Universality of soft and collinear factors in hard-scattering factorization

Universality in QCD factorization of parton densities, fragmentation functions, and soft factors is endangered by the process dependence of the directions of Wilson lines in their definitions. We find a choice of directions that is consistent with factorization and that gives universality between e^+e^- annihilation, semi-inclusive deep-inelastic scattering, and the Drell-Yan process. Universality is only modified by a time-reversal transformation of the soft function and parton densities between Drell-Yan and the other processes, whose only effect is the known reversal of sign for T-odd parton densities like the Sivers function. The modifications of the definitions needed to remove rapidity divergences with light-like Wilson lines do not affect the results.Comment: 4 pages. Extra references. Text and references as in published versio

Observing collapse in two colliding dipolar Bose-Einstein condensates

We study the collision of two Bose-Einstein condensates with pure dipolar interaction. A stationary pure dipolar condensate is known to be stable when the atom number is below a critical value. However, collapse can occur during the collision between two condensates due to local density fluctuations even if the total atom number is only a fraction of the critical value. Using full three-dimensional numerical simulations, we observe the collapse induced by local density fluctuations. For the purpose of future experiments, we present the time dependence of the density distribution, energy per particle and the maximal density of the condensate. We also discuss the collapse time as a function of the relative phase between the two condensates.Comment: 6 pages, 7 figure

Robust Entanglement through Macroscopic Quantum Jumps

We propose an entanglement generation scheme that requires neither the coherent evolution of a quantum system nor the detection of single photons. Instead, the desired state is heralded by a {\em macroscopic} quantum jump. Macroscopic quantum jumps manifest themselves as a random telegraph signal with long intervals of intense fluorescence (light periods) interrupted by the complete absence of photons (dark periods). Here we show that a system of two atoms trapped inside an optical cavity can be designed such that a dark period prepares the atoms in a maximally entangled ground state. Achieving fidelities above 0.9 is possible even when the single-atom cooperativity parameter C is as low as 10 and when using a photon detector with an efficiency as low as eta = 0.2.Comment: 5 pages, 4 figures, more detailed discussion of underlying physical effect, references update

Effect of frequency mismatched photons in quantum information processing

Many promising schemes for quantum information processing (QIP) rely on few-photon interference effects. In these proposals, the photons are treated as being indistinguishable particles. However, single photon sources are typically subject to variation from device to device. Thus the photons emitted from different sources will not be perfectly identical, and there will be some variation in their frequencies. Here, we analyse the effect of this frequency mismatch on QIP schemes. As examples, we consider the distributed QIP protocol proposed by Barrett and Kok, and Hong-Ou-Mandel interference which lies at the heart of many linear optical schemes for quantum computing. In the distributed QIP protocol, we find that the fidelity of entangled qubit states depends crucially on the time resolution of single photon detectors. In particular, there is no reduction in the fidelity when an ideal detector model is assumed, while reduced fidelities may be encountered when using realistic detectors with a finite response time. We obtain similar results in the case of Hong-Ou-Mandel interference -- with perfect detectors, a modified version of quantum interference is seen, and the visibility of the interference pattern is reduced as the detector time resolution is reduced. Our findings indicate that problems due to frequency mismatch can be overcome, provided sufficiently fast detectors are available.Comment: 14 pages, 8 figures. Comments welcome. v2: Minor changes. v3: Cleaned up 3 formatting error

Generalized polarizabilities of the nucleon studied in the linear sigma model (II)

In a previous paper virtual Compton scattering off the nucleon has been investigated in the one-loop approximation of the linear sigma model in order to determine the 3 scalar generalized polarizabilities. We have now extended this work and calculated the 7 vector polarizabilities showing up in the spin-dependent amplitude of virtual Compton scattering. The results fulfill 3 model-independent constraints recently derived. Compared to the constituent quark model there exist enormous differences for some of the vector polarizabilities. At vanishing three-momentum of the virtual photon, the analytical results of the sigma model and of chiral perturbation theory can be related. The influence of the $\pi^{0}$ exchange in the $t$ channel has been discussed in some detail. Besides, the vector polarizabilities determine 2 linear combinations of the third order spin-polarizabilities appearing in real Compton scattering.Comment: 17 pages, 4 figures, latex2e (Revtex), submitted to Z. Phys.

Dispersion relation formalism for virtual Compton scattering and the generalized polarizabilities of the nucleon

A dispersion relation formalism for the virtual Compton scattering (VCS) reaction on the proton is presented, which for the first time allows a dispersive evaluation of 4 generalized polarizabilities at a four-momentum transfer $Q^2 \leq$ 0.5 GeV$^2$. The dispersive integrals are calculated using a state-of-the-art pion photo- and electroproduction analysis. The dispersion formalism provides a new tool to analyze VCS experiments above pion threshold, thus increasing the sensitivity to the generalized polarizabilities of the nucleon.Comment: 4 pages, 2 figure

Structure analysis of the virtual Compton scattering amplitude at low energies

We analyze virtual Compton scattering off the nucleon at low energies in a covariant, model-independent formalism. We define a set of invariant functions which, once the irregular nucleon pole terms have been subtracted in a gauge-invariant fashion, is free of poles and kinematical zeros. The covariant treatment naturally allows one to implement the constraints due to Lorentz and gauge invariance, crossing symmetry, and the discrete symmetries. In particular, when applied to the $ep\to e'p'\gamma$ reaction, charge-conjugation symmetry in combination with nucleon crossing generates four relations among the ten originally proposed generalized polarizabilities of the nucleon.Comment: 19 pages, LaTeX2e/RevTeX, no figures, original sections IV.-VI. removed, to be discussed in a separate publication, none of the conclusions change

К разработке технологии возведения геокомпозитных охранных систем горных выработок

У статті розглянуто відмінності використання особливо тонко дисперсних в’яжучих (ОТДВ) у підземних умовах для створення елементів геокомпозитних конструкцій.In article the differences of the use especially thinly of dispersible astringent are considered in underground terms for creation elements of geocomposit constructions