11,012 research outputs found
Perfect State Transfer: Beyond Nearest-Neighbor Couplings
In this paper we build on the ideas presented in previous works for perfectly
transferring a quantum state between opposite ends of a spin chain using a
fixed Hamiltonian. While all previous studies have concentrated on
nearest-neighbor couplings, we demonstrate how to incorporate additional terms
in the Hamiltonian by solving an Inverse Eigenvalue Problem. We also explore
issues relating to the choice of the eigenvalue spectrum of the Hamiltonian,
such as the tolerance to errors and the rate of information transfer.Comment: 8 pages, 2 figures. Reorganised, more detailed derivations provided
and section on rate of information transfer adde
Partial Information Decomposition as a Unified Approach to the Specification of Neural Goal Functions
In many neural systems anatomical motifs are present repeatedly, but despite
their structural similarity they can serve very different tasks. A prime
example for such a motif is the canonical microcircuit of six-layered
neo-cortex, which is repeated across cortical areas, and is involved in a
number of different tasks (e.g.sensory, cognitive, or motor tasks). This
observation has spawned interest in finding a common underlying principle, a
'goal function', of information processing implemented in this structure. By
definition such a goal function, if universal, cannot be cast in
processing-domain specific language (e.g. 'edge filtering', 'working memory').
Thus, to formulate such a principle, we have to use a domain-independent
framework. Information theory offers such a framework. However, while the
classical framework of information theory focuses on the relation between one
input and one output (Shannon's mutual information), we argue that neural
information processing crucially depends on the combination of
\textit{multiple} inputs to create the output of a processor. To account for
this, we use a very recent extension of Shannon Information theory, called
partial information decomposition (PID). PID allows to quantify the information
that several inputs provide individually (unique information), redundantly
(shared information) or only jointly (synergistic information) about the
output. First, we review the framework of PID. Then we apply it to reevaluate
and analyze several earlier proposals of information theoretic neural goal
functions (predictive coding, infomax, coherent infomax, efficient coding). We
find that PID allows to compare these goal functions in a common framework, and
also provides a versatile approach to design new goal functions from first
principles. Building on this, we design and analyze a novel goal function,
called 'coding with synergy'. [...]Comment: 21 pages, 4 figures, appendi
Brick Walls and AdS/CFT
We discuss the relationship between the bulk-boundary correspondence in
Rehren's algebraic holography (and in other 'fixed-background' approaches to
holography) and in mainstream 'Maldacena AdS/CFT'. Especially, we contrast the
understanding of black-hole entropy from the viewpoint of QFT in curved
spacetime -- in the framework of 't Hooft's 'brick wall' model -- with the
understanding based on Maldacena AdS/CFT. We show that the brick-wall
modification of a Klein Gordon field in the Hartle-Hawking-Israel state on
1+2-Schwarzschild AdS (BTZ) has a well-defined boundary limit with the same
temperature and entropy as the brick-wall-modified bulk theory. One of our main
purposes is to point out a close connection, for general AdS/CFT situations,
between the puzzle raised by Arnsdorf and Smolin regarding the relationship
between Rehren's algebraic holography and mainstream AdS/CFT and the puzzle
embodied in the 'correspondence principle' proposed by Mukohyama and Israel in
their work on the brick-wall approach to black hole entropy. Working on the
assumption that similar results will hold for bulk QFT other than the Klein
Gordon field and for Schwarzschild AdS in other dimensions, and recalling the
first author's proposed resolution to the Mukohyama-Israel puzzle based on his
'matter-gravity entanglement hypothesis', we argue that, in Maldacena AdS/CFT,
the algebra of the boundary CFT is isomorphic only to a proper subalgebra of
the bulk algebra, albeit (at non-zero temperature) the (GNS) Hilbert spaces of
bulk and boundary theories are still the 'same' -- the total bulk state being
pure, while the boundary state is mixed (thermal). We also argue from the
finiteness of its boundary (and hence, on our assumptions, also bulk) entropy
at finite temperature, that the Rehren dual of the Maldacena boundary CFT
cannot itself be a QFT and must, instead, presumably be something like a string
theory.Comment: 54 pages, 3 figures. Arguments strengthened in the light of B.S. Kay
`Instability of Enclosed Horizons' arXiv:1310.739
Local Realism of Macroscopic Correlations
We show that for macroscopic measurements which cannot reveal full
information about microscopic states of the system, the monogamy of Bell
inequality violations present in quantum mechanics implies that practically all
correlations between macroscopic measurements can be described by local
realistic models. Our results hold for sharp measurement and arbitrary closed
quantum systems.Comment: 9 pages incl. one Appendix, 2 figure
The thermal and two-particle stress-energy must be ill-defined on the 2-d Misner space chronology horizon
We show that an analogue of the (four dimensional) image sum method can be
used to reproduce the results, due to Krasnikov, that for the model of a real
massless scalar field on the initial globally hyperbolic region IGH of
two-dimensional Misner space there exist two-particle and thermal Hadamard
states (built on the conformal vacuum) such that the (expectation value of the
renormalised) stress-energy tensor in these states vanishes on IGH. However, we
shall prove that the conclusions of a general theorem by Kay, Radzikowski and
Wald still apply for these states. That is, in any of these states, for any
point b on the Cauchy horizon and any neighbourhood N of b, there exists at
least one pair of non-null related points (x,x'), with x and x' in the
intersection of IGH with N, such that (a suitably differentiated form of) its
two-point function is singular. (We prove this by showing that the two-point
functions of these states share the same singularities as the conformal vacuum
on which they are built.) In other words, the stress-energy tensor in any of
these states is necessarily ill-defined on the Cauchy horizon.Comment: 6 pages, LaTeX, RevTeX, no figure
Distributional Modes for Scalar Field Quantization
We propose a mode-sum formalism for the quantization of the scalar field
based on distributional modes, which are naturally associated with a slight
modification of the standard plane-wave modes. We show that this formalism
leads to the standard Rindler temperature result, and that these modes can be
canonically defined on any Cauchy surface.Comment: 15 pages, RevTe
'She's like a daughter to me': insights into care, work and kinship from rural Russia
This article draws on ethnographic research into a state-funded homecare service in rural Russia. The article discusses intersections between care, work and kinship in the relationships between homecare workers and their elderly wards and explores the ways in which references to kinship, as a means of authenticating paid care and explaining its emotional content, reinforce public and private oppositions while doing little to relieve the tensions and conflicts of care work. The discussion brings together detailed empirical insights into local ideologies and practices as a way of generating new theoretical perspectives, which will be of relevance beyond the particular context of study
Targeting Mr Average: Participation, gender equity and school sport partnerships
The School Sport Partnership Programme (SSPP) is one strand of the national strategy for physical education and school sport in England, the physical education and school sport Club Links Strategy (PESSCL). The SSPP aims to make links between school physical education (PE) and out of school sports participation, and has a particular remit to raise the participation levels of several identified under-represented groups, of which girls and young women are one. National evaluations of the SSPP show that it is beginning to have positive impacts on young people's activity levels by increasing the range and provision of extra curricular activities (Office for Standards in Education (OFSTED), 2003, 2004, 2005; Loughborough Partnership, 2005, 2006). This paper contributes to the developing picture of the phased implementation of the programme by providing qualitative insights into the work of one school sport partnership with a particular focus on gender equity. The paper explores the ways in which gender equity issues have been explicitly addressed within the 'official texts' of the SSPP; how these have shifted over time and how teachers are responding to and making sense of these in their daily practice. Using participation observation, interview and questionnaire data, the paper explores how the coordinators are addressing the challenge of increasing the participation of girls and young women. The paper draws on Walby's (2000) conceptualisation of different kinds of feminist praxis to highlight the limitations of the coordinators' work. Two key themes from the data and their implications are addressed: the dominance of competitive sport practices and the PE professionals' views of targeting as a strategy for increasing the participation of under-represented groups. The paper concludes that coordinators work within an equality or difference discourse with little evidence of the transformative praxis needed for the programme to be truly inclusive. © 2008 Taylor & Francis
Spectropolarimetry of the Luminous Narrow-Line Seyfert Galaxies IRAS 20181-2244 and IRAS 13224-3809
We observed the narrow-line Seyfert 1 galaxies IRAS 20181-2244 and IRAS
13324-3809 with a new spectropolarimeter on the RC spectrograph at the CTIO 4m
telescope. Previously it had been suggested that IRAS 20181-2244 was a Type 2
QSO and thus might contain an obscured broad-line region which could be
detected by the presence of broad Balmer lines in the polarized flux. We found
the object to be polarized at about 2%, and constant with wavelength, (unlike
most narrow-line Seyfert 1s), but with no evidence of broad Balmer lines in
polarized flux. The spectropolarimetry indicates that the scattering material
is inside the BLR. IRAS 13224-3809, notable for its high variability in X-ray
and UV wavelengths, has a low polarization consistent with a Galactic
interstellar origin.Comment: 19 pages using (AASTEX) aaspp4.sty and 5 postscript figures To be
published in the Astrophysical Journa
Nature versus Nurture: The curved spine of the galaxy cluster X-ray luminosity -- temperature relation
The physical processes that define the spine of the galaxy cluster X-ray
luminosity -- temperature (L-T) relation are investigated using a large
hydrodynamical simulation of the Universe. This simulation models the same
volume and phases as the Millennium Simulation and has a linear extent of 500
h^{-1} Mpc. We demonstrate that mergers typically boost a cluster along but
also slightly below the L-T relation. Due to this boost we expect that all of
the very brightest clusters will be near the peak of a merger. Objects from
near the top of the L-T relation tend to have assembled much of their mass
earlier than an average halo of similar final mass. Conversely, objects from
the bottom of the relation are often experiencing an ongoing or recent merger.Comment: 8 pages, 7 figures, submitted to MNRA
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