2,011 research outputs found
The drive system of the Major Atmospheric Gamma-ray Imaging Cherenkov Telescope
The MAGIC telescope is an imaging atmospheric Cherenkov telescope, designed
to observe very high energy gamma-rays while achieving a low energy threshold.
One of the key science goals is fast follow-up of the enigmatic and short lived
gamma-ray bursts. The drive system for the telescope has to meet two basic
demands: (1) During normal observations, the 72-ton telescope has to be
positioned accurately, and has to track a given sky position with high
precision at a typical rotational speed in the order of one revolution per day.
(2) For successfully observing GRB prompt emission and afterglows, it has to be
powerful enough to position to an arbitrary point on the sky within a few ten
seconds and commence normal tracking immediately thereafter. To meet these
requirements, the implementation and realization of the drive system relies
strongly on standard industry components to ensure robustness and reliability.
In this paper, we describe the mechanical setup, the drive control and the
calibration of the pointing, as well as present measurements of the accuracy of
the system. We show that the drive system is mechanically able to operate the
motors with an accuracy even better than the feedback values from the axes. In
the context of future projects, envisaging telescope arrays comprising about
100 individual instruments, the robustness and scalability of the concept is
emphasized.Comment: 15 pages, 12 (10) figures, submitted to Astroparticle Physics, a high
resolution version of the paper (particularly fig. 1) is available at
http://publications.mppmu.mpg.de/2008/MPP-2008-101/FullText.pd
Fast initialization of a high-fidelity quantum register using optical superlattices
We propose a method for the fast generation of a quantum register of
addressable qubits consisting of ultracold atoms stored in an optical lattice.
Starting with a half filled lattice we remove every second lattice barrier by
adiabatically switching on a superlattice potential which leads to a long
wavelength lattice in the Mott insulator state with unit filling. The larger
periodicity of the resulting lattice could make individual addressing of the
atoms via an external laser feasible. We develop a Bose-Hubbard-like model for
describing the dynamics of cold atoms in a lattice when doubling the lattice
periodicity via the addition of a superlattice potential. The dynamics of the
transition from a half filled to a commensurately filled lattice is analyzed
numerically with the help of the Time Evolving Block Decimation algorithm and
analytically using the Kibble-Zurek theory. We show that the time scale for the
whole process, i.e. creating the half filled lattice and subsequent doubling of
the lattice periodicity, is significantly faster than adiabatic direct quantum
freezing of a superfluid into a Mott insulator for large lattice periods. Our
method therefore provides a high fidelity quantum register of addressable
qubits on a fast time scale.Comment: 22 pages, 9 figures, IOP style. Revised version to appear in NJ
The Quasi-1D S=1/2 Antiferromagnet Cs2CuCl4 in a Magnetic Field
Magnetic excitations of the quasi-1D S=1/2 Heisenberg antiferromagnet (HAF)
Cs2CuCl4 have been measured as a function of magnetic field using neutron
scattering. For T<0.62 K and B=0 T the weak inter-chain coupling produces 3D
incommensurate ordering. Fields greater than Bc =1.66 T, but less than the
field (~8 T) required to fully align the spins, are observed to decouple the
chains, and the system enters a disordered intermediate-field phase (IFP). The
IFP excitations are in agreement with the predictions of Muller et al. for the
1D S=1/2 HAF, and Talstra and Haldane for the related 1/r^2 chain (the
Haldane-Shastry model). This behaviour is inconsistent with linear spin-wave
theory.Comment: 10 pages, 4 encapsulated postscript figures, LaTeX, to be published
in PRL, e-mail comments to [email protected]
Work and Quantum Phase Transitions: Is there Quantum Latency?
We study the physics of quantum phase transitions from the perspective of
non-equilibrium thermodynamics. For first order quantum phase transitions, we
find that the average work done per quench in crossing the critical point is
discontinuous. This leads us to introduce the quantum latent work in analogy
with the classical latent heat of first order classical phase transitions. For
second order quantum phase transitions the irreversible work is closely related
to the fidelity susceptibility for weak sudden quenches of the system
Hamiltonian. We demonstrate our ideas with numerical simulations of first,
second, and infinite order phase transitions in various spin chain models.Comment: accepted in PR
Broadband study of blazar 1ES 1959+650 during flaring state in 2016
Aim : The nearby TeV blazar 1ES 1959+650 (z=0.047) was reported to be in
flaring state during June - July 2016 by Fermi-LAT, FACT, MAGIC and VERITAS
collaborations. We studied the spectral energy distributions (SEDs) in
different states of the flare during MJD 57530 - 57589 using simultaneous
multiwaveband data to understand the possible broadband emission scenario
during the flare. Methods : The UV/optical and X-ray data from UVOT and XRT
respectively on board Swift and high energy -ray data from Fermi-LAT
are used to generate multiwaveband lightcurves as well as to obtain high flux
states and quiescent state SEDs. The correlation and lag between different
energy bands is quantified using discrete correlation function. The synchrotron
self Compton (SSC) model was used to reproduce the observed SEDs during flaring
and quiescent states of the source. Results : A decent correlation is seen
between X-ray and high energy -ray fluxes. The spectral hardening with
increase in the flux is seen in X-ray band. The powerlaw index vs flux plot in
-ray band indicates the different emission regions for 0.1 - 3 GeV and
3-300 GeV energy photons. Two zone SSC model satisfactorily fits the observed
broadband SEDs. The inner zone is mainly responsible for producing synchrotron
peak and high energy -ray part of the SED in all states. The second
zone is mainly required to produce less variable optical/UV and low energy
-ray emission. Conclusions : Conventional single zone SSC model does
not satisfactorily explain broadband emission during observation period
considered. There is an indication of two emission zones in the jet which are
responsible for producing broadband emission from optical to high energy
-rays.Comment: 11 pages, 12 figures, Accepted in A&
The Demise of Regulation Q Differentials: Competition for Household Savins Between Commercial Banks and Savings and Loan Associations- A Note
William R. Reichenstein is a Visiting Professor of Finance at Southern Methodist University. Frederick H. Dorner is an Assistant Professor of Quantitative Methods at Trinity University
Quantum phase estimation with lossy interferometers
We give a detailed discussion of optimal quantum states for optical two-mode
interferometry in the presence of photon losses. We derive analytical formulae
for the precision of phase estimation obtainable using quantum states of light
with a definite photon number and prove that maximization of the precision is a
convex optimization problem. The corresponding optimal precision, i.e. the
lowest possible uncertainty, is shown to beat the standard quantum limit thus
outperforming classical interferometry. Furthermore, we discuss more general
inputs: states with indefinite photon number and states with photons
distributed between distinguishable time bins. We prove that neither of these
is helpful in improving phase estimation precision.Comment: 12 pages, 5 figure
Entangled states of trapped ions allow measuring the magnetic field gradient of a single atomic spin
Using trapped ions in an entangled state we propose detecting a magnetic
dipole of a single atom at distance of a few m. This requires a
measurement of the magnetic field gradient at a level of about 10
Tesla/m. We discuss applications e.g. in determining a wide variation of
ionic magnetic moments, for investigating the magnetic substructure of ions
with a level structure not accessible for optical cooling and detection,and for
studying exotic or rare ions, and molecular ions. The scheme may also be used
for measureing spin imbalances of neutral atoms or atomic ensembles trapped by
optical dipole forces. As the proposed method relies on techniques well
established in ion trap quantum information processing it is within reach of
current technology.Comment: 4 pages, 2 fi
Quantum computations with atoms in optical lattices: marker qubits and molecular interactions
We develop a scheme for quantum computation with neutral atoms, based on the
concept of "marker" atoms, i.e., auxiliary atoms that can be efficiently
transported in state-independent periodic external traps to operate quantum
gates between physically distant qubits. This allows for relaxing a number of
experimental constraints for quantum computation with neutral atoms in
microscopic potential, including single-atom laser addressability. We discuss
the advantages of this approach in a concrete physical scenario involving
molecular interactions.Comment: 15 pages, 14 figure
Phonon Life-times from first principles self consistent lattice dynamics
Phonon lifetime calculations from first principles usually rely on time
consuming molecular dynamics calculations, or density functional perturbation
theory (DFPT) where the zero temperature crystal structure is assumed to be
dynamically stable. Here a new and effective method for calculating phonon
lifetimes from first principles is presented, not limited to crystal structures
stable at 0 K, and potentially much more effective than most corresponding
molecular dynamics calculations. The method is based on the recently developed
self consistent lattice dynamical method and is here tested by calculating the
bcc phase phonon lifetimes of Li, Na, Ti and Zr, as representative examples.Comment: 4 pages, 4 figur
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