1,349 research outputs found
Installing a Fast Orbit Feedback at BESSY
In view of increased processing bandwidth at demanding experiments and the need for rapid compensation of noise spikes and new, yet unknown excitations, a fast orbit feedback aiming at noise suppression in the 1Hz 50Hz range has become mandatory for the 3rd generation light source BESSY II. The fast set point transmission plus the replacement of all corrector power supplies is scheduled as a first step. Later in combination with top up operation orbit stability can be further improved by replacing today s multiplexed analog beam position monitors by state of the art fast digital units. This paper describes how the pilot installation of a small subset of fast corrector power supplies allows to tune performance and study the benefits for today s most sensitive experiment
Exact Performance of Concatenated Quantum Codes
When a logical qubit is protected using a quantum error-correcting code, the
net effect of coding, decoherence (a physical channel acting on qubits in the
codeword) and recovery can be represented exactly by an effective channel
acting directly on the logical qubit. In this paper we describe a procedure for
deriving the map between physical and effective channels that results from a
given coding and recovery procedure. We show that the map for a concatenation
of codes is given by the composition of the maps for the constituent codes.
This perspective leads to an efficient means for calculating the exact
performance of quantum codes with arbitrary levels of concatenation. We present
explicit results for single-bit Pauli channels. For certain codes under the
symmetric depolarizing channel, we use the coding maps to compute exact
threshold error probabilities for achievability of perfect fidelity in the
infinite concatenation limit.Comment: An expanded presentation of the analytic methods and results from
quant-ph/0111003; 13 pages, 6 figure
Spectral line shape of resonant four-wave mixing induced by broad-bandwidth lasers
We present a theoretical and experimental study of the line shape of resonant four-wave mixing induced by broad-bandwidth laser radiation that revises the theory of Meacher, Smith, Ewart, and Cooper (MSEC) [Phys. Rev. A 46, 2718 (1992)]. We adopt the same method as MSEC but correct for an invalid integral used to average over the distribution of atomic velocities. The revised theory predicts a Voigt line shape composed of a homogeneous, Lorentzian component, defined by the collisional rate Γ, and an inhomogeneous, Doppler component, which is a squared Gaussian. The width of the inhomogeneous component is reduced by a factor of √2 compared to the simple Doppler width predicted by MSEC. In the limit of dominant Doppler broadening, the width of the homogeneous component is predicted to be 4Γ, whereas in the limit of dominant homogeneous broadening, the predicted width is 2Γ. An experimental measurement is reported of the line shape of the four-wave-mixing signal using a broad-bandwidth, "modeless", laser resonant with the Q1 (6) line of the A2 Σ - X2 Π(0,0) system of the hydroxyl radical. The measured widths of the Voigt components were found to be consistent with the predictions of the revised theory
Finite temperature quantum simulation of stabilizer Hamiltonians
We present a scheme for robust finite temperature quantum simulation of
stabilizer Hamiltonians. The scheme is designed for realization in a physical
system consisting of a finite set of neutral atoms trapped in an addressable
optical lattice that are controllable via 1- and 2-body operations together
with dissipative 1-body operations such as optical pumping. We show that these
minimal physical constraints suffice for design of a quantum simulation scheme
for any stabilizer Hamiltonian at either finite or zero temperature. We
demonstrate the approach with application to the abelian and non-abelian toric
codes.Comment: 13 pages, 2 figure
Landscape Study of Target Space Duality of (0,2) Heterotic String Models
In the framework of (0,2) gauged linear sigma models, we systematically
generate sets of perturbatively dual heterotic string compactifications. This
target space duality is first derived in non-geometric phases and then
translated to the level of GLSMs and its geometric phases. In a landscape
analysis, we compare the massless chiral spectra and the dimensions of the
moduli spaces. Our study includes geometries given by complete intersections of
hypersurfaces in toric varieties equipped with SU(n) vector bundles defined via
the monad construction.Comment: 40 pages, 6 figure
Moduli Stabilization and Inflationary Cosmology with Poly-Instantons in Type IIB Orientifolds
Equipped with concrete examples of Type IIB orientifolds featuring
poly-instanton corrections to the superpotential, the effects on moduli
stabilization and inflationary cosmology are analyzed. Working in the framework
of the LARGE volume scenario, the Kaehler modulus related to the size of the
four-cycle supporting the poly-instanton contributes sub-dominantly to the
scalar potential. It is shown that this Kaehler modulus gets stabilized and, by
displacing it from its minimum, can play the role of an inflaton. Subsequent
cosmological implications are discussed and compared to experimental data.Comment: 38 pages, 7 figures, Reference added, Typo fixed, Published versio
Measurements of OH and HO2 concentrations during the MCMA-2006 field campaign – Part 2: Model comparison and radical budget
Measurements of hydroxyl (OH) and hydroperoxy (HO2) radicals were made during the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These measurements provide a unique opportunity to test current models of atmospheric ROx (OH + HO2 + RO2) photochemistry under polluted conditions. A zero-dimensional box model based on the Regional Atmospheric Chemical Mechanism (RACM) was constrained by 10-min averages of 24 J-values and the concentrations of 97 chemical species. Several issues related to the ROx chemistry under polluted conditions are highlighted in this study: (i) Measured concentrations of both OH and HO2 were underpredicted during morning hours on a median campaign basis, suggesting a significant source of radicals is missing from current atmospheric models under polluted conditions, consistent with previous urban field campaigns. (ii) The model-predicted HO2/OH ratios underestimate the measurements for NO mixing ratios higher than 5 ppb, also consistent with previous urban field campaigns. This suggests that under high NOx conditions, the HO2 to OH propagation rate may be overestimated by the model or a process converting OH into HO2 may be missing from the chemical mechanism. On a daily basis (08:40 a.m.–06:40 p.m.), an analysis of the radical budget indicates that HONO photolysis, HCHO photolysis, O3-alkene reactions and dicarbonyls photolysis are the main radical sources. O3 photolysis contributes to less than 6% of the total radical production.Henry & Camille Dreyfus FoundationNational Science Foundation (U.S.) (ATM-9984152)National Science Foundation (U.S.) (0612738
- …