Referierte Publikationen

Spontaneous parametric down-conversion (SPDC) has been a reliable process for the generation of entangled photon pairs. In this process, a nonlinear quadratic crystal is pumped by a laser field in order to convert (high-energy) photons into correlated photon pairs whose efficient control plays an essential role in various applications of quantum information processing. In particular, the amount of entanglement has been successfully controlled by adjusting the spatial structure of the incident pump field. Here, we theoretically analyze how the entanglement of the down-converted two-photon state can be further enhanced by using Ince-Gaussian beams with well-defined ellipticity epsilon, i.e., solutions of the paraxial wave equation in elliptical coordinates. These spatially structured beams are quite universal as they include both the Laguerre-Gaussian beams for epsilon -> 0 as well as the Hermite-Gaussian beams for epsilon -> infinity. We demonstrate that the entanglement of the generated photon pairs in SPDC can be maximized by a proper choice of epsilon and that such an enhanced entanglement can be observed experimentally in terms of the Schmidt number.

We study nonlinear trident in laser pulses in the high-energy limit, where the initial electron experiences, in its rest frame, an electromagnetic field strength above Schwinger s critical field. At lower energies the dominant contribution comes from the two-step' part, but in the high-energy limit the dominant contribution comes instead from the one-step term. We obtain new approximations that explain the relation between the high-energy limit of trident and pair production by a Coulomb field, as well as the role of the Weizsacker-Williams approximation and why it does not agree with the high-chi limit of the locally-constant-field approximation. We also show that the next-to-leading order in the large-a(0) expansion is, in the high-energy limit, nonlocal and is numerically very important even for quite large a(0). We show that the small-a(0) perturbation series has a finite radius of convergence, but using Pade-conformal methods we obtain resummations that go beyond the radius of convergence and have a large numerical overlap with the large-a(0) approximation. We use Borel-Pade-conformal methods to resum the small-chi expansion and obtain a high precision up to very large chi. We also use newer resummation methods based on hypergeometric/Meijer-G and confluent hypergeometric functions.

We resonantly excite the K series of O5+ and O6+ up to principal quantum number n=11 with monochromatic x rays, producing K-shell holes, and observe their relaxation by soft-x-ray emission. Some photoabsorption resonances of O5+ reveal strong two-electron–one-photon (TEOP) transitions. We find that for the [(1s2s)15p3/2]3/2;1/2 states, TEOP relaxation is by far stronger than the radiative decay and competes with the usually much faster Auger decay path. This enhanced TEOP decay arises from a strong correlation with the near-degenerate upper states [(1s2p3/2)14s]3/2;1/2 of a Li-like satellite blend of the He-like Kα transition. Even in three-electron systems, TEOP transitions can play a dominant role, and the present results should guide further research on the ubiquitous and abundant many-electron ions where electronic energy degeneracies are far more common and configuration mixing is stronger.

The strong-field approximation (SFA) has been widely applied to model ionization processes in short and intense laser pulses. Several approaches have been suggested in order to overcome certain limitations of the original SFA formulation with regard to the representation of the initial bound and final continuum states of the emitted electron as well as a suitable description of the driving laser pulse. We here present a reformulation of the SFA in terms of partial waves and spherical tensor operators that supports a quite simple implementation and the comparison of different treatments of the active (photo)electron and the laser pulses. In particular, this reformulation helps to adapt the SFA to experimental setups, and it paves the way to extend the strong-field theory toward the study of nondipole contributions in light-atom interactions as well as of many-particle correlations in strong-field ionization processes. A series of detailed computations have been carried out in order to confirm the validity of the reformulation and to show how the representation of the bound and continuum states affects the predicted above-threshold ionization spectra and related observables.

Aims. Within the framework of compact-object accretion disks, we calculate plasma environment effects on the atomic structure and decay parameters used in the modeling of K lines in lowly charged iron ions, namely FeII-FeVIII.Methods. For this study, we used the fully relativistic multiconfiguration Dirac-Fock method approximating the plasma electron-nucleus and electron-electron screenings with a time-averaged Debye-Huckel potential.Results. We report modified ionization potentials, K-threshold energies, wavelengths, radiative emission rates, and Auger widths for plasmas characterized by electron temperatures and densities in the ranges 10(5)-10(7) K and 10(18)-10(22) cm(-3). In addition, we propose two universal fitting formulae to predict the IP and K-threshold lowerings in any elemental ion.Conclusions. We conclude that the high-resolution X-ray spectrometers onboard the future XRISM and ATHENA space missions will be able to detect the lowering of the K edges of these Fe ions due to the extreme plasma conditions occurring in the accretion disks around compact objects.

In this paper we derive and discuss the completely spin- and photon-polarization-dependent probability rates for nonlinear Compton scattering and nonlinear Breit-Wheeler pair production. The locally constant field approximation, which is essential for applications in plasma-QED simulation codes, is rigorously derived from the strong-field QED matrix elements in the Furry picture for a general plane-wave background field. We discuss important polarization correlation effects in the spectra of both processes. Asymptotic limits for both small and large values of $ćhi$ are derived and their spin and polarization dependence is discussed.

Generation of terahertz radiation by optical rectification of intense near-infrared laser pulses in N-benzyl-2-methyl-4-nitroaniline (BNA) is investigated in detail by carrying out a complete characterization of the terahertz radiation. We studied the scaling of THz yield with pump pulse repetition rate and fluence which enabled us to predict the optimal operating conditions for BNA crystals at room temperature for 800 nm pump wavelength. Furthermore, recording the transmitted laser spectrum allowed us to calculate the nonlinear refractive index of BNA at 800 nm.

The electron-capture process was studied for Xe54+ colliding with H2 molecules at the internal gas target of the Experimental Storage Ring (ESR) at GSI, Darmstadt. Cross-section values for electron capture into excited projectile states were deduced from the observed emission cross section of Lyman radiation, being emitted by the hydrogenlike ions subsequent to the capture of a target electron. The ion beam energy range was varied between 5.5 and 30.9 MeV/u by applying the deceleration mode of the ESR. Thus, electron-capture data were recorded at the intermediate and, in particular, the low-collision-energy regime, well below the beam energy necessary to produce bare xenon ions. The obtained data are found to be in reasonable qualitative agreement with theoretical approaches, while a commonly applied empirical formula significantly overestimates the experimental findings.

The on-going developments in laser acceleration of protons and light ions, as well as the production of strong bursts of neutrons and multi-MeV photons by secondary processes now provide a basis for novel high-flux nuclear physics experiments. While the maximum energy of protons resulting from Target Normal Sheath Acceleration is presently still limited to around 100MeV, the generated proton peak flux within the short laser-accelerated bunches can already today exceed the values achievable at the most advanced conventional accelerators by orders of magnitude. This paper consists of two parts covering the scientific motivation and relevance of such experiments and a first proof-of-principle demonstration. In the presented experiment pulses of 200J at ≈500fs duration from the PHELIX laser produced more than 10 12 protons with energies above 15MeV in a bunch of sub-nanosecond duration. They were used to induce fission in foil targets made of natural uranium. To make use of the nonpareil flux, these targets have to be very close to the laser acceleration source, since the particle density within the bunch is strongly affected by Coulomb explosion and the velocity differences between ions of different energy. The main challenge for nuclear detection with high-purity germanium detectors is given by the strong electromagnetic pulse caused by the laser-matter interaction close to the laser acceleration source. This was mitigated by utilizing fast transport of the fission products by a gas flow to a carbon filter, where the γ -rays were registered. The identified nuclides include those that have half-lives down to 39s. These results demonstrate the capability to produce, extract, and detect short-lived reaction products under the demanding experimental condition imposed by the high-power laser interaction. The approach promotes research towards relevant nuclear astrophysical studies at conditions currently only accessible at nuclear high energy density laser facilities.

Resonant Schottky cavity pickups have applications in the measurements of beam parameters in a storage ring. Apart from that they can be used in nondestructive in-ring decay studies of radioactive ion beams. In order to obtain the results of an experiment suitable Data Acquisition System (DAQ) is necessary. Several DAQs were used at Experimental Storage Ring (ESR) at GSI based on the different hardware and software solutions such as TCAP or NTCAP. The goal of this work is to design a prototype of a DAQ using open hardware and open source software defined radio (SDR) and conduct the test measurements at ESR.

We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within 8 % for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50 fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models.

We propose a setup, based on a periodically driven spin chain, that can realize a high-quality quantum router. We present two protocols, which utilize this setup, that can either generate highly entangled two-qubit states over an arbitrary distance or transfer single-qubit states with high fidelity to any desired location on the chain. In addition, we can execute several protocols at the same time and also store quantum states on the spin chain. Our protocols exploit the effect of coherent destruction of tunneling to control, which spins on the chain couple to each other. This control is acquired by suitably shaping the external driving field. The success of our protocols does not depend on the values of the couplings between the spins as long as they are finite and much smaller than the driving frequency. Our setup is scalable, robust against errors, and may be of practical use for future quantum information technologies.

Quantum electrodynamics (QED) is currently considered to be one of the most accurate theories of fundamental interactions. As its extraordinary precision offers unique scientific opportunities, e.g., search for new physics, stringent experimental tests of QED continue to be of high importance. To this end, highly charged ions represent an exceptional test-bed due to enhanced QED effects. Recently, forbidden transitions in F-like ions have been analyzed to few ppm precision, resolving previous discrepancies between theory and experiment. Here we further test the accuracy of QED calculations with three new (Re, Os, Ir), and two improved (Kr, W) measurements of the P1/22-P3/22 transition energy in F-like ions using the NIST electron-beam ion trap and extreme-ultraviolet and x-ray spectrometers. Good agreement between theoretical and experimental energies is found for all considered elements.