6,892 research outputs found
Algebraic Quantum Theory on Manifolds: A Haag-Kastler Setting for Quantum Geometry
Motivated by the invariance of current representations of quantum gravity
under diffeomorphisms much more general than isometries, the Haag-Kastler
setting is extended to manifolds without metric background structure. First,
the causal structure on a differentiable manifold M of arbitrary dimension
(d+1>2) can be defined in purely topological terms, via cones (C-causality).
Then, the general structure of a net of C*-algebras on a manifold M and its
causal properties required for an algebraic quantum field theory can be
described as an extension of the Haag-Kastler axiomatic framework.
An important application is given with quantum geometry on a spatial slice
within the causally exterior region of a topological horizon H, resulting in a
net of Weyl algebras for states with an infinite number of intersection points
of edges and transversal (d-1)-faces within any neighbourhood of the spatial
boundary S^2.Comment: 15 pages, Latex; v2: several corrections, in particular in def. 1 and
in sec.
Nonlinear magnetic field dependence of the conductance in d-wave NIS tunnel junctions
The ab-plane NIS-tunnelling conductance in d-wave superconductors shows a
zero-bias conductance peak which is predicted to split in a magnetic field. In
a pure d-wave superconductor the splitting is linear for fields small on the
scale of the thermodynamic critical field. The field dependence is shown to be
nonlinear, even at low fields, in the vicinity of a surface phase transition
into a local time-reversal symmetry breaking state. The field evolution of the
conductance is sensitive to temperature, doping, and the symmetry of the
sub-dominant pairing channel.Comment: 4 pages, 4 figure
Range imager performance comparison in homodyne and heterodyne operating modes
Range imaging cameras measure depth simultaneously for every pixel in a given field of view. In most implementations the basic operating principles are the same. A scene is illuminated with an intensity modulated light source and the reflected signal is sampled using a gain-modulated imager. Previously we presented a unique heterodyne range imaging system that employed a bulky and power hungry image intensifier as the high speed gain-modulation mechanism. In this paper we present a new range imager using an internally modulated image sensor that is designed to operate in heterodyne mode, but can also operate in homodyne mode. We discuss homodyne and heterodyne range imaging, and the merits of the various types of hardware used to implement these systems. Following this we describe in detail the hardware and firmware components of our new ranger. We experimentally compare the two operating modes and demonstrate that heterodyne operation is less sensitive to some of the limitations suffered in homodyne mode, resulting in better linearity and ranging precision characteristics. We conclude by showing various qualitative examples that demonstrate the system’s three-dimensional measurement performance
Shape and deformation measurement using heterodyne range imaging technology
Range imaging is emerging as a promising alternative technology for applications that require non-contact visual inspection of object deformation and shape. Previously, we presented a solid-state full-field heterodyne range imaging device capable of capturing three-dimensional images with sub-millimetre range resolution. Using a heterodyne indirect time-of-flight configuration, this system simultaneously measures distance (and intensity), for each pixel in a cameras field of view. In this paper we briefly describe our range imaging system, and its principle of operation. By performing measurements on several metal objects, we demonstrate the potential capabilities of this technology for surface profiling and deformation measurement. In addition to verifying system performance, the reported examples highlight some important system limitations. With these in mind we subsequently discuss the further developments required to enable the use of this device as a robust and practical tool in non-destructive testing and measurement applications
Experimental bound entanglement in a four-photon state
Entanglement [1, 2] enables powerful new quantum technologies [3-8], but in
real-world implementations, entangled states are often subject to decoherence
and preparation errors. Entanglement distillation [9, 10] can often counteract
these effects by converting imperfectly entangled states into a smaller number
of maximally entangled states. States that are entangled but cannot be
distilled are called bound entangled [11]. Bound entanglement is central to
many exciting theoretical results in quantum information processing [12-14],
but has thus far not been experimentally realized. A recent claim for
experimental bound entanglement is not supported by their data [15]. Here, we
consider a family of four-qubit Smolin states [16], focusing on a regime where
the bound entanglement is experimentally robust. We encode the state into the
polarization of four photons and show that our state exhibits both entanglement
and undistillability, the two defining properties of bound entanglement. We
then use our state to implement entanglement unlocking, a key feature of Smolin
states [16].Comment: 10 pages, 6 figures. For a simultaneously submitted related work see
arXiv:1005.196
Einstein and Brans-Dicke frames in multidimensional cosmology
Inhomogeneous multidimensional cosmological models with a higher dimensional
space-time manifold M= M_0 x M_1 ...x M_n are investigated under dimensional
reduction to a D_0-dimensional effective non-minimally coupled sigma-model
which generalizes the familiar Brans-Dicke model.
It is argued that the Einstein frame should be considered as the physical
one. The general prescription for the Einstein frame reformulation of known
solutions in the Brans-Dicke frame is given. As an example, the reformulation
is demonstrated explicitly for the generalized Kasner solutions where it is
shown that in the Einstein frame there are no solutions with inflation of the
external space.Comment: 27 pages, Revte
A power-saving modulation technique for time-of-flight range imaging sensors
Time-of-flight range imaging cameras measure distance and intensity simultaneously for every pixel in an image. With the continued advancement of the technology, a wide variety of new depth sensing applications are emerging; however a number of these potential applications have stringent electrical power constraints that are difficult to meet with the current state-of-the-art systems. Sensor gain modulation contributes a significant proportion of the total image sensor power consumption, and as higher spatial resolution range image sensors operating at higher modulation frequencies (to achieve better measurement precision) are developed, this proportion is likely to increase. The authors have developed a new sensor modulation technique using resonant circuit concepts that is more power efficient than the standard mode of operation. With a proof of principle system, a 93–96% reduction in modulation drive power was demonstrated across a range of modulation frequencies from 1–11 MHz. Finally, an evaluation of the range imaging performance revealed an improvement in measurement linearity in the resonant configuration due primarily to the more sinusoidal shape of the resonant electrical waveforms, while the average precision values were comparable between the standard and resonant operating modes
Integrable Multicomponent Perfect Fluid Multidimensional Cosmology II: Scalar Fields
We consider anisotropic cosmological models with an universe of dimension 4
or more, factorized into n>1 Ricci-flat spaces, containing an m-component
perfect fluid of m non-interacting homogeneous minimally coupled scalar fields
under special conditions. We describe the dynamics of the universe: It has a
Kasner-like behaviour near the singularity and isotropizes during the expansion
to infinity.
Some of the considered models are integrable, and classical as well as
quantum solutions are found. Some solutions produce inflation from "nothing".
There exist classical asymptotically anti-de Sitter wormholes, and quantum
wormholes with discrete spectrum.Comment: 28 pages, LaTeX, subm. to Gen. Rel. Gra
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