Sampling and reconstruction of operators

We study the recovery of operators with bandlimited Kohn-Nirenberg symbol from the action of such operators on a weighted impulse train, a procedure we refer to as operator sampling. Kailath, and later Kozek and the authors have shown that operator sampling is possible if the symbol of the operator is bandlimited to a set with area less than one. In this paper we develop explicit reconstruction formulas for operator sampling that generalize reconstruction formulas for bandlimited functions. We give necessary and sufficient conditions on the sampling rate that depend on size and geometry of the bandlimiting set. Moreover, we show that under mild geometric conditions, classes of operators bandlimited to an unknown set of area less than one-half permit sampling and reconstruction. A similar result considering unknown sets of area less than one was independently achieved by Heckel and Boelcskei. Operators with bandlimited symbols have been used to model doubly dispersive communication channels with slowly-time-varying impulse response. The results in this paper are rooted in work by Bello and Kailath in the 1960s.Comment: Submitted to IEEE Transactions on Information Theor

Exact Results on Dynamical Decoupling by π\pi-Pulses in Quantum Information Processes

The aim of dynamical decoupling consists in the suppression of decoherence by appropriate coherent control of a quantum register. Effectively, the interaction with the environment is reduced. In particular, a sequence of $\pi$ pulses is considered. Here we present exact results on the suppression of the coupling of a quantum bit to its environment by optimized sequences of $\pi$ pulses. The effect of various cutoffs of the spectral density of the environment is investigated. As a result we show that the harder the cutoff is the better an optimized pulse sequence can deal with it. For cutoffs which are neither completely hard nor very soft we advocate iterated optimized sequences.Comment: 12 pages and 3 figure

Generic susceptibilities of the half-filled Hubbard model in infinite dimensions

Around a metal-to-insulator transition driven by repulsive interaction (Mott transition) the single particle excitations and the collective excitations are equally important. Here we present results for the generic susceptibilities at zero temperature in the half-filled Hubbard model in infinite dimensions. Profiting from the high resolution of dynamic density-matrix renormalization at all energies, results for the charge, spin and Cooper-pair susceptibilities in the metallic and the insulating phase are computed. In the insulating phase, an almost saturated local magnetic moment appears. In the metallic phase a pronounced low-energy peak is found in the spin response.Comment: 12 pages, 12 figures; slight changes and one additional figure due to referees' suggestion

Three dimensional generalization of the J1J_1-J2J_2 Heisenberg model on a square lattice and role of the interlayer coupling JcJ_c

A possibility to describe magnetism in the iron pnictide parent compounds in terms of the two-dimensional frustrated Heisenberg $J_1$-$J_2$ model has been actively discussed recently. However, recent neutron scattering data has shown that the pnictides have a relatively large spin wave dispersion in the direction perpendicular to the planes. This indicates that the third dimension is very important. Motivated by this observation we study the $J_1$-$J_2$-$J_c$ model that is the three dimensional generalization of the $J_1$-$J_2$ Heisenberg model for $S = 1/2$ and S = 1. Using self-consistent spin wave theory we present a detailed description of the staggered magnetization and magnetic excitations in the collinear state. We find that the introduction of the interlayer coupling $J_c$ suppresses the quantum fluctuations and strengthens the long range ordering. In the $J_1$-$J_2$-$J_c$ model, we find two qualitatively distinct scenarios for how the collinear phase becomes unstable upon increasing $J_1$. Either the magnetization or one of the spin wave velocities vanishes. For $S = 1/2$ renormalization due to quantum fluctuations is significantly stronger than for S=1, in particular close to the quantum phase transition. Our findings for the $J_1$-$J_2$-$J_c$ model are of general theoretical interest, however, the results show that it is unlikely that the model is relevant to undoped pnictides.Comment: 11 pages, 10 figures. Updated version, several references adde

Towards a common object model and API for accelerator controls

An Object-Oriented Application Programming Interface (OO API) can provide applications with an abstract model of the components of an accelerator. The main question is how to encapsulate different control systems into one single abstract model. The abstract model of an 00 API can be described in a formal way via object models in order to clarify the semantic issues, to describe the important concepts (device, attributes, ...), and to decompose the objects up to the granularity where the model of some objects can be shared between labs. A C++ API (as well as C API) can be derived from the object-model. This paper presents a common object model which is derived from the object-model. This paper presents a common object model which is derived from both the current CERN-PS model and the current ERSF model. We describe the technical difficulties we encountered in migrating existing control systems into a shared but usable model. We also aim to increase the universality of the model by taking into account the CDEV library, as well as CORBA. A high-level description of the model will be presented with examples of the derived API

Emotional and rational disease acceptance in patients with depression and alcohol addiction

<p>Abstract</p> <p>Background</p> <p>The concept of a rational respectively emotional acceptance of disease is highly valued in the treatment of patients with depression or addiction. Due to the importance of this concept for the long-term course of disease, there is a strong interest to develop a tool to identify the levels and factors of acceptance. We thus intended to test an instrument designed to assess the level of positive psychological wellbeing and coping, particularly emotional disease acceptance and life satisfaction</p> <p>Methods</p> <p>In an anonymous cross-sectional survey enrolling 115 patients (51% female, 49% male; mean age 47.6 ± 10.0 years) with depression and/or alcohol addiction, the ERDA questionnaire was tested.</p> <p>Results</p> <p>Factor analysis of the 29-item construct (Cronbach's alpha = 0.933) revealed a 4-factor solution, which explained 59.4% of variance: (1) Positive Life Construction, Contentedness and Well-Being; (2) Conscious Dealing with Illness; (3) Rejection of an Irrational Dealing with Disease; (4) Disease Acceptance. Two factors could be ascribed to a rational, and two to an emotional acceptance. All factors correlated negatively with Depression and Escape, while several aspects of Life Satisfaction" (i.e. myself, overall life, where I live, and future prospects) correlated positively. The highest factor scores were found for the rational acceptance styles (i.e. Conscious Dealing with Illness; Disease Acceptance). Emotional acceptance styles were not valued in a state of depression. Escape from illness was the strongest predictor for several acceptance aspects, while life satisfaction was the most relevant predictor for "Positive Life Construction, Contentedness and Well-Being".</p> <p>Conclusion</p> <p>The ERDA questionnaire was found to be a reliable and valid assessment of disease acceptance strategies in patients with depressive disorders and drug abuses. The results indicate the preferential use of rational acceptance styles even in depression. Disease acceptance should not be regarded as a coping style with an attitude of fatalistic resignation, but as a complex and active process of dealing with a chronic disease. One may assume that an emotional acceptance of disease will result in a therapeutic coping process associated with higher level of life satisfaction and overall quality of life.</p

Constraints on Lorentz Invariance Violation using INTEGRAL/IBIS observations of GRB041219A

One of the experimental tests of Lorentz invariance violation is to measure the helicity dependence of the propagation velocity of photons originating in distant cosmological obejcts. Using a recent determination of the distance of the Gamma-Ray Burst GRB 041219A, for which a high degree of polarization is observed in the prompt emission, we are able to improve by 4 orders of magnitude the existing constraint on Lorentz invariance violation, arising from the phenomenon of vacuum birefringence.Comment: 5 pages, 3 figures, accepted for publication as a Rapid Communication in Physical Review

One-Watt level mid-IR output, singly resonant, continuous-wave optical parametric oscillator pumped by a monolithic diode laser

We report more than 1.1 Watt of idler power at 3373 nm in a singly resonant optical parametric oscillator (SRO), directly pumped by a single-frequency monolithic tapered diode laser. The SRO is based on a periodically poled MgO:LiNbO3 crystal in a four mirror cavity and is excited by 8.05 W of 1062 nm radiation. The SRO pump power at threshold is 4 W. The internal slope-efficiency and conversion efficiency reach 89% and 44% respectively. The signal and idler waves are temperature tuned in the range of 1541 to 1600 nm and 3154 to 3415 nm respectively. To the best of our knowledge, this is the highest output obtained for a diode pumped optical parametric oscillator (OPO), and the first time a SRO is directly pumped by a monolithic tapered diode laser

INTEGRAL discovery of persistent hard X-ray emission from the Soft Gamma Ray Repeater SGR 1806-20

We report the discovery of persistent hard X-ray emission extending up to 150 keV from the soft gamma-ray repeater SGR 1806-20 using data obtained with the INTEGRAL satellite in 2003-2004. Previous observations of hard X-rays from objects of this class were limited to short duration bursts and rare transient episodes of strongly enhanced luminosity (``flares''). The emission observed with the IBIS instrument above 20 keV has a power law spectrum with photon index in the range 1.5-1.9 and a flux of 3 milliCrabs, corresponding to a 20-100 keV luminosity of ~10^36 erg s^-1 (for a distance of 15 kpc). The spectral hardness and the luminosity correlate with the level of source activity as measured from the number of emitted bursts.Comment: 5 pages, 3 figures, Revised version accepted for publication in Astronomy and Astrophysics Letter