4,168 research outputs found
Higher Gauge Theory and Gravity in (2+1) Dimensions
Non-abelian higher gauge theory has recently emerged as a generalization of
standard gauge theory to higher dimensional (2-dimensional in the present
context) connection forms, and as such, it has been successfully applied to the
non-abelian generalizations of the Yang-Mills theory and 2-form
electrodynamics. (2+1)-dimensional gravity, on the other hand, has been a
fertile testing ground for many concepts related to classical and quantum
gravity, and it is therefore only natural to investigate whether we can find an
application of higher gauge theory in this latter context. In the present paper
we investigate the possibility of applying the formalism of higher gauge theory
to gravity in (2+1) dimensions, and we show that a nontrivial model of
(2+1)-dimensional gravity coupled to scalar and tensorial matter fields - the
model - can be formulated both as a standard gauge theory and
as a higher gauge theory. Since the model has a very rich structure - it admits
as solutions black-hole BTZ-like geometries, particle-like geometries as well
as Robertson-Friedman-Walker cosmological-like expanding geometries - this
opens a wide perspective for higher gauge theory to be tested and understood in
a relevant gravitational context. Additionally, it offers the possibility of
studying gravity in (2+1) dimensions coupled to matter in an entirely new
framework.Comment: 22 page
Anisotropic Diffusion Limited Aggregation
Using stochastic conformal mappings we study the effects of anisotropic
perturbations on diffusion limited aggregation (DLA) in two dimensions. The
harmonic measure of the growth probability for DLA can be conformally mapped
onto a constant measure on a unit circle. Here we map preferred directions
for growth of angular width to a distribution on the unit circle which
is a periodic function with peaks in such that the width
of each peak scales as , where defines the
``strength'' of anisotropy along any of the chosen directions. The two
parameters map out a parameter space of perturbations that allows a
continuous transition from DLA (for or ) to needle-like fingers
as . We show that at fixed the effective fractal dimension of
the clusters obtained from mass-radius scaling decreases with
increasing from to a value bounded from below by
. Scaling arguments suggest a specific form for the dependence
of the fractal dimension on for large , form which compares
favorably with numerical results.Comment: 6 pages, 4 figures, submitted to Phys. Rev.
Recovery of entanglement lost in entanglement manipulation
When an entangled state is transformed into another one with probability one
by local operations and classical communication, the quantity of entanglement
decreases. This letter shows that entanglement lost in the manipulation can be
partially recovered by an auxiliary entangled pair. As an application, a
maximally entangled pair can be obtained from two partially entangled pairs
with probability one. Finally, this recovery scheme reveals a fundamental
property of entanglement relevant to the existence of incomparable states.Comment: 4 pages, 2 figures, REVTeX; minor correction
Giant Spin Seebeck Effect through an Interface Organic Semiconductor
Interfacing an organic semiconductor C60 with a non-magnetic metallic thin
film (Cu or Pt) has created a novel heterostructure that is ferromagnetic at
ambient temperature, while its interface with a magnetic metal (Fe or Co) can
tune the anisotropic magnetic surface property of the material. Here, we
demonstrate that sandwiching C60 in between a magnetic insulator (Y3Fe5O12:
YIG) and a non-magnetic, strong spin-orbit metal (Pt) promotes highly efficient
spin current transport via the thermally driven spin Seebeck effect (SSE).
Experiments and first principles calculations consistently show that the
presence of C60 reduces significantly the conductivity mismatch between YIG and
Pt and the surface perpendicular magnetic anisotropy of YIG, giving rise to
enhanced spin mixing conductance across YIG/C60/Pt interfaces. As a result, a
600% increase in the SSE voltage (VLSSE) has been realized in YIG/C60/Pt
relative to YIG/Pt. Temperature-dependent SSE voltage measurements on
YIG/C60/Pt with varying C60 layer thicknesses also show an exponential increase
in VLSSE at low temperatures below 200 K, resembling the temperature evolution
of spin diffusion length of C60. Our study emphasizes the important roles of
the magnetic anisotropy and the spin diffusion length of the intermediate layer
in the SSE in YIG/C60/Pt structures, providing a new pathway for developing
novel spin-caloric materials
Quantum entanglement can be simulated without communication
It has recently been shown that all causal correlations between two parties
which output each one bit, a and b, when receiving each one bit, x and y, can
be expressed as convex combinations of local correlations (i.e., correlations
that can be simulated with local random variables) and non-local correlations
of the form a+b=xy mod 2. We show that a single instance of the latter
elementary non-local correlation suffices to simulate exactly all possible
projective measurements that can be performed on the singlet state of two
qubits, with no communication needed at all. This elementary non-local
correlation thus defines some unit of non-locality, which we call a nl-bit.Comment: 4 pages RevTex, 3 eps figure
Generalized quantum measurements and local realism
The structure of a local hidden variable model for experiments involving
sequences of measurements rigorously is analyzed. Constraints imposed by local
realism on the conditional probabilities of the outcomes of such measurement
schemes are explicitly derived. The violation of local realism in the case of
``hidden nonlocality'' is illustrated by an operational example.Comment: Revtex, 12 pages; Some modifications of introduction has been made; a
note stating that part of results had been obtained earlier by other authors,
has been added; one postscript figure available at request from
[email protected]
On the volume of the set of mixed entangled states
A natural measure in the space of density matrices describing N-dimensional
quantum systems is proposed. We study the probability P that a quantum state
chosen randomly with respect to the natural measure is not entangled (is
separable). We find analytical lower and upper bounds for this quantity.
Numerical calculations give P = 0.632 for N=4 and P=0.384 for N=6, and indicate
that P decreases exponentially with N. Analysis of a conditional measure of
separability under the condition of fixed purity shows a clear dualism between
purity and separability: entanglement is typical for pure states, while
separability is connected with quantum mixtures. In particular, states of
sufficiently low purity are necessarily separable.Comment: 10 pages in LaTex - RevTex + 4 figures in eps. submitted to Phys.
Rev.
Processing and Linking Audio Events in Large Multimedia Archives: The EU inEvent Project
In the inEvent EU project [1], we aim at structuring, retrieving, and sharing large archives of networked, and dynamically changing, multimedia recordings, mainly consisting of meetings, videoconferences, and lectures. More specifically, we are developing an integrated system that performs audiovisual processing of multimedia recordings, and labels them in terms of interconnected âhyper-events â (a notion inspired from hyper-texts). Each hyper-event is composed of simpler facets, including audio-video recordings and metadata, which are then easier to search, retrieve and share. In the present paper, we mainly cover the audio processing aspects of the system, including speech recognition, speaker diarization and linking (across recordings), the use of these features for hyper-event indexing and recommendation, and the search portal. We present initial results for feature extraction from lecture recordings using the TED talks. Index Terms: Networked multimedia events; audio processing: speech recognition; speaker diarization and linking; multimedia indexing and searching; hyper-events. 1
Are the laws of entanglement theory thermodynamical?
We argue that on its face, entanglement theory satisfies laws equivalent to
thermodynamics if the theory can be made reversible by adding certain bound
entangled states as a free resource during entanglement manipulation. Subject
to plausible assumptions, we prove that this is not the case in general, and
discuss the implications of this for the thermodynamics of entanglement.Comment: 4 pages, 1 figure, Revtex4; to appear in Phys. Rev. Let
Mixed-state entanglement and distillation: is there a ``bound'' entanglement in nature?
It is shown that if a mixed state can be distilled to the singlet form, it
must violate partial transposition criterion [A. Peres, Phys. Rev. Lett. 76,
1413 (1996)]. It implies that there are two qualitatively different types of
entanglement: ``free'' entanglement which is distillable, and ``bound''
entanglement which cannot be brought to the singlet form useful for quantum
communication purposes. Possible physical meaning of the result is discussed.Comment: RevTeX, 4 page
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