Tailoring superradiance to design artificial quantum systems

Cooperative phenomena arising due to the coupling of individual atoms via the radiation field are a cornerstone of modern quantum and optical physics. Recent experiments on x-ray quantum optics added a new twist to this line of research by exploiting superradiance in order to construct artificial quantum systems. However, so far, systematic approaches to deliberately design superradiance properties are lacking, impeding the desired implementation of more advanced quantum optical schemes. Here, we develop an analytical framework for the engineering of single-photon superradiance in extended media applicable across the entire electromagnetic spectrum, and show how it can be used to tailor the properties of an artificial quantum system. This "reverse engineering" of superradiance not only provides an avenue towards non-linear and quantum mechanical phenomena at x-ray energies, but also leads to a unified view on and a better understanding of superradiance across different physical systems.Comment: 6 pages + supplemental materia

Radiative Corrections to Multi-Level Mollow-Type Spectra

This paper is concerned with two rather basic phenomena: the incoherent fluorescence spectrum of an atom driven by an intense laser field and the coupling of the atom to the (empty) modes of the radiation field. The sum of the many-photon processes gives rise to the inelastic part of the atomic fluorescence, which, for a two-level system, has a well-known characteristic three-peak structure known as the Mollow spectrum. From a theoretical point of view, the Mollow spectrum finds a natural interpretation in terms of transitions among laser-dressed states which are the energy eigenstates of a second-quantized two-level system strongly coupled to a driving laser field. As recently shown, the quasi-energies of the laser-dressed states receive radiative corrections which are nontrivially different from the results which one would expect from an investigation of the coupling of the bare states to the vacuum modes. In this article, we briefly review the basic elements required for the analysis of the dynamic radiative corrections, and we generalize the treatment of the radiative corrections to the incoherent part of the steady-state fluorescence to a three-level system consisting of 1S, 3P and 2S states.Comment: Dedicated to Prof. H. Walther on the occasion of his 70th birthda

Yoctosecond photon pulses from quark-gluon plasmas

Present ultra-fast laser optics is at the frontier between atto- and zeptosecond photon pulses, giving rise to unprecedented applications. We show that high-energetic photon pulses down to the yoctosecond timescale can be produced in heavy ion collisions. We focus on photons produced during the initial phase of the expanding quark-gluon plasma. We study how the time evolution and properties of the plasma may influence the duration and shape of the photon pulse. Prospects for achieving double peak structures suitable for pump-probe experiments at the yoctosecond timescale are discussed.Comment: 4 pages, 2 figures; final version as accepted by PR

Comment in response to the public consultation paper issued by the European Commission on 28th January 2011 on taxation problems that arise when dividends are distributed across borders to individual investors and possible solutions

The EU Commission identifies two major shortcomings of the cross‐border taxation of dividends received by portfolio shareholders in its consultation paper. Firstly, withholding taxes may be levied and credited in a discriminatory manner which may constitute an infringement of the free movement of capital set out in Articles 63 to 66 of the Treaty on the Functioning of the European Union. Secondly, the imposition of withholding taxes may result in double taxation and as a consequence impede investors from investing in foreign corporations. Hence, the benefits of international risk diversification are not fully exploited which may result in welfare losses.1 The parent‐subsidiary‐directive2 addresses this issue by prohibiting the imposition of dividends paid to substantial shareholdings. A comparable relief is, however, not available for portfolio and individual investors. In turn, withholding taxes on interest have been considerably reduced and even abolished by many member states whereas withholding taxes on dividends are still common and significant.3 This report therefore discusses how the above highlighted issues concerning the cross‐border taxation of dividends received by individuals can be resolved. The taxation of portfolio investments by corporations which is also left out of the scope of the parent‐subsidiary‐directive is not addressed in the following as this issue requires a separate analysis

Effective tax rates under IP tax planning

Tax planning with intangibles has become one of the most popular and most vividly debated topics in international taxation. We incorporate various intellectual property (IP) tax planning models into forward-looking measures of effective tax rates, namely the disposal of intangibles to low-tax subsidiaries, intra-group licensing arrangements, and intra-group contract R&D. In doing so, we draw upon the methodology put forward by Devereux and Griffith and amend this model by considering a research & development (R&D) investment which is carried out by a parent company, whereby the resulting intangible is exploited by a foreign subsidiary. We point out analytically under which conditions IP tax planning achieves the objective of reducing the effective average tax rate of the group. We find that the disposal of intangibles to low-tax subsidiaries does not achieve this tax planning objective, if the true value of the asset is subject to tax upon the disposal. We show to what extent the parent must understate the value of the intangible in order to reduce the group’s tax burden. We furthermore point out that contract R&D may generally achieve a significant lower effective tax burden. We present cost of capital and effect average tax rates to illustrate these findings

Romania's development to a low-tax country: Effective corporate tax burden in Romania from 1992 to 2010 and Romania's current ranking among the eastern European member states

We trace back Romania's development to a low-tax country among the Member States of the European Union by analysing the major tax law changes in corporate taxation since 1992. We find that the significant reduction of the corporate income tax rate from 45% in 1992 to 16% since 2005 has not been accompanied by a comprehensive broadening of the corporate income tax base as prevalent in many longstanding Member States of the EU and the OECD. Our analysis is not limited to a comprehensive description of the development of corporate taxation in Romania, but goes on with a numerical analysis of the tax burdens at different periods of time which constitute milestones in the development of corporate taxation in Romania. For this purpose, we apply the European Tax Analyzer, which is a computer-based model firm approach. We find that the average company tax burden of the underlying model company has dropped significantly by almost 65% since 1992. Furthermore, our numerical analysis does not confirm the tax base broadening policy. As a result, Romania holds position two among the group of Central and Eastern European EU Member States. --corporate taxation,effective tax burden,transition economy,EU accession countries,tax reform,tax-rate-cum-base-broadening reform

Streaking At High Energies With Electrons And Positrons

State-of-the-art attosecond metrology deals with the detection and characterization of photon pulses with typical energies up to the hundreds of eV and time resolution of several tens of attoseconds. Such short pulses are used for example to control the motion of electrons on the atomic scale or to measure inner-shell atomic dynamics. The next challenge of time-resolving the inner-nuclear dynamics, transient meson states and resonances requires photon pulses below attosecond duration and with energies exceeding the MeV scale. Here we discuss a detection scheme for time-resolving high-energy gamma ray pulses down to the zeptosecond timescale. The scheme is based on the concept of attosecond streak imaging, but instead of conversion of photons into electrons in a nonlinear medium, the high-energy process of electron-positron pair creation is utilized. These pairs are produced in vacuum through the collision of a test pulse to be characterized with an intense laser pulse, and they acquire additional energy and momentum depending on their phase in the streaking pulse at the moment of production. A coincidence measurement of the electron and positron momenta after the interaction provides information on the pair production phase within the streaking pulse. We examine the limitations imposed by quantum radiation reaction in multiphoton Compton scattering on this detection scheme, and discuss other necessary conditions to render the scheme feasible in the upcoming Extreme Light Infrastructure (ELI) laser facility.Comment: 6 pages, 2 figures, contribution to the AIP proceedings of "Light at Extreme Intensities" (LEI 2011), Szeged, Hungary, Nov 14-18, 201

Fiscal Investment Climate and the Cost of Capital in Germany

Measured in terms of the cost of capital, the investment climate in Germany currently ranks 24th Key Messages out of the 28 EU member states. However, this represents an improvement over the year 2000, when Germany ranked 27th. In fact, among the EU-28, Germany has seen the most significant decline in the cost of capital between 2000 and 2014, from 7.7% to 6.5%. This decline is mainly attributable to considerable reductions in the country’s corporate tax rate. The comprehensive corporate tax reforms enacted in 2001 and 2008 thus appear to have succeeded in improving the investment climate in Germany. Nevertheless, in comparison to most other EU member states, the cost of capital in Germany is still quite high. In order to further improve the investment climate in Germany, the (temporary) re-introduction of reduced-balance tax depreciation for movable fixed assets is currently being discussed. However, our analysis indicates that this would only slightly decrease the cost of capital from 6.5% to 6.3%. When we additionally account for the personal taxation of individual investors, a different picture emerges: The overall cost of capital in Germany currently amounts to 6.0%, and could be reduced considerably by taxing dividends and capital gains at lower rates than interest income

VENUS meets SEMAT : How do they compare?

SEMAT (Software Engineering Methods And Theory) is an initiative to define a generic foundation for software engineering as a rigorous discipline. The so-called SEMAT kernel provides a thinking framework for software engineers that is not constrained to certain methods and processes but aims to encompass all kinds of proven principles and best practices. Our own interdisciplinary VENUS development method is designed to achieve similar generality and compatibility objectives, although the chosen application domain in VENUS has a much narrower scope. In this paper we compare the VENUS development method with SEMAT. The main contributions are positioning the VENUS development concepts within the SEMAT conceptual framework, and investigating whether SEMAT is an appropriate framework for dealing with inherently interdisciplinary development processes. In the end we present suggestions for the improvement of both approaches.Sociedad Argentina de Informática e Investigación Operativa (SADIO