14,965 research outputs found
Compact Nuclei in Galaxies at Moderate Redshift: I. Imaging and Spectroscopy
This study explores the space density and properties of active galaxies to
z=0.8. We have investigated the frequency and nature of unresolved nuclei in
galaxies at moderate redshift as indicators of nuclear activity such as Active
Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged
galaxies with multi-component models using maximum likelihood estimate
techniques to determine the best model fit. We select those galaxies requiring
an unresolved, point source component in the galaxy nucleus, in addition to a
disk and/or bulge component, to adequately model the galaxy light. We have
searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies
containing compact nuclei. In our survey of 1033 galaxies, the fraction
containing an unresolved nuclear component greater than 3% of the total galaxy
light is 16+/-3% corrected for incompleteness and 9+/-1% for nuclei greater
than 5% of the galaxy light. Spectroscopic redshifts have been obtained for 35
of our AGN/starburst candidates and photometric redshifts are estimated to an
accuracy of sigma_z=0.1 for the remaining sample. In this paper, the first of
two in this series, we present the selected HST imaged galaxies having
unresolved nuclei and discuss the selection procedure. We also present the
ground-based spectroscopy for these galaxies as well as the photometric
redshifts estimated for those galaxies without spectra.Comment: 56 pages, 22 figures, to appear in ApJ Supplement Series, April 199
Entanglement requirements for implementing bipartite unitary operations
We prove, using a new method based on map-state duality, lower bounds on
entanglement resources needed to deterministically implement a bipartite
unitary using separable (SEP) operations, which include LOCC (local operations
and classical communication) as a particular case. It is known that the Schmidt
rank of an entangled pure state resource cannot be less than the Schmidt rank
of the unitary. We prove that if these ranks are equal the resource must be
uniformly (maximally) entangled: equal nonzero Schmidt coefficients. Higher
rank resources can have less entanglement: we have found numerical examples of
Schmidt rank 2 unitaries which can be deterministically implemented, by either
SEP or LOCC, using an entangled resource of two qutrits with less than one ebit
of entanglement.Comment: 7 pages Revte
Complex temperatures zeroes of partition function in spin-glass models
An approximate method is proposed for investigating complex-temperature
properties of real-dimensional spin-glass models. The method uses the
complex-temperature data of the ferromagnetic model on the same lattice. The
universality line in the complex-temperature space is obtained.Comment: latex, corrected some misprint
Consistent Resolution of Some Relativistic Quantum Paradoxes
A relativistic version of the (consistent or decoherent) histories approach
to quantum theory is developed on the basis of earlier work by Hartle, and used
to discuss relativistic forms of the paradoxes of spherical wave packet
collapse, Bohm's formulation of Einstein-Podolsky-Rosen, and Hardy's paradox.
It is argued that wave function collapse is not needed for introducing
probabilities into relativistic quantum mechanics, and in any case should never
be thought of as a physical process. Alternative approaches to stochastic time
dependence can be used to construct a physical picture of the measurement
process that is less misleading than collapse models. In particular, one can
employ a coarse-grained but fully quantum mechanical description in which
particles move along trajectories, with behavior under Lorentz transformations
the same as in classical relativistic physics, and detectors are triggered by
particles reaching them along such trajectories. States entangled between
spacelike separate regions are also legitimate quantum descriptions, and can be
consistently handled by the formalism presented here. The paradoxes in question
arise because of using modes of reasoning which, while correct for classical
physics, are inconsistent with the mathematical structure of quantum theory,
and are resolved (or tamed) by using a proper quantum analysis. In particular,
there is no need to invoke, nor any evidence for, mysterious long-range
superluminal influences, and thus no incompatibility, at least from this
source, between relativity theory and quantum mechanics.Comment: Latex 42 pages, 7 figures in text using PSTrick
Graviton mediated photon-photon scattering in general relativity
In this paper we consider photon-photon scattering due to self-induced
gravitational perturbations on a Minkowski background. We focus on four-wave
interaction between plane waves with weakly space and time dependent
amplitudes, since interaction involving a fewer number of waves is excluded by
energy-momentum conservation. The Einstein-Maxwell system is solved
perturbatively to third order in the field amplitudes and the coupling
coefficients are found for arbitrary polarizations in the center of mass
system. Comparisons with calculations based on quantum field theoretical
methods are made, and the small discrepances are explained.Comment: 5 pages, 3 figure
Expanding, axisymmetric pure-radiation gravitational fields with a simple twist
New expanding, axisymmetric pure-radiation solutions are found, exploiting
the analogy with the Euler-Darboux equation for aligned colliding plane waves.Comment: revtex, 5 page
Generalised Kundt waves and their physical interpretation
We present the complete family of space-times with a non-expanding,
shear-free, twist-free, geodesic principal null congruence (Kundt waves) that
are of algebraic type III and for which the cosmological constant ()
is non-zero. The possible presence of an aligned pure radiation field is also
assumed. These space-times generalise the known vacuum solutions of type N with
arbitrary and type III with . It is shown that there
are two, one and three distinct classes of solutions when is
respectively zero, positive and negative. The wave surfaces are plane,
spherical or hyperboloidal in Minkowski, de Sitter or anti-de Sitter
backgrounds respectively, and the structure of the family of wave surfaces in
the background space-time is described. The weak singularities which occur in
these space-times are interpreted in terms of envelopes of the wave surfaces.Comment: 16 pages including 2 figures. To appear in Classical and Quantum Gra
Impulsive spherical gravitational waves
Penrose's identification with warp provides the general framework for
constructing the continuous form of impulsive gravitational wave metrics. We
present the 2-component spinor formalism for the derivation of the full family
of impulsive spherical gravitational wave metrics which brings out the power in
identification with warp and leads to the simplest derivation of exact
solutions. These solutions of the Einstein vacuum field equations are obtained
by cutting Minkowski space into two pieces along a null cone and re-identifying
them with warp which is given by an arbitrary non-linear holomorphic
transformation. Using 2-component spinor techniques we construct a new metric
describing an impulsive spherical gravitational wave where the vertex of the
null cone lies on a world-line with constant acceleration
An introduction to the spectrum, symmetries, and dynamics of spin-1/2 Heisenberg chains
Quantum spin chains are prototype quantum many-body systems. They are
employed in the description of various complex physical phenomena. The goal of
this paper is to provide an introduction to the subject by focusing on the time
evolution of a Heisenberg spin-1/2 chain and interpreting the results based on
the analysis of the eigenvalues, eigenstates, and symmetries of the system. We
make available online all computer codes used to obtain our data.Comment: 8 pages, 3 figure
Classicality of quantum information processing
The ultimate goal of the classicality programme is to quantify the amount of
quantumness of certain processes. Here, classicality is studied for a
restricted type of process: quantum information processing (QIP). Under special
conditions, one can force some qubits of a quantum computer into a classical
state without affecting the outcome of the computation. The minimal set of
conditions is described and its structure is studied. Some implications of this
formalism are the increase of noise robustness, a proof of the quantumness of
mixed state quantum computing and a step forward in understanding the very
foundation of QIP.Comment: Minor changes, published in Phys. Rev. A 65, 42319 (2002
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