206 research outputs found
S-Matrix for AdS from General Boundary QFT
The General Boundary Formulation (GBF) is a new framework for studying
quantum theories. After concise overviews of the GBF and Schr\"odinger-Feynman
quantization we apply the GBF to resolve a well known problem on Anti-deSitter
spacetime where due to the lack of temporally asymptotic free states the usual
S-matrix cannot be defined. We construct a different type of S-matrix plus
propagators for free and interacting real Klein-Gordon theory.Comment: 4 pages, 5 figures, Proceedings of LOOPS'11 Madrid, to appear in IOP
Journal of Physics: Conference Series (JPCS
The Unruh-deWitt Detector and the Vacuum in the General Boundary formalism
We discuss how to formulate a condition for choosing the vacuum state of a
quantum scalar field on a timelike hyperplane in the general boundary
formulation (GBF) using the coupling to an Unruh-DeWitt detector. We explicitly
study the response of an Unruh-DeWitt detector for evanescent modes which occur
naturally in quantum field theory in the presence of the equivalent of a
dielectric boundary. We find that the physically correct vacuum state has to
depend on the physical situation outside of the boundaries of the spacetime
region considered. Thus it cannot be determined by general principles
pertaining only to a subset of spacetime.Comment: Version as published in CQ
Towards the graviton from spinfoams: the 3d toy model
Recently, a proposal has appeared for the extraction of the 2-point function
of linearised quantum gravity, within the spinfoam formalism. This relies on
the use of a boundary state, which introduces a semi-classical flat geometry on
the boundary. In this paper, we investigate this proposal considering a toy
model in the (Riemannian) 3d case, where the semi-classical limit is better
understood. We show that in this limit the propagation kernel of the model is
the one for the harmonic oscillator. This is at the origin of the expected 1/L
behaviour of the 2-point function. Furthermore, we numerically study the short
scales regime, where deviations from this behaviour occur.Comment: 8 pages, 2 figures; v3 revised versio
A simple background-independent hamiltonian quantum model
We study formulation and probabilistic interpretation of a simple
general-relativistic hamiltonian quantum system. The system has no unitary
evolution in background time. The quantum theory yields transition
probabilities between measurable quantities (partial observables). These
converge to the classical predictions in the limit. Our main tool
is the kernel of the projector on the solutions of Wheeler-deWitt equation,
which we analyze in detail. It is a real quantity, which can be seen as a
propagator that propagates "forward" as well as "backward" in a local parameter
time. Individual quantum states, on the other hand, may contain only "forward
propagating" components. The analysis sheds some light on the interpretation of
background independent transition amplitudes in quantum gravity
Quantum Entanglement of Electromagnetic Fields in Non-inertial Reference Frames
Recently relativistic quantum information has received considerable attention
due to its theoretical importance and practical application. Especially,
quantum entanglement in non-inertial reference frames has been studied for
scalar and Dirac fields. As a further step along this line, we here shall
investigate quantum entanglement of electromagnetic fields in non-inertial
reference frames. In particular, the entanglement of photon helicity entangled
state is extensively analyzed. Interestingly, the resultant logarithmic
negativity and mutual information remain the same as those for inertial
reference frames, which is completely different from that previously obtained
for the particle number entangled state.Comment: more explanatory material added in the introduction, version to
appear in Journal of Physics
Least squares optimization: From theory to practice
Nowadays, Nonlinear Least-Squares embodies the foundation of many Robotics and Computer Vision systems. The research community deeply investigated this topic in the last few years, and this resulted in the development of several open-source solvers to approach constantly increasing classes of problems. In this work, we propose a unified methodology to design and develop efficient Least-Squares Optimization algorithms, focusing on the structures and patterns of each specific domain. Furthermore, we present a novel open-source optimization system that addresses problems transparently with a different structure and designed to be easy to extend. The system is written in modern C++ and runs efficiently on embedded systemsWe validated our approach by conducting comparative experiments on several problems using standard datasets. The results show that our system achieves state-of-the-art performances in all tested scenarios
Spatially asymptotic S-matrix from general boundary formulation
We construct a new type of S-matrix in quantum field theory using the general
boundary formulation. In contrast to the usual S-matrix the space of free
asymptotic states is located at spatial rather than at temporal infinity.
Hence, the new S-matrix applies to situations where interactions may remain
important at all times, but become negligible with distance. We show that the
new S-matrix is equivalent to the usual one in situations where both apply.
This equivalence is mediated by an isomorphism between the respective
asymptotic state spaces that we construct. We introduce coherent states that
allow us to obtain explicit expressions for the new S-matrix. In our formalism
crossing symmetry becomes a manifest rather than a derived feature of the
S-matrix.Comment: 27 pages, LaTeX + revtex4; v2: various corrections, references
update
Background independence in a nutshell
We study how physical information can be extracted from a background
independent quantum system. We use an extremely simple `minimalist' system that
models a finite region of 3d euclidean quantum spacetime with a single
equilateral tetrahedron. We show that the physical information can be expressed
as a boundary amplitude. We illustrate how the notions of "evolution" in a
boundary proper-time and "vacuum" can be extracted from the background
independent dynamics.Comment: 19 pages, 19 figure
Graviton propagator in loop quantum gravity
We compute some components of the graviton propagator in loop quantum
gravity, using the spinfoam formalism, up to some second order terms in the
expansion parameter.Comment: 41 pages, 6 figure
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