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M. Braun

Publications and source records attributed to M. Braun.

At least 19 recordsLinked to original sources
arXiv2026

Pauli Cloners for Pauli Channels

We present a quantum circuit architecture for the one-to-two cloning of $N$-qubit registers. It implements the broad class of Pauli cloners by extending the Niu--Griffiths architecture to multi-qubit systems. In the single-qubit case, we provide explicit constructions for asymmetric universal, phase covariant and biased cloners. We explore the fundamental relationship between Pauli errors, mutually unbiased bases and Pauli cloning. Furthermore, we demonstrate how Pauli cloners can be tailored to specific noise models in the context of quantum communication, especially quantum key distribution.

quant-ph↗
arXiv2021

Electron Beam Irradiation of Gallium Nitride-on-Silicon Betavoltaics Fabricated with a Triple Mesa Etch

A process for growing gallium nitride (GaN) vertical p-i-n homojunctions on (111) silicon (Si) substrates using metalorganic chemical vapor deposition (MOCVD) was developed, and a triple mesa etch technique was used to fabricate efficient betavoltaic energy converters. Monte Carlo simulation platform CASINO was used to model beta radiation penetration into GaN to aid device design. The resulting devices were tested under irradiation from a scanning electron microscope (SEM) electron beam (e-beam) tuned to imitate the energies of the 63Ni beta emission spectrum. Based on current-voltage (I-V) measurements taken under e-beam illumination, a maximum open-circuit voltage of 412 mV and a maximum short-circuit current density of 407 nA/cm2 were measured. A high fill factor (FF) of 0.77 and power conversion efficiency of 6.6% were obtained. Additionally, the proposed triple mesa etch technique used to create these betavoltaics has potential for further use in fabricating many types of electronic devices using a wide variety of material platforms.

physics.app-ph↗
arXiv2020

Application of Local Approaches to the Assessment of Fatigue Test results obtained for Welded Joints at Sub-Zero Temperatures

Several studies have found significant increase in the fatigue strength of welded joints of structural steels at sub-zero temperatures. This study addresses the research by investigating the applicability of local fatigue assessment methods to welded joints exposed to sub-zero temperatures. For this purpose, fatigue test results of two fillet weld details with weld toe and weld root failure are evaluated at a range of temperatures using a variety of structural hot-spot and notch stress approaches, then are compared to the nominal stress approach. Large differences in prediction accuracy are found for the analysed assessment methods and both failure locations.

physics.app-ph↗
arXiv2010

Simultaneous time-optimal control of the inversion of two spin 1/2 particles

We analyze the simultaneous time-optimal control of two-spin systems. The two non coupled spins which differ in the value of their chemical offsets are controlled by the same magnetic fields. Using an appropriate rotating frame, we restrict the study to the case of opposite shifts. We then show that the optimal solution of the inversion problem in a rotating frame is composed of a pulse sequence of maximum intensity and is similar to the optimal solution for inverting only one spin by using a non-resonant control field in the laboratory frame. An example is implemented experimentally using techniques of Nuclear Magnetic Resonance.

quant-ph↗
arXiv2010

Singular extremals for the time-optimal control of dissipative spin 1/2 particles

We consider the time-optimal control by magnetic fields of a spin 1/2 particle in a dissipative environment. This system is used as an illustrative example to show the role of singular extremals in the control of quantum systems. We analyze a simple case where the control law is explicitly determined. We experimentally implement the optimal control using techniques of nuclear magnetic resonance. To our knowledge, this is the first experimental demonstration of singular extremals in quantum systems with bounded control amplitudes.

quant-ph↗
arXiv2004

Pomeron Vertices in Perturbative QCD in Diffractive Scattering

We analyse the momentum space triple Pomeron vertex in perturbative QCD. In addition to the standard form of this vertex which is used in the context of total cross-sections at high energies and in the QCD reggeon field theory, there exists an alternative form which has to be used in the study of high-mass diffraction. We review and analyse the relation between these two versions. We discuss some implications for the BK-equation. In the second part of our paper we extend this analysis to the Pomeron-Odderon-Odderon vertex.

hep-ph↗
arXiv2003

Influence of a Uniform Current on Collective Magnetization Dynamics in a Ferromagnetic Metal

We discuss the influence of a uniform current, $\vec{j} $, on the magnetization dynamics of a ferromagnetic metal. We find that the magnon energy $ε(\vec{q})$ has a current-induced contribution proportional to $\vec{q}\cdot \vec{\cal J}$, where $\vec{\cal J}$ is the spin-current, and predict that collective dynamics will be more strongly damped at finite ${\vec j}$. We obtain similar results for models with and without local moment participation in the magnetic order. For transition metal ferromagnets, we estimate that the uniform magnetic state will be destabilized for $j \gtrsim 10^{9} {\rm A} {\rm cm}^{-2}$. We discuss the relationship of this effect to the spin-torque effects that alter magnetization dynamics in inhomogeneous magnetic systems.

cond-mat.mtrl-sci↗
arXiv2003

Multi-Channel Inverse Scattering Problem on the Line: Thresholds and Bound States

We consider the multi-channel inverse scattering problem in one-dimension in the presence of thresholds and bound states for a potential of finite support. Utilizing the Levin representation, we derive the general Marchenko integral equation for N-coupled channels and show that, unlike to the case of the radial inverse scattering problem, the information on the bound state energies and asymptotic normalization constants can be inferred from the reflection coefficient matrix alone. Thus, given this matrix, the Marchenko inverse scattering procedure can provide us with a unique multi-channel potential. The relationship to supersymmetric partner potentials as well as possible applications are discussed. The integral equation has been implemented numerically and applied to several schematic examples showing the characteristic features of multi-channel systems. A possible application of the formalism to technological problems is briefly discussed.

quant-ph↗
arXiv2001

ISOPHOT - Photometric Calibration of Point Sources

All observations by the aperture photometer (PHT-P) and the far-infrared (FIR) camera section (PHT-C) of ISOPHOT included reference measurements against stable internal fine calibration sources (FCS) to correct for temporal drifts in detector responsivities. The FCSs were absolutely calibrated in-orbit against stars, asteroids and planets, covering wavelengths from 3.2 to 240 micron. We present the calibration concept for point sources within a flux-range from 60 mJy up to 4500 Jy for staring and raster observations in standard configurations and discuss the requisite measurements and the uncertainties involved. In this process we correct for instrumental effects like nonlinearities, signal transients, time variable dark current, misalignments and diffraction effects. A set of formulae is developed that describes the calibration from signal-level to flux densities. The scatter of 10 to 20 % of the individual data points around the derived calibration relations is a measure of the consistency and typical accuracy of the calibration. The reproducibility over longer periods of time is better than 10 %. The calibration tables and algorithms have been implemented in the final versions of the software for offline processing and interactive analysis.

astro-ph↗
arXiv2001

Finite formation time effects in inclusive and semi-inclusive electro-disintegration of few-body nuclei

Finite Formation Time (FFT) effects in the exclusive reaction ^4He(e,e'p)^3H at high values of Q^2 are introduced and discussed. It is shown that the minimum in the momentum distributions predicted by the Plane Wave Impulse Approximation (PWIA), which is filled by the Glauber-type Final State Interaction (FSI), is completely recovered when FFT effects are taken into account. The semi-inclusive process ^4 He(e,e'p)X is also investigated.

nucl-th↗
arXiv1999

The bootstrap for impact factors and the gluon wave function

Using the results recently obtained for the non-forward quark and gluon impact factors, it is shown that their form in the gluon colour channel is consistent with the ``third bootstrap condition'', namely, that they should be proportional to the gluon wave function. The gluon wave function found from this assumption is used to write the full bootstrap condition for the gluonic potential in the next-to-leading order.

hep-ph↗
arXiv1999

Transverse Momentum Dependence of Cumulative Pions

In the framework of the recently proposed QCD based parton model for the cumulative phenomena in the interactions with nuclei the dependence of the cumulative pion production rates on the transverse momentum is studied. The mean value of the transverse momentum is found to grow with x in the cumulative region. The obtained results are in agreement with the available experimental data.

hep-ph↗
arXiv1998

Photoexcitation of the Nucleon in the Constituent Quark Model

Photoexcitation of the nucleon is studied in the framework of the constituent quark model with nonrelativistic quark-quark interactions. Utilizing the dominance of the pairwise structure of the relevant three-quark potential, the bound and excited state wave functions have been constructed using the Faddeev-type Integrodifferential Equation Approach. The method is based on the Hyperspherical Harmonics expansion and takes two-body correlations exactly into account and therefore reliable wave functions, required in photo processes, can be obtained. The integrated photoabsorption cross sections for the E1-and M1-transitions are calculated and compared with experimental values as well as with those of other theoretical calculations. Our results, in overall, are in good to very good agreement with the experimental values.

nucl-th↗
arXiv1997

Vibrational Frequencies of the 2p^2A^"_2 and 3d^2E^" States of the Triatomic Deuterium Molecule

We investigated the vibrational energies in the 2p^2A^"_2 and 3d^2E^" states of the triatomic deuterium molecule D_3. The experiments were performed using a fast neutral beam photoionization spectrometer recently developed at Freiburg. A depletion type optical double-resonance scheme using two pulsed dye lasers was applied. The measured vibrational frequencies of the 2p^2A^"_2 state of D_3 are compared to those of H_3 and to theoretical values calculated from an ab initio potential energy surface. The data give insight into the importance of the coupling between the valence electron and the ion core.

physics.chem-ph↗
arXiv1997

Nuclear Fusion via Triple Collisions in Solar Plasma

We consider several nuclear fusion reactions that take place at the center of the sun, which are omitted in the standard pp-chain model. More specifically the reaction rates of the nonradiative production of ^3He, ^7Be, and ^8B nuclei in triple collisions involving electrons are estimated within the framework of the adiabatic approximation. These rates are compared with those of the corresponding binary fusion reactions.

nucl-th↗
arXiv1997

The system of four reggeized gluons and the three-pomeron vertex in the high colour limit

The amplitude for 4 interacting reggeized gluons is studied in the high-colour limit. The leading order amplitude is explicitly shown to reduce to a pair of reggeons, i.e to a single BFKL pomeron. The next-to-leading order diffractive amplitude is found to split into a double pomeron exchange and triple pomeron contribution. The obtained three-pomeron vertex is different from the originally proposed in [2].

hep-ph↗
arXiv1997

Eta-meson light nucleus scattering and Charge Symmetry Breaking

The η-meson collision with light nuclei, ^2H, ^3H, ^3He, and ^4He, is considered on the basis of a microscopic approach which utilises Faddeev type and Lippmann-Schwinger equations. The nuclear hamiltonian is approximated by the finite rank approximation which amounts to the coherent approximation. The η-nucleus scattering length and the resonance and bound--state poles of the amplitude describing elastic scattering of η-meson by these nuclei are obtained. For each of the nuclei considered, the minimal factor enhancing the ηN--attraction that moves the poles to the quasi--bound state area of the complex k--plane, is determined. It is shown that within the existing uncertainties in the elementary ηN iteraction all these nuclei can support a quasi--bound state which can result in a formation of an eta-mesic nucleus. Various aspects of the eta-nucleus physics are discussed with specific emphasis given to the η--nucleus effective interaction in configuration space (constructed via the Marchenko inverse scattering method), to the Okubo--Zweig--Iizuka (OZI) rule, and to the charge symmetry breaking problem.

nucl-th↗