451 research outputs found
Interactive Visualization of the Largest Radioastronomy Cubes
3D visualization is an important data analysis and knowledge discovery tool,
however, interactive visualization of large 3D astronomical datasets poses a
challenge for many existing data visualization packages. We present a solution
to interactively visualize larger-than-memory 3D astronomical data cubes by
utilizing a heterogeneous cluster of CPUs and GPUs. The system partitions the
data volume into smaller sub-volumes that are distributed over the rendering
workstations. A GPU-based ray casting volume rendering is performed to generate
images for each sub-volume, which are composited to generate the whole volume
output, and returned to the user. Datasets including the HI Parkes All Sky
Survey (HIPASS - 12 GB) southern sky and the Galactic All Sky Survey (GASS - 26
GB) data cubes were used to demonstrate our framework's performance. The
framework can render the GASS data cube with a maximum render time < 0.3 second
with 1024 x 1024 pixels output resolution using 3 rendering workstations and 8
GPUs. Our framework will scale to visualize larger datasets, even of Terabyte
order, if proper hardware infrastructure is available.Comment: 15 pages, 12 figures, Accepted New Astronomy July 201
Optical absorption of spin ladders
We present a theory of phonon-assisted optical two-magnon absorption in
two-leg spin-ladders. Based on the strong intra-rung-coupling limit we show
that collective excitations of total spin S=0, 1 and 2 exist outside of the
two-magnon continuum. It is demonstrated that the singlet collective state has
a clear signature in the optical spectrum.Comment: 4 pages, 3 figure
Mean-Field Theory for Spin Ladders Using Angular-Momentum Coupled Bases
We study properties of two-leg Heisenberg spin ladders in a mean-field
approximation using a variety of angular-momentum coupled bases. The mean-field
theory proposed by Gopalan, Rice, and Sigrist, which uses a rung basis, assumes
that the mean-field ground state consists of a condensate of spin-singlets
along the rungs of the ladder. We generalize this approach to larger
angular-momentum coupled bases which incorporate---by their mere definition---a
substantial fraction of the important short-range structure of these materials.
In these bases the mean-field ground-state remains a condensate of spin
singlet---but now with each involving a larger fraction of the spins in the
ladder. As expected, the ``purity'' of the ground-state, as judged by the
condensate fraction, increases with the size of the elementary block defining
the basis. Moreover, the coupling to quasiparticle excitations becomes weaker
as the size of the elementary block increases. Thus, the weak-coupling limit of
the theory becomes an accurate representation of the underlying mean-field
dynamics. We illustrate the method by computing static and dynamic properties
of two-leg ladders in the various angular-momentum coupled bases.Comment: 28 pages with 8 figure
The Effect of Stochastic Noise on Quantum State Transfer
We consider the effect of classical stochastic noise on control laser pulses
used in a scheme for transferring quantum information between atoms, or quantum
dots, in separate optical cavities via an optical connection between cavities.
We develop a master equation for the dynamics of the system subject to
stochastic errors in the laser pulses, and use this to evaluate the sensitivity
of the transfer process to stochastic pulse shape errors for a number of
different pulse shapes. We show that under certain conditions, the sensitivity
of the transfer to the noise depends on the pulse shape, and develop a method
for determining a pulse shape that is minimally sensitive to specific errors.Comment: 10 pages, 9 figures, to appear in Physical Review
Decoherence and Programmable Quantum Computation
An examination of the concept of using classical degrees of freedom to drive
the evolution of quantum computers is given. Specifically, when externally
generated, coherent states of the electromagnetic field are used to drive
transitions within the qubit system, a decoherence results due to the back
reaction from the qubits onto the quantum field. We derive an expression for
the decoherence rate for two cases, that of the single-qubit Walsh-Hadamard
transform, and for an implementation of the controlled-NOT gate. We examine the
impact of this decoherence mechanism on Grover's search algorithm, and on the
proposals for use of error-correcting codes in quantum computation.Comment: submitted to Phys. Rev. A 35 double-spaced pages, 2 figures, in LaTe
Excitation spectrum of the S=1/2 quantum spin ladder with frustration: elementary quasiparticles and many-particle bound states
We study the excitation spectrum of the two-chain S=1/2 Heisenberg spin
ladder with additional inter-chain second-neighbor frustrating interactions.
The one and two-particle excitations are analyzed by using a mapping of the
model onto a Bose gas of hard-core triplets. We find that low-lying singlet and
triplet two-particle bound states are present and their binding energy
increases with increasing frustration. In addition, many-particle bound states
are found by a combination of variational and exact diagonalization techniques.
We prove that the larger the number of bound quasiparticles the larger the
binding energy. Thus the excitation spectrum has a complex structure and
consists of elementary triplets and collective many-particle singlet and
triplet excitations which generally mix with the elementary ones.
The model exhibits a quantum phase transition from an antiferromagnetic
ladder phase (small frustration) into Haldane phase (effectively ferromagnetic
ladder for large frustration). We argue that near the transition point the
spectrum in both triplet and singlet channels becomes gapless. The excitation
wave function is dominated by large-size bound states which leads to the
vanishing of the quasiparticle residue.Comment: RevTeX, 23 pages, 12 figure
Augmented Reality in Astrophysics
Augmented Reality consists of merging live images with virtual layers of
information. The rapid growth in the popularity of smartphones and tablets over
recent years has provided a large base of potential users of Augmented Reality
technology, and virtual layers of information can now be attached to a wide
variety of physical objects. In this article, we explore the potential of
Augmented Reality for astrophysical research with two distinct experiments: (1)
Augmented Posters and (2) Augmented Articles. We demonstrate that the emerging
technology of Augmented Reality can already be used and implemented without
expert knowledge using currently available apps. Our experiments highlight the
potential of Augmented Reality to improve the communication of scientific
results in the field of astrophysics. We also present feedback gathered from
the Australian astrophysics community that reveals evidence of some interest in
this technology by astronomers who experimented with Augmented Posters. In
addition, we discuss possible future trends for Augmented Reality applications
in astrophysics, and explore the current limitations associated with the
technology. This Augmented Article, the first of its kind, is designed to allow
the reader to directly experiment with this technology.Comment: 15 pages, 11 figures. Accepted for publication in Ap&SS. The final
publication will be available at link.springer.co
Phase Transitions Between Topologically Distinct Gapped Phases in Isotropic Spin Ladders
We consider various two-leg ladder models exhibiting gapped phases. All of
these phases have short-ranged valence bond ground states, and they all exhibit
string order. However, we show that short-ranged valence bond ground states
divide into two topologically distinct classes, and as a consequence, there
exist two topologically distinct types of string order. Therefore, not all
gapped phases belong to the same universality class. We show that phase
transitions occur when we interpolate between models belonging to different
topological classes, and we study the nature of these transitions.Comment: 11 pages, 16 postscript figure
A Monte Carlo Study of Correlations in Quantum Spin Ladders
We study antiferromagnetic spin--1/2 Heisenberg ladders, comprised of
chains () with ratio of inter-- to
intra--chain couplings. From measurements of the correlation function we deduce
the correlation length . For even , the static structure factor
exhibits a peak at a temperature below the corresponding spin gap. Results for
isotropically coupled ladders () are compared to those for
the single chain and the square lattice. For , the
correlation function of the two--chain ladder is in excellent agreement with
analytic results from conformal field theory, and exhibits simple
scaling behavior.Comment: 4 pages, 5 EPS figures, submitted to Phys. Rev. Let
Hole Dispersions for Antiferromagnetic Spin-1/2 Two-Leg Ladders by Self-Similar Continuous Unitary Transformations
The hole-doped antiferromagnetic spin-1/2 two-leg ladder is an important
model system for the high- superconductors based on cuprates. Using the
technique of self-similar continuous unitary transformations we derive
effective Hamiltonians for the charge motion in these ladders. The key
advantage of this technique is that it provides effective models explicitly in
the thermodynamic limit. A real space restriction of the generator of the
transformation allows us to explore the experimentally relevant parameter
space. From the effective Hamiltonians we calculate the dispersions for single
holes. Further calculations will enable the calculation of the interaction of
two holes so that a handle of Cooper pair formation is within reach.Comment: 16 pages, 26 figure
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