowed the generation of 12 fs (rms) electron pulses, the shortest to our knowledge. Lastly, stability of the laser--electron synchronisation over many hours is also demonstrated on a sub-five-femtosecond scale through in-situ measurement and subsequent compensation for the entire range of possible long-term drifts. This shows that incorporating these techniques can allow ultrafast electron diffraction experiments to observe the fastest reversible atomic-scale light--matter interaction dynamics.

In this thesis we study geometric corrections to the low-energy effective actions of string theory. More concretely, we compute higher-derivative corrections to the couplings of three-dimensional, N = 2 supergravity theories and interpret the induced α′-corrections in N = 1, minimal supergravity theories in four dimensions, in the framework of F-theory. These allow for chiral spectra and are therefore phenomenological relevant. We analyzed higher-derivative corrections to M-theory, accessible through its low-energy effective theory, given by eleven-dimensional supergravity. The next to leading order terms to eleven-dimensional supergravity carry eight-derivatives, and are suppressed by lM6 compared to the classical terms, with lM being the eleven-dimensional Planck-Length - the only scale in eleven dimensions. These corrections are lifted from IIA supergravity corrections, which are derived from string scattering amplitudes. The common theme of this thesis is to compactify the bosonic sector of the eleven-dimensional supergravity action, including all known eight-derivative corrections, on a supersymmetric background to find a 3d, N = 2 theory, which then can be lifted to a 4d, N = 1 theory. This goal is approached in several steps. In the classical reduction of eleven-dimensional supergravity the metric background is a direct product of the external space, consisting of two space and one time dimension and the internal eight spacelike-dimensional Calabi-Yau manifold. However, when considering higher-derivative corrections the background has to be altered by introducing a dependence of the external space on the warp- factor, which is a function of the internal space. We find an explicit warped background solution to the eleven-dimensional E.O.M.’s including non-vanishing flux. To check the background for its supersymmetry features one would need to consider the eleven-dimensional gravitino variations at this order in lM . However, these are not known, which leads us to propose higher-order lM -corrected gravitino variations consistent with our background solution. As a next step we dimensionally reduce the bosonic sector of the eleven-dimensional supergravity action including all eight-derivative terms on this warped background and analyze the resulting three- dimensional theory. In this context the interplay of the warp-factor and the higher-derivative terms is of crucial importance. To identify the N = 2 properties of the resulting three-dimensional theory obtained by dimensional reduction, we compare it to the canonical from of three-dimensional N = 2 supergravtiy. We conclude that the reduced action is compatible with N = 2 supersymmetry and give a proposal for the K ̈ahler potential and the complex coordinates, which receive lM6 corrections. Besides a warp-factor contribution, the K ̈ahler potential receives a correction proportional to the third Chern- 2 form of the zeroth order internal background, being the Calabi-Yau fourfold. The complex coordinates are defined as divisor-integrals and are corrected by a warp-factor dependent term as well as one related to the non-harmonic part of the fourth Chern-form, of the zeroth order Calabi-Yau manifold. Thus the couplings of the resulting theory receive besides the warp-factor, in particular geometric corrections of order lM6 . In the first part of this thesis we study a simplified setup, only considering a subset of the relevant eight-derivative corrections in eleven dimensions. Furthermore, we do compactify on the classical background, consisting of the internal Calabi-Yau fourfold without warping and fluxes, to gain a three-dimensional theory. However, we use the M/F-theory duality to uplift the yielded corrections, which results in corrections to the couplings of the four-dimensional theory. In the weak coupling limit we find that these are sourced by the self-intersection curves of D7-branes.

Fri, 18 Sep 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18790/ https://edoc.ub.uni-muenchen.de/18790/1/Duca_Lucia.pdf Duca, Lucia ddc:530, ddc: