41,376 research outputs found
The Unruh Quantum Otto Engine
We introduce a quantum heat engine performing an Otto cycle by using the
thermal properties of the quantum vacuum. Since Hawking and Unruh, it has been
established that the vacuum space, either near a black hole or for an
accelerated observer, behaves as a bath of thermal radiation. In this work, we
present a fully quantum Otto cycle, which relies on the Unruh effect for a
single quantum bit (qubit) in contact with quantum vacuum fluctuations. By
using the notions of quantum thermodynamics and perturbation theory we obtain
that the quantum vacuum can exchange heat and produce work on the qubit.
Moreover, we obtain the efficiency and derive the conditions to have both a
thermodynamic and a kinematic cycle in terms of the initial populations of the
excited state, which define a range of allowed accelerations for the Unruh
engine.Comment: 31 pages, 11 figure
Nonviolation of Bell's Inequality in Translation Invariant Systems
The nature of quantum correlations in strongly correlated systems has been a
subject of intense research. In particular, it has been realized that
entanglement and quantum discord are present at quantum phase transitions and
able to characterize it. Surprisingly, it has been shown for a number of
different systems that qubit pairwise states, even when highly entangled, do
not violate Bell's inequalities, being in this sense local. Here we show that
such a local character of quantum correlations is in fact general for
translation invariant systems and has its origins in the monogamy trade-off
obeyed by tripartite Bell correlations. We illustrate this result in a quantum
spin chain with a soft breaking of translation symmetry. In addition, we extend
the monogamy inequality to the -qubit scenario, showing that the bound
increases with and providing examples of its saturation through uniformly
generated random pure states.Comment: Published erratum added at the en
Collapse of Primordial Clouds
We present here studies of collapse of purely baryonic Population III objects
with masses ranging from to . A spherical Lagrangian
hydrodynamic code has been written to study the formation and evolution of the
primordial clouds, from the beginning of the recombination era () until the redshift when the collapse occurs. All the relevant processes
are included in the calculations, as well as, the expansion of the Universe. As
initial condition we take different values for the Hubble constant and for the
baryonic density parameter (considering however a purely baryonic Universe), as
well as different density perturbation spectra, in order to see their influence
on the behavior of the Population III objects evolution. We find, for example,
that the first mass that collapses is for ,
and with the mass scale . For
we obtain for the first
mass that collapses. The cooling-heating and photon drag processes have a key
role in the collapse of the clouds and in their thermal history. Our results
show, for example, that when we disregard the Compton cooling-heating, the
collapse of the objects with masses occurs earlier. On
the other hand, disregarding the photon drag process, the collapse occurs at a
higher redshift.Comment: 10 pages, MN plain TeX macros v1.6 file, 9 PS figures. Also available
at http://www.iagusp.usp.br/~oswaldo (click "OPTIONS" and then "ARTICLES").
MNRAS in pres
Fourier Eigenfunctions, Uncertainty Gabor Principle and Isoresolution Wavelets
Shape-invariant signals under Fourier transform are investigated leading to a
class of eigenfunctions for the Fourier operator. The classical uncertainty
Gabor-Heisenberg principle is revisited and the concept of isoresolution in
joint time-frequency analysis is introduced. It is shown that any Fourier
eigenfunction achieve isoresolution. It is shown that an isoresolution wavelet
can be derived from each known wavelet family by a suitable scaling.Comment: 6 pages, XX Simp\'osio Bras. de Telecomunica\c{c}\~oes, Rio de
Janeiro, Brazil, 2003. Fixed typo
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