846 research outputs found
Can non-private channels transmit quantum information?
We study the power of quantum channels with little or no capacity for private
communication. Because privacy is a necessary condition for quantum
communication, one might expect that such channels would be of little use for
transmitting quantum states. Nevertheless, we find strong evidence that there
are pairs of such channels that, when used together, can transmit far more
quantum information than the sum of their individual private capacities.
Because quantum transmissions are necessarily private, this would imply a large
violation of additivity for the private capacity. Specifically, we present
channels which display either (1) A large joint quantum capacity but very small
individual private capacities or (2) a severe violation of additivity for the
Holevo information.Comment: We both think so. 4 pages and 3 figures explain wh
Finite states in four dimensional quantum gravity. The isotropic minisuperspace Asktekar--Klein--Gordon model
In this paper we construct the generalized Kodama state for the case of a
Klein--Gordon scalar field coupled to Ashtekar variables in isotropic
minisuperspace by a new method. The criterion for finiteness of the state stems
from a minisuperspace reduction of the quantized full theory, rather than the
conventional techniques of reduction prior to quantization. We then provide a
possible route to the reproduction of a semiclassical limit via these states.
This is the result of a new principle of the semiclassical-quantum
correspondence (SQC), introduced in the first paper in this series. Lastly, we
examine the solution to the minisuperspace case at the semiclassical level for
an isotropic CDJ matrix neglecting any quantum corrections and examine some of
the implications in relation to results from previous authors on semiclassical
orbits of spacetime, including inflation. It is suggested that the application
of nonperturbative quantum gravity, by way of the SQC, might potentially lead
to some predictions testable below the Planck scale.Comment: 26 pages. Accepted for publication by Class. Quantum Grav. journa
The linearization of the Kodama state
We study the question of whether the linearization of the Kodama state around
classical deSitter spacetime is normalizable in the inner product of the theory
of linearized gravitons on deSitter spacetime. We find the answer is no in the
Lorentzian theory. However, in the Euclidean theory the corresponding
linearized Kodama state is delta-functional normalizable. We discuss whether
this result invalidates the conjecture that the full Kodama state is a good
physical state for quantum gravity with positive cosmological constant.Comment: 14 pages, statement on the corresponding Yang-Mills case correcte
Link Invariants of Finite Type and Perturbation Theory
The Vassiliev-Gusarov link invariants of finite type are known to be closely
related to perturbation theory for Chern-Simons theory. In order to clarify the
perturbative nature of such link invariants, we introduce an algebra V_infinity
containing elements g_i satisfying the usual braid group relations and elements
a_i satisfying g_i - g_i^{-1} = epsilon a_i, where epsilon is a formal variable
that may be regarded as measuring the failure of g_i^2 to equal 1.
Topologically, the elements a_i signify crossings. We show that a large class
of link invariants of finite type are in one-to-one correspondence with
homogeneous Markov traces on V_infinity. We sketch a possible application of
link invariants of finite type to a manifestly diffeomorphism-invariant
perturbation theory for quantum gravity in the loop representation.Comment: 11 page
Reconstructing Quantum Geometry from Quantum Information: Spin Networks as Harmonic Oscillators
Loop Quantum Gravity defines the quantum states of space geometry as spin
networks and describes their evolution in time. We reformulate spin networks in
terms of harmonic oscillators and show how the holographic degrees of freedom
of the theory are described as matrix models. This allow us to make a link with
non-commutative geometry and to look at the issue of the semi-classical limit
of LQG from a new perspective. This work is thought as part of a bigger project
of describing quantum geometry in quantum information terms.Comment: 16 pages, revtex, 3 figure
Development of a formalism of movable cellular automaton method for numerical modeling of fracture of heterogeneous elastic-plastic materials
A general approach to realization of models of elasticity, plasticity and fracture of heterogeneousmaterials within the framework of particle-based numerical methods is proposed in the paper. It is based onbuilding many-body forces of particle interaction, which provide response of particle ensemble correctlyconforming to the response (including elastic-plastic behavior and fracture) of simulated solids. Implementationof proposed approach within particle-based methods is demonstrated by the example of the movable cellularautomaton (MCA) method, which integrates the possibilities of particle-based discrete element method (DEM)and cellular automaton methods. Emergent advantages of the developed approach to formulation of manybodyinteraction are discussed. Main of them are its applicability to various realizations of the concept ofdiscrete elements and a possibility to realize various rheological models (including elastic-plastic or visco-elasticplastic)and models of fracture to study deformation and fracture of solid-phase materials and media.Capabilities of particle-based modeling of heterogeneous solids are demonstrated by the problem of simulationof deformation and fracture of particle-reinforced metal-ceramic composites
A perspective on the landscape problem
I discuss the historical roots of the landscape problem and propose criteria
for its successful resolution. This provides a perspective to evaluate the
possibility to solve it in several of the speculative cosmological scenarios
under study including eternal inflation, cosmological natural selection and
cyclic cosmologies.Comment: Invited contribution for a special issue of Foundations of Physics
titled: Forty Years Of String Theory: Reflecting On the Foundations. 31
pages, no figure
Quantum symmetry, the cosmological constant and Planck scale phenomenology
We present a simple algebraic argument for the conclusion that the low energy
limit of a quantum theory of gravity must be a theory invariant, not under the
Poincare group, but under a deformation of it parameterized by a dimensional
parameter proportional to the Planck mass. Such deformations, called
kappa-Poincare algebras, imply modified energy-momentum relations of a type
that may be observable in near future experiments. Our argument applies in both
2+1 and 3+1 dimensions and assumes only 1) that the low energy limit of a
quantum theory of gravity must involve also a limit in which the cosmological
constant is taken very small with respect to the Planck scale and 2) that in
3+1 dimensions the physical energy and momenta of physical elementary particles
is related to symmetries of the full quantum gravity theory by appropriate
renormalization depending on Lambda l^2_{Planck}. The argument makes use of the
fact that the cosmological constant results in the symmetry algebra of quantum
gravity being quantum deformed, as a consequence when the limit \Lambda
l^2_{Planck} -> 0 is taken one finds a deformed Poincare invariance. We are
also able to isolate what information must be provided by the quantum theory in
order to determine which presentation of the kappa-Poincare algebra is relevant
for the physical symmetry generators and, hence, the exact form of the modified
energy-momentum relations. These arguments imply that Lorentz invariance is
modified as in proposals for doubly special relativity, rather than broken, in
theories of quantum gravity, so long as those theories behave smoothly in the
limit the cosmological constant is taken to be small.Comment: LaTex, 19 page
Constraints on the quantum gravity scale from kappa - Minkowski spacetime
We compare two versions of deformed dispersion relations (energy vs momenta
and momenta vs energy) and the corresponding time delay up to the second order
accuracy in the quantum gravity scale (deformation parameter). A general
framework describing modified dispersion relations and time delay with respect
to different noncommutative kappa -Minkowski spacetime realizations is firstly
proposed here and it covers all the cases introduced in the literature. It is
shown that some of the realizations provide certain bounds on quadratic
corrections, i.e. on quantum gravity scale, but it is not excluded in our
framework that quantum gravity scale is the Planck scale. We also show how the
coefficients in the dispersion relations can be obtained through a
multiparameter fit of the gamma ray burst (GRB) data.Comment: 9 pages, final published version, revised abstract, introduction and
conclusion, to make it clear to general reade
Covariant quantization of membrane dynamics
A Lorentz covariant quantization of membrane dynamics is defined, which also
leaves unbroken the full three dimensional diffeomorphism invariance of the
membrane. Among the applications studied are the reduction to string theory,
which may be understood in terms of the phase space and constraints, and the
interpretation of physical,zero-energy states. A matrix regularization is
defined as in the light cone gauged fixed theory but there are difficulties
implementing all the gauge symmetries. The problem involves the
non-area-preserving diffeomorphisms which are realized non-linearly in the
classical theory. In the quantum theory they do not seem to have a consistent
implementation for finite N. Finally, an approach to a genuinely background
independent formulation of matrix dynamics is briefly described.Comment: Latex, 21 pages, no figure
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