160 research outputs found
Anomalies of weakened decoherence criteria for quantum histories
The theory of decoherent histories is checked for the requirement of
statistical independence of subsystems. Strikingly, this is satisfied only when
the decoherence functional is diagonal in both its real a n d imaginary parts.
In particular, the condition of consistency (or weak decoherence) required for
the assignment of probabilities appears to be ruled out. The same conclusion is
obtained independently, by claiming a plausible dynamical robustness of
decoherent histories.Comment: 3pp, submitted to Phys. Rev. Let
Micrometre-scale refrigerators
A superconductor with a gap in the density of states or a quantum dot with
discrete energy levels is a central building block in realizing an electronic
on-chip cooler. They can work as energy filters, allowing only hot
quasiparticles to tunnel out from the electrode to be cooled. This principle
has been employed experimentally since the early 1990s in investigations and
demonstrations of micrometre-scale coolers at sub-kelvin temperatures. In this
paper, we review the basic experimental conditions in realizing the coolers and
the main practical issues that are known to limit their performance. We give an
update of experiments performed on cryogenic micrometre-scale coolers in the
past five years
Localization of Relative-Position of Two Atoms Induced by Spontaneous Emission
We revisit the back-action of emitted photons on the motion of the relative
position of two cold atoms. We show that photon recoil resulting from the
spontaneous emission can induce the localization of the relative position of
the two atoms through the entanglement between the spatial motion of individual
atoms and their emitted photons. The result provides a more realistic model for
the analysis of the environment-induced localization of a macroscopic object.Comment: 8 pages and 4 figure
Many worlds in one
A generic prediction of inflation is that the thermalized region we inhabit
is spatially infinite. Thus, it contains an infinite number of regions of the
same size as our observable universe, which we shall denote as \O-regions. We
argue that the number of possible histories which may take place inside of an
\O-region, from the time of recombination up to the present time, is finite.
Hence, there are an infinite number of \O-regions with identical histories up
to the present, but which need not be identical in the future. Moreover, all
histories which are not forbidden by conservation laws will occur in a finite
fraction of all \O-regions. The ensemble of \O-regions is reminiscent of
the ensemble of universes in the many-world picture of quantum mechanics. An
important difference, however, is that other \O-regions are unquestionably
real.Comment: 9 pages, 2 figures, comments and references adde
A physical distinction between a covariant and non covariant reduction process in relativistic quantum theories
Causality imposes strong restrictions on the type of operators that may be
observables in relativistic quantum theories. In fact, causal violations arise
when computing conditional probabilities for certain partial causally connected
measurements using the standard non covariant procedure. Here we introduce
another way of computing conditional probabilities, based on an intrinsic
covariant relational order of the events, which differs from the standard one
when these type of measurements are included. This alternative procedure is
compatible with a wider and very natural class of operators without breaking
causality. If some of these measurements could be implemented in practice as
predicted by our formalism, the non covariant, conventional approach should be
abandoned. Furthermore, the description we promote here would imply a new
physical effect where interference terms are suppressed as a consequence of the
covariant order in the measurement process.Comment: 7 pages, latex file, 1 ps figure. Major presentation changes. To
appear in New Journal of Physic
Entanglement of a Double Dot with a Quantum Point Contact
Entanglement between particle and detector is known to be inherent in the
measurement process. Gurvitz recently analyzed the coupling of an electron in a
double dot (DD) to a quantum point contact (QPC) detector. In this paper we
examine the dynamics of entanglement that result between the DD and QPC. The
rate of entanglement is optimized as a function of coupling when the electron
is initially in one of the dots. It decreases asymptotically towards zero with
increased coupling. The opposite behavior is observed when the DD is initially
in a superposition: the rate of entanglement increases unboundedly as the
coupling is increased. The possibility that there are conditions for which
measurement occurs versus entanglement is considered
Electron Wave Filters from Inverse Scattering Theory
Semiconductor heterostructures with prescribed energy dependence of the
transmittance can be designed by combining: {\em a)} Pad\'e approximant
reconstruction of the S-matrix; {\em b)} inverse scattering theory for
Schro\"dinger's equation; {\em c)} a unitary transformation which takes into
account the variable mass effects. The resultant continuous concentration
profile can be digitized into an easily realizable rectangular-wells structure.
For illustration, we give the specifications of a 2 narrow band-pass 12 layer
filter with the high energy peak more than {\em twice
narrower} than the other.Comment: 4 pages, Revtex with one eps figur
Extraction of scattering lengths from final-state interactions
A recently proposed method based on dispersion theory, that allows to extract
the scattering length of a hadronic two-body system from corresponding
final-state interactions, is generalized to the situation where the Coulomb
interaction is present. The steps required in a concrete practical application
are discussed in detail. In addition a thorough examination of the accuracy of
the proposed method is presented and a comparison is made with results achieved
with other methods like the Jost-function approach based on the effective-range
approximation. Deficiencies of the latter method are pointed out. The
reliability of the dispersion theory method for extracting also the effective
range is investigated.Comment: 16 pages, 6 figures, some corrections to text, to appear in Phys.
Rev.
Quantum origin of the primordial fluctuation spectrum and its statistics
The usual account for the origin of cosmic structure during inflation is not
fully satisfactory, as it lacks a physical mechanism capable of generating the
inhomogeneity and anisotropy of our Universe, from an exactly homogeneous and
isotropic initial state associated with the early inflationary regime. The
proposal in [A. Perez, H. Sahlmann, and D. Sudarsky, Classical Quantum Gravity,
23, 2317, (2006)] considers the spontaneous dynamical collapse of the wave
function, as a possible answer to that problem. In this work, we review briefly
the difficulties facing the standard approach, as well as the answers provided
by the above proposal and explore their relevance to the investigations
concerning the characterization of the primordial spectrum and other
statistical aspects of the cosmic microwave background and large-scale matter
distribution. We will see that the new approach leads to novel ways of
considering some of the relevant questions, and, in particular, to distinct
characterizations of the non-Gaussianities that might have left imprints on the
available data.Comment: 27 pages. Revision to match the published versio
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