13,922 research outputs found
The academic and industrial embrace of space-time methods
[Guest Editors introduction to: Special issue on space-time transmission, reception, coding and signal processing]
Every episode of the classic 1966–1969 television series Star Trek begins with Captain Kirk’s (played by William Shatner) famous words : “Space: The final frontier….” While space may not be the final frontier for the information and communication theory community, it is proving to be an important and fruitful one.
In the information theory community, the notion of space can be broadly defined as the simultaneous use of multiple, possibly coupled, channels. The notions of space–time and multiple-input multiple-output (MIMO) channels are therefore often used interchangeably. The connection between space and MIMO is most transparent when we view the multiple channels as created by two or more spatially separated antennas at a wireless transmitter or receiver.
A large component of the current interest in space–time methods can be attributed to discoveries in the late 1980s and early 1990s that a rich wireless scattering environment can be beneficial when multiple antennas are used on a point-to-point link. We now know that adding antennas in a rich environment provides proportional increases in point-to-point data rates, without extra transmitted power or bandwidth
A quantum computational semantics for epistemic logical operators. Part I: epistemic structures
Some critical open problems of epistemic logics can be investigated in the framework
of a quantum computational approach. The basic idea is to interpret sentences like
“Alice knows that Bob does not understand that π is irrational” as pieces of quantum information
(generally represented by density operators of convenient Hilbert spaces). Logical
epistemic operators (to understand, to know. . .) are dealt with as (generally irreversible)
quantum operations, which are, in a sense, similar to measurement-procedures. This approach
permits us to model some characteristic epistemic processes, that concern both human
and artificial intelligence. For instance, the operation of “memorizing and retrieving
information” can be formally represented, in this framework, by using a quantum teleportation
phenomenon
A generalized Kramers-Kronig transform for Casimir effect computations
Recent advances in experimental techniques now permit to measure the Casimir
force with unprecedented precision. In order to achieve a comparable precision
in the theoretical prediction of the force, it is necessary to accurately
determine the electric permittivity of the materials constituting the plates
along the imaginary frequency axis. The latter quantity is not directly
accessible to experiments, but it can be determined via dispersion relations
from experimental optical data. In the experimentally important case of
conductors, however, a serious drawback of the standard dispersion relations
commonly used for this purpose, is their strong dependence on the chosen
low-frequency extrapolation of the experimental optical data, which introduces
a significant and not easily controllable uncertainty in the result. In this
paper we show that a simple modification of the standard dispersion relations,
involving suitable analytic window functions, resolves this difficulty, making
it possible to reliably determine the electric permittivity at imaginary
frequencies solely using experimental optical data in the frequency interval
where they are available, without any need of uncontrolled data extrapolations.Comment: 10 pages, 6 encapsulated figures. A few typos corrected, some
references added. The new version matches the one accepted for publication on
Phys. Rev.
Domain wall suppression in trapped mixtures of Bose-Einstein condensates
The ground state energy of a binary mixture of Bose-Einstein condensates can
be estimated for large atomic samples by making use of suitably regularized
Thomas-Fermi density profiles. By exploiting a variational method on the trial
densities the energy can be computed by explicitly taking into account the
normalization condition. This yields analytical results and provides the basis
for further improvement of the approximation. As a case study, we consider a
binary mixture of Rb atoms in two different hyperfine states in a double
well potential and discuss the energy crossing between density profiles with
different numbers of domain walls, as the number of particles and the
inter-species interaction vary.Comment: 9 page
Making precise predictions of the Casimir force between metallic plates via a weighted Kramers-Kronig transform
The possibility of making precise predictions for the Casimir force is
essential for the theoretical interpretation of current precision experiments
on the thermal Casimir effect with metallic plates, especially for sub-micron
separations. For this purpose it is necessary to estimate very accurately the
dielectric function of a conductor along the imaginary frequency axis. This
task is complicated in the case of ohmic conductors, because optical data do
not usually extend to sufficiently low frequencies to permit an accurate
evaluation of the standard Kramers-Kronig integral used to compute . By making important improvements in the results of a previous paper by
the author, it is shown that this difficulty can be resolved by considering
suitable weighted dispersions relations, which strongly suppress the
contribution of low frequencies. The weighted dispersion formulae presented in
this paper permit to estimate accurately the dielectric function of ohmic
conductors for imaginary frequencies, on the basis of optical data extending
from the IR to the UV, with no need of uncontrolled data extrapolations towards
zero frequency that are instead necessary with standard Kramers-Kronig
relations. Applications to several sets of data for gold films are presented to
demonstrate viability of the new dispersion formulae.Comment: 18 pages, 15 encapsulated figures. In the revised version important
improvements have been made, which affect the main conclusions of the pape
Sine-Gordon solitons, auxiliary fields, and singular limit of a double pendulums chain
We consider the continuum version of an elastic chain supporting topological
and non-topological degrees of freedom; this generalizes a model for the
dynamics of DNA recently proposed and investigated by ourselves. In a certain
limit, the non-topological degrees of freedom are frozen, and the model reduces
to the sine-Gordon equations and thus supports well-known topological soliton
solutions. We consider a (singular) perturbative expansion around this limit
and study in particular how the non-topological field assume the role of an
auxiliary field. This provides a more general framework for the slaving of this
degree of freedom on the topological one, already observed elsewhere in the
context of the mentioned DNA model; in this framework one expects such
phenomenon to arise in a quite large class of field-theoretical models.Comment: 18 pages, 2 figure
A theory of electromagnetic fluctuations for metallic surfaces and van der Waals interactions between metallic bodies
A new general expression is derived for the fluctuating electromagnetic field
outside a metal surface, in terms of its surface impedance. It provides a
generalization to real metals of Lifshitz theory of molecular interactions
between dielectric solids. The theory is used to compute the radiative heat
transfer between two parallel metal surfaces at different temperatures. It is
shown that a measurement of this quantity may provide an experimental
resolution of a long-standing controversy about the effect of thermal
corrections on the Casimir force between real metal plates.Comment: 4 pages, 2 figures; typos corrected, minor changes to match the
published version in Physical Review Letter
Planck-scale modifications to Electrodynamics characterized by a space-like symmetry-breaking vector
In the study of Planck-scale ("quantum-gravity induced") violations of
Lorentz symmetry, an important role was played by the deformed-electrodynamics
model introduced by Myers and Pospelov. Its reliance on conventional effective
quantum field theory, and its description of symmetry-violation effects simply
in terms of a four-vector with nonzero component only in the time-direction,
rendered it an ideal target for experimentalists and a natural concept-testing
ground for many theorists. At this point however the experimental limits on the
single Myers-Pospelov parameter, after improving steadily over these past few
years, are "super-Planckian", {\it i.e.} they take the model out of actual
interest from a conventional quantum-gravity perspective. In light of this we
here argue that it may be appropriate to move on to the next level of
complexity, still with vectorial symmetry violation but adopting a generic
four-vector. We also offer a preliminary characterization of the phenomenology
of this more general framework, sufficient to expose a rather significant
increase in complexity with respect to the original Myers-Pospelov setup. Most
of these novel features are linked to the presence of spatial anisotropy, which
is particularly pronounced when the symmetry-breaking vector is space-like, and
they are such that they reduce the bound-setting power of certain types of
observations in astrophysics
Entanglement Trapping in Structured Environments
The entanglement dynamics of two independent qubits each embedded in a
structured environment under conditions of inhibition of spontaneous emission
is analyzed, showing entanglement trapping. We demonstrate that entanglement
trapping can be used efficiently to prevent entanglement sudden death. For the
case of realistic photonic band-gap materials, we show that high values of
entanglement trapping can be achieved. This result is of both fundamental and
applicative interest since it provides a physical situation where the
entanglement can be preserved and manipulated, e.g. by Stark-shifting the qubit
transition frequency outside and inside the gap.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Lett. on Friday 16 May
200
Configurational entropy of hard spheres
We numerically calculate the configurational entropy S_conf of a binary
mixture of hard spheres, by using a perturbed Hamiltonian method trapping the
system inside a given state, which requires less assumptions than the previous
methods [R.J. Speedy, Mol. Phys. 95, 169 (1998)]. We find that S_conf is a
decreasing function of packing fraction f and extrapolates to zero at the
Kauzmann packing fraction f_K = 0.62, suggesting the possibility of an ideal
glass-transition for hard spheres system. Finally, the Adam-Gibbs relation is
found to hold.Comment: 10 pages, 6 figure
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