Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 02/05

The cosmic origin and evolution is encoded in the large-scale matter distribution

observed in astronomical surveys. Galaxy redshift surveys have become in the

recent years one of the best probes for cosmic large-scale structures. They are

complementary to other information sources like the cosmic microwave background, since they

trace a different epoch of the

Universe, the time after reionization at which the Universe

became transparent, covering about the last twelve billion years.

Regarding that the Universe is about

thirteen billion years old, galaxy

surveys cover a huge range of time, even if the sensitivity limitations of the

detectors do not permit to reach the furthermost sources in the transparent

Universe. This makes galaxy surveys extremely interesting for cosmological evolution studies.

The observables, galaxy position in the sky, galaxy ma

gnitude and redshift, however, give an incomplete representation of the real

structures in the Universe, not only due to the limitations and

uncertainties in the measurements, but also due to their biased

nature. They trace the underlying continuous dark matter field only partially

being a discrete sample of the luminous baryonic distribution.

In addition, galaxy catalogues are plagued by many complications. Some have a

physical foundation, as mentioned before, others are due to the

observation process. The problem of reconstructing the underlying density

field, which permits to make cosmological studies, thus requires a

statistical approach.

This thesis describes a cosmic cartography project.

The necessary concepts, mathematical frame-work, and numerical algorithms are

thoroughly analyzed.

On that basis a Bayesian software tool is implemented. The resulting Argo-code allows to

investigate the characteristics of the large-scale cosmological structure with unprecedented

accuracy and flexibility. This is achieved by jointly estimating the large-scale density along

with a variety of other parameters ---such as the cosmic flow, the small-scale peculiar velocity

field, and the power-spectrum--- from the information provided by galaxy redshift

surveys. Furthermore, Argo is capable of dealing with many observational issues like

mask-effects, galaxy selection criteria, blurring and noise in a very efficient

implementation of an operator based formalism which was carefully derived for this purpose.

Thanks to the achieved high efficiency of Argo the application of iterative sampling algorithms

based on Markov Chain Monte Carlo is now possible. This will ultimately lead to a full

description of the matter distribution with all its relevant parameters like velocities,

power spectra, galaxy bias, etc., including the associated uncertainties. Some applications

are shown, in which such techniques are used.

A rejection sampling scheme is successfully applied to correct for the observational

redshift-distortions effect which is especially severe in regimes of non-linear structure

formation, causing the so-called finger-of-god effect.

Also a Gibbs-sampling algorithm for power-spectrum determination is presented

and some preliminary results are shown in which the correct level and shape of

the power-spectrum is recovered solely from the data.

We present in an additional appendix the gravitational collapse and subsequent neutrino-driven

explosion of the low-mass end of stars that undergo core-collapse Supernovae.

We obtain results which are for the first time compatible with the Crab Nebula.

Entangled photons are at the heart of experimental quantum physics. They were used for the first fundamental tests of quantum theory, and became a basic building block for many novel quantum protocols, such as quantum cryptography, dense coding or teleportation. Therefore, the efficient generation of entangled photons, as well as their distribution and accurate analysis are of paramount importance, particularly with regard to the practicability of many applications of quantum communication. This thesis deals largely with the problem of efficient generation of photonic entanglement with the principal aim of developing a

bright source of polarization-entangled photon pairs, which meets the requirements for reliable and economic operation of quantum communication prototypes and demonstrators. Our approach uses a correlated photon-pair emission in nonlinear process of spontaneous parametric down-conversion pumped by light coming from a compact and cheap blue laser diode. Two alternative source configurations are examined within the thesis. The first makes use of a well established concept of degenerate non-collinear emission from a single type-II nonlinear crystal and the second relies on a novel method where the emissions from two adjacent type-I phase-matched nonlinear crystals operated in collinear non-degenerate regime are coherently overlapped. The latter approach showed to be more effective, yielding a total detected rate of almost 10^6 pairs/s at >98 % quantum interference visibility of polarization correlations. This performance, together with the almost free of alignment operation of the system, suggest that it is an especially promising candidate for

many future practical applications, including quantum cryptography, detector calibration or use in undergraduate lab courses. The second issue addressed within the thesis is the simplification and practical implementation of quantum-assisted solutions to multiparty communication tasks. While the recent rapid progress in the development of bright entangled photon-pair sources has been followed with ample experimental reports on two-party quantum communication tasks, the practical implementations of tasks for more than two parties have been held back, so far. This is mainly due to the requirement of multiparty entangled states, which are

very difficult to be produced with current methods and moreover suffer from a high noise. We show that entanglement is not the only non-classical resource endowing the quantum multiparty information processing its power. Instead, only the sequential communication and transformation of a single qubit can be sufficient to accomplish certain tasks. This we prove for two distinct communication tasks, secret sharing and communication complexity. Whereas the goal of the first is to split a cryptographic key among several parties in a way that its reconstruction requires their collaboration, the latter aims at reducing the amount of communication during distributed computational tasks. Importantly, our qubit-assisted solutions to the problems are feasible with state-of-the-art technology. This we clearly demonstrate in the laboratory implementation for 6 and 5 parties, respectively, which is to the best of our knowledge the highest number of actively performing parties in a quantum protocol ever implemented. Thus, by successfully solving and implementing a cryptographic task as well as a task originating in computer science, we clearly illustrate the potential to introduce multiparty communication problems into real life.

Die vorliegende Arbeit “Suche nach Cepheid-artigen Veränderlichen Sternen in nördlichen Zwerggalaxien der Lokalen Gruppe” faßt die Ergebnisse des VarStarDwarves-Projektes für alle “blauen”, periodisch Variablen Sterne (VS) zusammen. Die Zielsetzung des VarStarDwarves-Projekts, das seit 1999 an der Universitäts-Sternwarte München betrieben wird, läßt sich am besten durch ein Zitat aus dem zugrundeliegenden DFG-Antrag (Ho1812) erläutern: “Die Bestimmung der Sternentstehungsgeschichte von Zwerggalaxien (ZG) erlaubt, wesentliche Randbedingungen für die Modelle der Evolution von Galaxien abzuleiten. Eine Grundvoraussetzung hierzu ist die Kenntnis der genauen Distanz der ZG. Die Beobachtung hinreichend großer Sample von Veränderlichen Sternen ermöglicht es, diese Distanzen wesentlich präziser als bisher zu bestimmen. Um diese großen Sample zu erhalten, sollen mit für Microlensing entwickelten Bildverarbeitungsmethoden VS in nahen ZG bis weit unter die bis jetzt erreichten Detektionsgrenzen nachgewiesen werden. Darüberhinaus werden die gefundenen VS auch Hinweise zur stellaren Zusammensetzung und damit zum Entwicklungszustand der ZG geben.” Die Ergebnisse für langperiodische, “rote” VS wurden bereits in der Arbeit von Snigula [2006] präsentiert.

Im Rahmen des VarStarDwarves-Projekts wurden umfangreiche Modernisierungen und Verbesserungen an den Beobachtungsinstrumenten und dem Umfeld des Observatoriums Wendelstein durchgeführt und darüberhinaus die Entwicklung und der Bau einer neuen, modernen Zweikanal-CCD-Kamera initiiert. Auch diese Aspekte werden hier dokumentiert und die erreichten Fortschritte aufgezeigt (siehe hierzu Kapitel 2.2, 2.3 und 3 sowie die Anhänge A bis C). Diese Dissertation untergliedert sich weiter in eine Einleitung, die neben einer kurzen Vorstellung der mit dem Projekt befaßten Kollegen und ihrer diesbezüglichen Aufgabenbereichen v.a. eine historische Einführung in die Gebiete der Lokalen Zwerggalaxien sowie der “blauen” pulsationsveränderlichen Sterne bietet (Kap. 1), einen Überblick über Kandidatenauswahl und die durchgeführten Beobachtungen (Kap. 2.1, 2.4 und 2.5), sowie eine detaillierte Erläuterung der aufwendigen, z.T. ebenfalls im Rahmen des Projekts entwickelten Datenanalyseverfahren (Kap. 4).

Die wesentlichen wissenschaftlichen Ergebnisse der Arbeit sind neue, unabhängige Entfernungsbestimmungen mittels δ Cephei Perioden-Leuchtkraftbeziehungen zu zwei der sechs VarStarDwarves Zwerge, die Bestätigung der Entfernung eines weiteren Zwergs, sowie die Entdeckung etlicher weiterer VS-Kandidaten, deren endgültige Bedeutungen erst durch weitere Beobachtungen bestimmt werden können, wobei aber durchaus mit weitreichenden Konsequenzen für das physikalische Verständnis der entsprechenden VS oder ihrer “Muttergalaxie” zu rechnen ist (Kap. 5).

In the age of information and globalisation, secure communication as well as the protection of sensitive data against unauthorised access are of utmost importance. Quantum cryptography currently provides the only way to exchange a cryptographic key between two parties in an unconditionally secure fashion. Owing to losses and noise of today's optical fibre and detector technology, at present quantum cryptography is limited to distances below a few 100 km. In principle, larger distances could be subdivided into shorter segments, but the required quantum repeaters are still beyond current technology. An alternative approach for bridging larger distances is a satellite-based system, that would enable secret key exchange between two arbitrary points on the globe using free-space optical communication.

The aim of the presented experiment was to investigate the feasibility of satellite-based global quantum key distribution. In this context, a free-space quantum key distribution experiment over a real distance of 144 km was performed. The transmitter and the receiver were situated in 2500 m altitude on the Canary Islands of La Palma and Tenerife, respectively. The small and compact transmitter unit generated attenuated laser pulses, that were sent to the receiver via a 15-cm optical telescope. The receiver unit for polarisation analysis and detection of the sent pulses was integrated into an existing mirror telescope designed for classical optical satellite communications. To ensure the required stability and efficiency of the optical link in the presence of atmospheric turbulence, the two telescopes were equipped with a bi-directional automatic tracking system.

Still, due to stray light and high optical attenuation, secure key exchange would not be possible using attenuated pulses in connection with the standard BB84 protocol. The photon number statistics of attenuated pulses follows a Poissonian distribution. Hence, by removing a photon from all pulses containing two or more photons, an eavesdropper could measure its polarisation without disturbing the polarisation state of the remaining pulse. In this way, he can gain information about the key without introducing detectable errors. To protect against such attacks, the presented experiment employed the recently developed method of using additional "decoy" states, i.e., the the intensity of the pulses created by the transmitter were varied in a random manner. By analysing the detection probabilities of the different pulses individually, a photon-number-splitting attack can be detected. Thanks to the decoy-state analysis, the secrecy of the resulting quantum key could be ensured despite the Poissonian nature of the emitted pulses. For a channel attenuation as high as 35 dB, a secret key rate of up to 250 bit/s was achieved.

Our outdoor experiment was carried out under real atmospheric conditions and with a channel attenuation comparable to an optical link from ground to a satellite in low earth orbit. Hence, it definitely shows the feasibility of satellite-based quantum key distribution using a technologically comparatively simple system.

Fulgide gehören zu den photochromen molekularen Schaltern auf der Basis eines 6-elektrozyklischen Systems. Ihre ausgeprägte Photochromie wird von einer photoinduzierten, reversiblen Ringöffnung bestimmt: Das farbige Isomer mit geschlossenem Ring wird dabei durch sichtbares Licht in ein farbloses, offenes Isomer überführt. Mithilfe der transienten Absorptionsspektroskopie im UV/VIS und im mittleren IR konnten in der vorliegenden Arbeit entscheidende Fortschritte im Verständnis der perizyklischen Ringöffnung von Fulgiden und von chemisch nahe verwandten Fulgimiden erreicht werden.

Erstmals konnte die Kinetik der Ringöffnungsreaktion im Detail vermessen und damit der Reaktionsweg der Ringöffnung aufgeklärt werden: Die Messergebnisse deuten auf einem Modell mit einer direkten photochemischen Umwandlung auf der Zeitskala von wenigen Pikosekunden und ohne langlebige Intermediate hin.

Nach Anregung des geschlossenen Isomers in den angeregten S2-Zustand alternative Reaktionspfade abläuft. Die Mehrheit der S2-angeregten Moleküle beschreiten einen direkten Reaktionsweg, während nur etwa 40% die bekannte Abfolge (Kasha-Regel) der molekularen Prozesse für Photoreaktionen aus höheren elektronischen Zuständen durchlaufen: Zunächst

erfolgt eine Relaxation in den niedrigsten elektronisch angeregten Zustand und von dort die „normale“ S1-Photochemie.

Weiter werden die Auswirkungen optischer und thermischer Überschussenergie auf die Quanteneffizienz der Ringöffnungsreaktion der Fulgide untersucht. Ein einfaches Modell auf der Basis der Arrhenius-Gleichung wird entwickelt, das davon ausgeht, dass beide Arten der Energiezufuhr eine äquivalente photochemische Wirkung zeigen, dass also optische Überschussenergie sehr schnell thermalisiert. Zentrales Element ist dabei ein Konversionskoeffizient, der es in linearer Näherung erlaubt, optische Überschussenergie in zusätzliche thermische Energie

umzurechnen.

Im Zusammenhang mit potentiellen Anwendungen ist es gelungen, den Prototyp eines extrem schnellen, rein optischen Schreib/Lese/Lösch/Lese Operationszyklus auf molekularer Basis zu realisieren. Dieses „proof of principle“ Experiment demonstriert, dass die einzelnen Elemente des Zyklus basierend auf photochromen Fulgiden mit einer Taktrate von 250 GHz kombiniert werden können.

The topic of this thesis is the correspondence of open and closed strings. Already for a long time it has been evident that those both sectors of string theory do not only couple to each other, but it is also possible to identify excitations of the closed string in the open string sector. This correspondence has been shown in a multitude of examples, which indicates a deep connection.

In this work we consider different aspects of k-essence theories, which are scalar field theories described by the generally covariant and Lorentz invariant action with non-canonical kinetic terms. It is shown that, because of the intrinsic nonlinearity, these theories can have rather unexpected and unusual physical properties. As in

the usual relativistic field theories, any nontrivial dynamical solution of the k-essence equation of motion spontaneously breaks the Lorentz invariance. Thus the perturbations around such solutions propagate

in a "new aether". In contrast to the usual relativistic field theories, these perturbations propagate in an "effective" metric which is different from the usual gravitational metric. This effective metric can be interpreted as the so-called "emergent spacetime".

In this thesis we show that the dynamics of the perturbations can be described by the action which is generally covariant with respect to the reparameterization of this "emergent spacetime". The most

interesting manifestation of this "emergent spacetime" is that perturbations can propagate faster than light. We show that despite the superluminal propagation the causal paradoxes do not arise in these theories, and in this respect these theories are not less safe than General Relativity. This superluminal propagation of perturbations

has interesting consequences for cosmology and black hole physics. In particular, it is possible to construct models of inflation with an enhanced production of gravitational waves. This in turn can be verified in the nearest future by the observations of the B-mode polarization of the Cosmic Microwave Background Radiation (CMBR).

Moreover, we have shown that there exist k-essence models which in principle allow to obtain information from the region beyond the black hole horizon. This information is encoded in the perturbations around the solution describing the stationary accretion of the k-essence onto the black hole.

In addition, we discuss the possible dynamical violation of the Null Energy Condition (NEC) in the k-essence models. The violation of NEC by the dynamical Dark Energy (DE) is not excluded and even slightly preferred by observations Moreover, the violation of NEC plays a crucial role in the pre-Big-Bang scenarios. We have shown that in the framework of general k-essence the dynamical violation of NEC is physically implausible.

Particles of micrometer size externally introduced in plasmas usually find their positions of levitation in the plasma sheath, where the gravity force is compensated by the strong electric field. Here due to electrostatic interaction they form different structures, which are interesting objects for the investigation of strongly coupled systems and critical phenomena. Because of the low

damping (e.g. in comparison to colloidal suspension) it is possible to measure the dynamics up to the relevant highest frequency (e.g. Einstein frequency) at the most elementary level of single particle motion. The task of this work was to analyze the three dimensional structure, dynamical processes and the limit of the cooperative behavior in small plasma crystals. In addition to the

study of the systems formed, the immersed particles themselves may be used for diagnostics of the plasma environment: estimation of parameters or monitoring of the processes inside plasma. The laboratory experiments are performed in two radio-frequency (RF) plasma reactors with

parallel plate electrodes, where the lower electrode is a so-called "adaptive electrode". This electrode is segmented into 57 small "pixels" independently driven in DC (direct current) and/or RF voltage. When RF voltage is applied to one of these pixels, a bright localized glow, "secondary

plasma ball", appears above. Three dimensional dust crystals with less than 100 particles are formed inside this "plasma ball" - the ideal conditions for the investigation of the transition from cluster systems to collective systems. The investigation of the particle interactions in crystals is performed with an optical diagnostic, which allows determination of all three particle coordinates simultaneously with time resolution of 0.04 sec.

The experimental results are:

1. The binary interaction among particles in addition to the repelling Coulomb force exhibits also an attractive part, which is experimentally determined for the first time.

2. Analysis of the dynamical evolution shows the tendency of the systems to approach the state with minimum energy by rearranging particles inside.

3. The measured 63 particles' crystal vibrations are in close agreement with vibrations of a drop with surface tension. This indicates that even a 63 particle crystal already exhibits properties normally associated with the cooperative regime.

The possibility to use levitated particles as a new powerful diagnostic of the plasma sheath region is proposed. The existence of different equilibrium positions of microparticles suspended in an Oxygen discharge provides evidence of a structured electronegative plasma sheath, a feature so far only mathematically and numerically investigated.