The possibility of creating a scaleable quantum network by interconverting photonic and atomic qubits shows great promise. The fundamental requirement for such a network is deterministic control over the emission and absorption of photons from single atoms. This thesis reports on the experi-mental construction of a photon source that can emit single-photons with arbitrary spatio-temporal shape, phase, and frequency. The photon source itself is a strongly-coupled atom cavity system based on a single ⁸⁷ Rb atom within a macroscopic high-finesse Fabry-Perot cavity. It operates intermittently for periods of up to 100µs, with single-photon repetition rates of 1.0 MHz and an efficiency of almost 80%. Atoms are loaded into the cavity using an atomic fountain, with the upper turning point near the centre of the cavity mode. This ensures long interaction times without any disturbances introduced by trapping potentials. The photons’ indistinguishability was tested, with a two-photon Hong-Ou-Mandel visibility of 87%. This ability to both generate, and control, the photons’ properties, for example producing photons with symmetric or multi-peaked spatio-temporal shapes, allows for the production of photons in an n-time-bin superposition state where each time-bin has an arbitrarily defined amplitude and phase. These photons can be used as photonic qubits, qutrits and qquads, and their properties have been tested using a small linear-optics network.

For cavity quantum electrodynamics systems (cavity-QED) to play a role in quantum information processing applications and in quantum networks, they must be robust and scalable in addition to having a suitable method for the generation, processing and storage of quantum bits. One solution is to develop a composite system that couples a nitrogen vacancy (NV) center in diamond to a whispering gallery mode supported by a fused silica microsphere. Such a system is motivated by the optical and electron-spin properties of the NV center. The NV center is the leading spin-qubit and exhibits atomic like linewidths at cryogenic temperatures and has spin coherence times greater than milliseconds at room temperature. These long coherence times, coupled with nanosecond scale spin readout and manipulation times, allow for millions of quantum operations to be processed. Silica whispering gallery resonators are the only class of microresonators with quality factor high enough to reach the strong coupling regime, which is necessary for some quantum information processing applications. Integrating these two components into a system that could position a diamond nanopillar near the surface of a deformed-double stemmed microsphere system, with nanometer precision, at 10 K was a major achievement of this research. Cavity resonances in deformed microspheres can be excited with a free-space coupling technique which simplifies their integration into cryogenic environments. In these intentionally deformed resonators, an enhanced evanescent field decay length was observed at specific locations along the ray orbit. The double-stem arrangement enables the cavity resonance to be tuned over 450 GHz, with sub-10 MHz resolution, at 10 K. These two features, the enhanced decay length and broad range tuning with high resolution, are indispensible tools for cavity-QED studies with silica microspheres. Diamond nanopillars were fabricated from single crystal diamond with diameters as small as 140 nm in order to maintain a high quality factor. Studies were conducted on NV centers in nanopillars and bulk diamond to determine their suitability for cavity-QED applications. In an attempt to increase the light-matter interaction between NV centers and whispering gallery modes, diamond substrates were optically characterized that were irradiated with nitrogen ions.

The theory of quantum electrodynamics (QED) was born out of an attempt to merge Einsteins theory of special relativity and quantum mechanics. Einsteins energy/mass equivalence together with Heisenberg's uncertainty principle allows for particle pairs to be spontaneously created and annihilated in vacuum. These spontaneous fluctuations gives the quantum vacuum properties analogous to that of a nonlinear medium. Although these fluctuations in general does not give note of themselves, effects due to their presence can be stimulated or enhanced through external means, such as boundary conditions or electromagnetic fields. Whereas QED has been very well tested in the high-energy, low-intensity regime using particle accelerators, the opposite regime where the photon energy is low but instead the intensity is high is still to a large degree not investigated. This is expected to change with the rapid progress of modern high-power laser-systems. In this thesis we begin by studying the QED effect of photon-photon scattering. This process has so far not been successfully verified experimentally, but we show that this may change already with present day laser powers. We also study QED effects due to strong magnetic fields. In particular, we obtain an analytical description for vacuum birefringence valid at arbitrary field strengths. Astrophysics already offer environments where QED processes may be influential, e.g. in neutron star and magnetar environments. For astrophysical purposes we investigate how effects of QED can be implemented in plasma models. In particular, we study QED dispersive effects due to weak rapidly oscillating fields, nonlinear effects due to slowly varying strong fields, as well as QED effects in strongly magnetized plasmas. Effects of quantum dispersion and the electron spin has also been included in an extended plasma description, of particular interest for dense and/or strongly magnetized systems.

The prospect of a universal quantum computer is alluring, yet formidable. Smaller scale quantum information processing, however, has been demonstrated. Quantum networks, interlinking flying and stationary qubits, and linear optical quantum computing (LOQC) are both good candidates for scaling up such computations. A strongly coupled atom-cavity system is a promising approach for applications in these fields, both as a node in a quantum network, and as a source of photons for LOQC. This thesis demonstrates the versatile capabilities of an atom-cavity system comprising a single ⁸⁷Rb atom within a macroscopic high-finesse Fabry-P&eacute;rot cavity. It operates intermittently for periods of up to 100 &mu;s, with single-photon repetition rates of 1 MHz and an intra-cavity production efficiency of up to 85%. Exploiting the long coherence time of around 500 ns, the photons are subdivided into d time bins, with arbitrary amplitudes and phases, thus encoding arbitrary qudits. High fidelity quantum logic is shown, operating a controlled-NOT gate integrated into a photonic chip with a classical fidelity of 95.9^+1.4<sub style='position: relative; left: -1.6em;'>-1.7</sub> %. Additionally, the generation of entanglement is verified and non-classical correlations between events separated by periods exceeding the travel time across the chip by three orders of magnitude are observed. Photonic quantum simulation is performed, using temporally encoded qudits to mimic the correlation statistics of both fermions and anyons, in addition to bosons. Finally measurement-based quantum feedback is demonstrated and used to actively control the routing of temporal qubits.

No description available.

Depuis les années 1930, on sait que le noyau des atomes est composé de deux types de particules: les protons et les neutrons. Ces deux particules sont très similaires: d'une part le neutron est subtilement plus lourd (un pour mille) que le proton et d'autre part le proton porte une charge électrique positive tandis que le neutron est neutre. La petite différence de masse entre le neutron et le proton fourni l'énergie suffisante pour autoriser désintégration où un neutron se désintègre en un proton en émettant un électron et un anti-neutrino électronique. Aussi, le fait que le proton ne se désintègre pas assure la stabilité de l'atome d'hydrogène. De plus, on sait empiriquement que les paramètres de la désintégration déterminent la composition des noyaux d'atomes stables plus lourds que l'hydrogène. Il est donc raisonnable de penser que si la différence de masse entre le neutron et le proton était de signe opposé ou seulement légèrement différente, l'Univers visible serait surement très différent de celui que l'on connait. Il est donc essentiel de comprendre l'origine de cette différence de masse à partir des principes premiers de la physique. C'est à ce problème, et à des problèmes liés à celui-ci, qu'essaye de répondre ce travail. Dans la compréhension actuelle de la physique, les neutrons et les protons sont des particules composées de particules élémentaires appelées quark up (symbole u) et quark down (symbole d). Le proton est un état lié uud et le neutron est un état lié udd. Les quarks up et down sont deux particules similaires: elles sont toutes deux légères (de l'ordre de quelques MeV) et leurs charges électriques sont différentes. / .

Diese Arbeit beschreibt eine ausfŸhrliche Untersuchung der physikalischen Eigenschaften von Mikrokugelresonatoren aus Quarzglas. Diese Resonatoren unterstŸtzen sogennante whispering-gallery Moden (WGM), die GŸten so hoch bis 109 bieten. Als experimentelle Hilfsmittel wurden ein Nahfeld- und ein Konfokalmikroskop benutzt, um die Struktur der Moden bezŸglich der Topographie des Resonators eindeutig zu identifizieren, oder um einzelne Quantenemitter zu detektieren und anzuregen. Die resonante †berhšhung des elektromagnetischen Feldes in den Moden des Resonators wurde ausgenutzt, um stimulierte Raman-Streuung mit extrem niedrigem Schwellenwert im Quarzglas zu beobachten. Ein Rekordschwellenwert von 4.5 Mikrowatts wurde gemessen. Mittels einer Nahfeldsonde wurde die Modenstruktur des Mikro-Ramanlasers gemessen. Mikroresonatoren stellen einen Grundbaustein der Resonator-Quantenelektrodynamik dar. In dieser Arbeit wurde die Kopplung von einem einzelnen strahlenden Dipol an die WGM sowohl theoretisch als auch experimentell untersucht. Die kontrollierte Kopplung von einem einzelnen Nanoteilchen an die WGM eines Mikrokugelresonators wurde nachgewiesen. Erste Ergebnisse in der Kopplung eines einzelnen Emitters an die Moden des Resonators wurden erzielt. Die resonante Wechselwirkung mit Resonatormoden wurde ausgenutzt, um den Photonentransfer zwischen zwei Nanoteilchen dramatisch zu verstŠrken. Schlie§lich wurde die bislang unbeachtete Analogie zwischen dem Quantensystem eines einzelnen Emitters in Wechselwirkung mit einer einzelnen Resonatormode und dem klassischen System zweier gekoppelten Moden experimentell untersucht. Es wurde bewiesen, wie die aus der Resonatorquantenelektrodynamik bekannten Kopplungsregime der starken und schwachen Kopplung in Analogie auch an einem klassischen System beobachtet werden kšnnen. Der †bergang von schwacher zu starker Kopplung wurde beobachtet, und bislang gemessene unerwartet hohe Kopplungsraten konnten einfach erklŠrt werden. / This work presents an extensive study of the physical properties of silica microsphere resonators, which support whispering-gallery modes (WGMs). These modes feature Q-factors as high as 109 corresponding to a finesse of 3 millions for spheres with a diameter of about 80 micrometers. These are to date among the highest available Q-factors, leading to cavity lifetimes of up to few microseconds. A near-field microscope and a confocal microscope are used as tools to unequivocally identify the mode structure related to the sphere topography, and for excitation and detection of single quantum emitters. The high field enhancement of the cavity modes is exploited to observe ultra-low threshold stimulated Raman scattering in silica glass. A record ultra-low threshold of 4.5 microwatts was recorded. The mode structure of the laser is investigated by means of a near-field probe, and the interaction of the probe itself with the lasing properties is investigated in a systematic way. Microcavities also one of the building blocks of Cavity QED. Here, the coupling of a radiative dipole to the whispering-gallery modes has been studied both theoretically and experimentally. The controlled coupling of a single nanoparticle to the WGMs is demonstrated, and first results in coupling a single quantum emitter to the modes of a microsphere are reported. The resonant interaction with these modes is exploited to enhance photon exchange between two nanoparticles. Finally a novel analogy between a system composed of a single atom interacting with one cavity mode on one side and intramodal coupling in microsphere resonators induced by a near-field probe on the other side is presented and experimentally explored. The induced coupling regimes reflect the different regimes of weak and strong coupling typical of Cavity QED. The transition between the two coupling regimes is observed, and a previously observed unexpectedly large coupling rate is explained.

Moderne Lasereinrichtungen stellen hochintensives Licht mit sehr kurzer zeitlicher Struktur zur Verfügung. Damit bringen diese Einrichtungen die Phänomene in die Laboratorien, welche normalerweise nur in der Nähe von stark strahlenden Sternen im Weltall zu finden sind. Bezüglich der Streuprozesse von Teilchen innerhalb dieser extremen Lichtquellen gibt es eine Vielzahl an theoretischen Untersuchungen. Vorwiegend geschehen diese unter der Verwendung der Starkfeld-Quantenelektrodynamik, einer Theorie zur quanten- theoretischen Beschreibung von elektromagnetischen Wechselwirkungen innerhalb eines kohärenten hochintensiven Feldes, welches als semi-klassisches Hintergrundfeld beschrieben wird. Zum Beispiel zeigte die theoretische Behandlung des Compton-Prozesses (die inelastis- che Elektron-Photon-Streuung) oder des Breit-Wheeler-Prozesses (der Paarproduktion in der Kollision von zwei Photonen) innerhalb der Starkfeld-Quantenelektrodynamik eine große Menge an neuen nicht-linearen Effekten und Phänomen, welche stellenweise in zukun- ftsweisenden Experimenten nachgewiesen werden konnten. Von großem Interesse und auch zentrales Untersuchungsobjekt der vorliegenden Arbeit ist ebenso der Trident-Prozess: ein Prozess zweiter Ordnung in der (Starkfeld-) Quan- tenelektrodynamik, bei dem ein Elektron-Positron-Paar innerhalb der Kollision eines Photonstrahls (z.B. erzeugt von einem Laser) und eines gegenläufigen Elektronenstrahls entsteht. Allerdings ist der Trident-Prozess im Zusammenhang mit hochintensiven Feldern nicht ausschließlich das Produkt seiner Teile, den erwähnten Compton- und Breit-Wheeler- Prozessen, vielmehr erzeugt das Vorhandensein des intermediären Photons durch seine virtuellen und reellen Beträge überaus komplizierte Strukturen. In den letzten Jahren gab es daher eine große Menge an theoretischen Beiträgen zur nicht-linearen Behandlung des Trident-Prozesses bezüglich eines weiten Bereichs an Eigenschaften der verwendeten Lichtquelle. Jedoch ist der nicht-lineare Trident-Prozess wegen seiner anspruchsvollen mathematischen Natur bisher nicht als völlig verstanden anzusehen. In der vorliegen- den Arbeit liegt der Fokus auf der Abhängigkeit des Trident-Prozesses von den kurzen zeitlichen Strukturen der verwendeten Lichtquellen bei hohen Energien. Grob gesprochen bedeutet dies, dass die kurz gepulsten Strukturen der modernen Lichtquellen zu breiten Spektren der Photonstrahlen führen, welche sich dann auch in den betrachteten Prozessen widerspiegeln. Demfolgend wird in der vorliegenden Arbeit eine neue Approximation an die Starkfeld-Quantenelektrodynamik erarbeitet, welche in der Lage ist, die spektralen Abhängigkeiten in den Prozessen zu beschreiben, die in Laser-Elektron-Kollisionen bei hohen Energien vorzufinden sind. Diese neue Approximation wird dann auf den Trident- Prozess angewendet und es werden die neuen Strukturen herausgearbeitet, welche durch das breite Spektrum der betrachteten Lichtquelle entstehen. Ferner werden bestehende oder geplante extreme Lichtquellen dahingehend untersucht, in welcher Weise diese, kombiniert mit einem passendem Elektronenstrahl, sensitiv für die vorgestellten spektralen Effekte im Trident-Prozess sind. Abschließend werden weitere mögliche Anwendungsbereiche der neuen Approximation diskutiert.:1 Introduction 1 2 Strong-field quantum electrodynamics 11 2.1 Description of the laser field 12 2.2 Background field approximation 18 2.3 Momentum space rules of strong-field QED 25 2.4 Ward identity and gauge invariance 34 2.5 Strong-field trident process 36 3 Pulsed-perturbative quantum electrodynamics 43 3.1 Approaches and approximations to strong-field QED 43 3.2 Momentum space rules in pulsed-perturbative QED 46 3.3 Spectrum of the background field 52 4 Pulsed-perturbative trident process 57 4.1 Matrixelement and cross section 57 4.2 Total cross section 72 4.3 Inclusive positron distributions 75 4.4 Exclusive electron distributions 81 4.5 Experimental capability 93 5 Summary and Outlook 97 Appendix 101 A Relativistic Kinematics 103 A.1 Preliminary remarks 103 A.2 Coordinate systems 104 A.3 Frames of reference 109 A.4 Kinematics of 2→3 processes 111 B Feynman rules of QED 121 C Perturbative trident pair production 125 C.1 Matrixelement and cross section 125 C.2 Numerical implementation and comparison to literature 129 C.3 Differential cross sections in transverse coordinates 132 C.4 Darkphotons 134 D Useful mathematical statements 139 Bibliography 153 / Modern laser facilities provide highly intense light with a very short temporal structure, which brings the phenomena originally found near the strong radiating stars in the universe into the laboratory. Accordingly, there are, among others, wide theoretical investigations w.r.t. scattering processes of particles impinging this extreme light sources. This has been done by applying the strong-field quantum electrodynamics, which is a theory of electromagnetic interactions within coherent highly intense light treated as a semi-classical background field. For instance, the treatment of the Compton process (inelastic electron- photon scattering) and the Breit-Wheeler process (pair production of a collision of two photons) with strong-field quantum electrodynamics revealed a vast amount of novel non-linear structures and phenomena, which were to some extent experimentally verified. Of particular interest and the central object of investigation within this thesis is also the trident process: a second order process in (strong-field) quantum electrodynamics producing an electron-positron pair within the collision of a photon beam (e.g. produced by a laser) with a counter-propagating electron. However, in the context of highly intense fields, the trident process is more than the product of its parts, the mentioned Compton and Breit-Wheeler process, since the intermediate photon yields both virtual and real contributions producing exceedingly complicated structures. Over the last years, there are several theoretical contributions to the non-linear treatment of the trident process w.r.t. a wide range of laser properties, but the trident process has not yet been fully understood due to its demanding mathematical nature. Within the present thesis, we focus on the dependence of the trident process to the short temporal structures of the involved light source at high energies. Loosely speaking, this means the short pulsed structure of modern light sources provide a wide energy spectrum of the respective photons, which is imprinted on the considered scattering processes. Accordingly, we elaborate a new approximation to strong-field quantum electrodynamics capable to describe the spectral dependence of processes within laser-electron collisions at high energies. Then we apply this new approximation to the trident process and reveal the novel structures generated by the spectrum of the light source. Therefore, we provide an analysis of the spectral impact to the trident process involving the total cross section as well as several inclusive and exclusive distributions of its final particles. Consequently, we examine in principle the experimental capabilities of present or planed extreme light sources by combining them with a suitable electron beam, whether they are sensitive to the encountered spectral effects of the trident process and discuss further applications of the newly introduced approximation.:1 Introduction 1 2 Strong-field quantum electrodynamics 11 2.1 Description of the laser field 12 2.2 Background field approximation 18 2.3 Momentum space rules of strong-field QED 25 2.4 Ward identity and gauge invariance 34 2.5 Strong-field trident process 36 3 Pulsed-perturbative quantum electrodynamics 43 3.1 Approaches and approximations to strong-field QED 43 3.2 Momentum space rules in pulsed-perturbative QED 46 3.3 Spectrum of the background field 52 4 Pulsed-perturbative trident process 57 4.1 Matrixelement and cross section 57 4.2 Total cross section 72 4.3 Inclusive positron distributions 75 4.4 Exclusive electron distributions 81 4.5 Experimental capability 93 5 Summary and Outlook 97 Appendix 101 A Relativistic Kinematics 103 A.1 Preliminary remarks 103 A.2 Coordinate systems 104 A.3 Frames of reference 109 A.4 Kinematics of 2→3 processes 111 B Feynman rules of QED 121 C Perturbative trident pair production 125 C.1 Matrixelement and cross section 125 C.2 Numerical implementation and comparison to literature 129 C.3 Differential cross sections in transverse coordinates 132 C.4 Darkphotons 134 D Useful mathematical statements 139 Bibliography 153

With the constantly increasing precision of experimental data acquired at the current collider experiments Tevatron and LHC the theoretical uncertainty on the prediction of multiparticle final states has to decrease accordingly in order to have meaningful tests of the underlying theories such as the Standard Model. A pure leading order calculation, defined in the perturbative expansion of said theory in the interaction constant, represents the classical limit to such a quantum field theory and was already found to be insufficient at past collider experiments, e.g. LEP or Hera. Such a leading order calculation can be systematically improved in various limits. If the typical scales of a process are large and the respective coupling constants are small, the inclusion of fixed-order higher-order corrections then yields quickly converging predictions with much reduced uncertainties. In certain regions of the phase space, still well within the perturbative regime of the underlying theory, a clear hierarchy of the inherent scales, however, leads to large logarithms occurring at every order in perturbation theory. In many cases these logarithms are universal and can be resummed to all orders leading to precise predictions in these limits. Multiparticle final states now exhibit both small and large scales, necessitating a description using both resummed and fixed-order results. This thesis presents the consistent combination of two such resummation schemes with fixed-order results. The main objective therefor is to identify and properly treat terms that are present in both formulations in a process and observable independent manner. In the first part the resummation scheme introduced by Yennie, Frautschi and Suura (YFS), resumming large logarithms associated with the emission of soft photons in massive Qed, is combined with fixed-order next-to-leading matrix elements. The implementation of a universal algorithm is detailed and results are studied for various precision observables in e.g. Drell-Yan production or semileptonic B meson decays. The results obtained for radiative tau and muon decays are also compared to experimental data. In the second part the resummation scheme introduced by Dokshitzer, Gribov, Lipatov, Altarelli and Parisi (DGLAP), resumming large logarithms associated with the emission of collinear partons applicable to both Qcd and Qed, is combined with fixed-order next-to-leading matrix elements. While the focus rests on its application to Qcd corrections, this combination is discussed in detail and the implementation is presented. The resulting predictions are evaluated and compared to experimental data for a multitude of processes in four different collider environments. This formulation has been further extended to accommodate real emission corrections to beyond next-to-leading order radiation otherwise described only by the DGLAP resummation. Its results are also carefully evaluated and compared to a wide range of experimental data.

With the constantly increasing precision of experimental data acquired at the current collider experiments Tevatron and LHC the theoretical uncertainty on the prediction of multiparticle final states has to decrease accordingly in order to have meaningful tests of the underlying theories such as the Standard Model. A pure leading order calculation, defined in the perturbative expansion of said theory in the interaction constant, represents the classical limit to such a quantum field theory and was already found to be insufficient at past collider experiments, e.g. LEP or Hera. Such a leading order calculation can be systematically improved in various limits. If the typical scales of a process are large and the respective coupling constants are small, the inclusion of fixed-order higher-order corrections then yields quickly converging predictions with much reduced uncertainties. In certain regions of the phase space, still well within the perturbative regime of the underlying theory, a clear hierarchy of the inherent scales, however, leads to large logarithms occurring at every order in perturbation theory. In many cases these logarithms are universal and can be resummed to all orders leading to precise predictions in these limits. Multiparticle final states now exhibit both small and large scales, necessitating a description using both resummed and fixed-order results. This thesis presents the consistent combination of two such resummation schemes with fixed-order results. The main objective therefor is to identify and properly treat terms that are present in both formulations in a process and observable independent manner. In the first part the resummation scheme introduced by Yennie, Frautschi and Suura (YFS), resumming large logarithms associated with the emission of soft photons in massive Qed, is combined with fixed-order next-to-leading matrix elements. The implementation of a universal algorithm is detailed and results are studied for various precision observables in e.g. Drell-Yan production or semileptonic B meson decays. The results obtained for radiative tau and muon decays are also compared to experimental data. In the second part the resummation scheme introduced by Dokshitzer, Gribov, Lipatov, Altarelli and Parisi (DGLAP), resumming large logarithms associated with the emission of collinear partons applicable to both Qcd and Qed, is combined with fixed-order next-to-leading matrix elements. While the focus rests on its application to Qcd corrections, this combination is discussed in detail and the implementation is presented. The resulting predictions are evaluated and compared to experimental data for a multitude of processes in four different collider environments. This formulation has been further extended to accommodate real emission corrections to beyond next-to-leading order radiation otherwise described only by the DGLAP resummation. Its results are also carefully evaluated and compared to a wide range of experimental data.:1. Introduction 1.1 Event generators 1.2 The event generator Sherpa 1.3 Outline of this thesis Part I YFS resummation & fixed order calculations 2 Yennie-Frautschi-Suura resummation 2.1 Resummation of virtual photon corrections 2.2 Resummation of real emission corrections 2.3 The Yennie-Frautschi-Suura form factor 3 A process independent implementation in Sherpa 3.1 The Algorithm 3.1.1 The master formula 3.1.2 Phase space transformation 3.1.3 Mapping of momenta 3.1.4 Event generation 3.2 Higher Order Corrections 3.2.1 Approximations for real emission matrix elements 3.2.2 Real emission corrections 3.2.3 Virtual emission corrections 4 The Z lineshape and radiative lepton decay corrections 4.1 The Z lineshape 4.1.1 Radiation pattern 4.1.2 Numerical stability 4.2 Radiative lepton decays 4.3 Summary and conclusions 5 Electroweak corrections to semileptonic B decays 5.1 Tree-level decay 5.2 Next-to-leading order corrections 5.2.1 Matching of different energy regimes 5.2.2 Short-distance next-to-leading order corrections 5.2.3 Long-distance next-to-leading order corrections 5.2.4 Structure dependent terms 5.2.5 Soft-resummation and inclusive exponentiation 5.3 Methods 5.3.1 BLOR 5.3.2 Sherpa/Photons 5.3.3 PHOTOS 5.4 Results 5.4.1 Next-to-leading order corrections to decay rates 5.4.2 Next-to-leading order corrections to differential rates 5.4.3 Influence of explicit short-distance terms 5.5 Summary and conclusions Part II DGLAP resummation & fixed order calculations 6 DGLAP resummation & approximate higher order corrections 6.1 Dokshitzer-Gribov-Lipatov-Altarelli-Parisi resummation 6.1.1 The naive parton model 6.1.2 QCD corrections to the parton model 6.1.3 Factorisation and the collinear counterterm 6.1.4 The DGLAP equations 6.2 Parton evolution 6.2.1 Approximate real emission cross sections 6.2.2 Parton evolution 6.2.3 Scale choices for the running coupling 6.3 Soft emission corrections 7 The reinterpretation and automisation of the POWHEG method 7.1 Decomposition of the real-emission cross sections 7.2 Construction of a parton shower 7.3 Matrix element corrections to parton showers 7.4 The reformulation of the POWHEG method 7.4.1 Approximate NLO cross sections 7.4.2 The POWHEG method and its accuracy 7.5 The single-singularity projectors 7.6 Theoretical ambiguities 7.7 MC@NLO 7.8 Realisation of the POWHEG method in the Sherpa Monte Carlo 7.8.1 Matrix elements and subtraction terms 7.8.2 The parton shower 7.8.3 Implementation & techniques 7.8.4 Automatic identification of Born zeros 7.9 Results for processes with trivial colour structures 7.9.1 Process listing 7.9.2 Tests of internal consistency 7.9.3 Comparison with tree-level matrix-element parton-shower merging 7.9.4 Comparison with experimental data 7.9.5 Comparison with existing POWHEG 7.10 Results for processes with non-trivial colour structures 7.10.1 Comparison with experimental data 7.11 Summary and conclusions 8 MENLOPS 8.1 Improving parton showers with higher-order matrix elements 8.1.1 The POWHEG approach 8.1.2 The ME+PS approach 8.2 Merging POWHEG and ME+PS - The MENLOPS 8.3 Results 8.3.1 Merging Systematics 8.3.2 ee -> jets 8.3.3 Deep-inelastic lepton-nucleon scattering 8.3.4 Drell-Yan lepton-pair production 8.3.5 W+jets Production 8.3.6 Higgs boson production 8.3.7 W-pair+jets production 8.4 Summary and conclusions Summary Appendix A Details on the YFS resummation implementation A.1 The YFS-Form-Factor A.1.1 Special cases A.2 A.2.1 Avarage photon multiplicity A.2.2 Photon energy A.2.3 Photon angles A.2.4 Photons from multipoles A.3 Massive dipole splitting functions A.3.1 Final State Emitter, Final State Spectator A.3.2 Final State Emitter, Initial State Spectator A.3.3 Initial State Emitter, Final State Spectator B Formfactors and higher order matrix elements for semileptonic B decays B.1 Form factor models of exclusive semileptonic B meson decays B.1.1 Form factors for B -> D l nu B.1.2 Form factors for B -> pi l nu B.1.3 Form factors for B -> D0* l nu B.2 NLO matrix elements B.2.1 Real emission matrix elements B.2.2 Virtual emission matrix elements B.3 Scalar Integrals B.3.1 General definitions B.3.2 Tadpole integrals B.3.3 Bubble integrals B.3.4 Triangle integrals C Explicit form of the leading order Altarelli-Parisi splitting functions C.1 Collinear limit of real emission matrix elements C.1.1 q -> gq splittings C.1.2 q -> qg splittings C.1.3 g -> qq splittings C.1.4 g -> gg splittings Bibliography