700 research outputs found
On time and the quantum-to-classical transition in Jordan-Brans-Dicke quantum gravity
Any quantum theory of gravity which treats the gravitational constant as a
dynamical variable has to address the issue of superpositions of states
corresponding to different eigenvalues. We show how the unobservability of such
superpositions can be explained through the interaction with other
gravitational degrees of freedom (decoherence). The formal framework is
canonically quantized Jordan-Brans-Dicke theory. We discuss the concepts of
intrinsic time and semiclassical time as well as the possibility of tunneling
into regions corresponding to a negative gravitational constant. We calculate
the reduced density matrix of the Jordan-Brans-Dicke field and show that the
off-diagonal elements can be sufficiently suppressed to be consistent with
experiments. The possible relevance of this mechanism for structure formation
in extended inflation is briefly discussed.Comment: 10 pages, Latex, ZU-TH 15/93, BUTP-93/1
What is the best strategy for the prevention of transfusion-transmitted malaria in sub-Saharan African countries where malaria is endemic?
The transmission of malaria by blood transfusion was one of the first recorded incidents of transfusion-transmitted infections (TTIs). Although the World Health Organization (WHO) recommends that blood for transfusion should be screened for TTIs, malaria screening is not performed in most malaria-endemic countries in sub-Saharan Africa (SSA). The transfusion of infected red blood cells may lead to severe post-transfusion clinical manifestations of malaria, which could be rapidly fatal. Ensuring that blood supply in endemic countries is free from malaria is highly problematical, as most of the donors may potentially harbour low levels of malaria parasites. Pre-transfusion screening within endemic settings has been identified as a cost-effective option for prevention of transfusion-transmitted malaria (TTM). But currently, there is no screening method that is practical, affordable and suitably sensitive for use by blood banks in SSA. Even if this method was available, rejection of malaria-positive donors would considerably jeopardize the blood supply and increase morbidity and mortality, especially among pregnant women and children who top the scale of blood transfusion users in SSA. In this context, the systematic prophylaxis of recipients with anti-malarials could constitute a good alternative, as it prevents any deferral of donor units as well as the occurrence of TTM. With the on-going programme, namely the Affordable Medicine Facility - Malaria, there is an increase in the availability of low-priced artemisinin-based combination therapy that can be used for systematic prophylaxis. It appears nonetheless an urgent need to conduct cost-benefit studies in order to evaluate each of the TTM preventive methods. This approach could permit the design and implementation of an evidence-based measure of TTM prevention in SSA, advocating thereby its widespread use in the region
Kondo effect in Complex Quantum Dots in the presence of an oscillating and fluctuating gate signal
We show how the charge input signal applied to the gate electrode in a double
and triple quantum dot may be converted to a pulse in the Kondo cotunneling
current being a spin response of a nano-device under a strong Coulomb blockade.
The stochastic component of the input signal results in the infrared cutoff of
Kondo transmission. The stochastization of the orbital component of the Kondo
effect in triple quantum dots results in a noise-induced SU(4) - SU(2) quantum
transition.Comment: 16 pages, 12 figure
Interacting classical and quantum ensembles
A consistent description of interactions between classical and quantum
systems is relevant to quantum measurement theory, and to calculations in
quantum chemistry and quantum gravity. A solution is offered here to this
longstanding problem, based on a universally-applicable formalism for ensembles
on configuration space. This approach overcomes difficulties arising in
previous attempts, and in particular allows for backreaction on the classical
ensemble, conservation of probability and energy, and the correct classical
equations of motion in the limit of no interaction. Applications include
automatic decoherence for quantum ensembles interacting with classical
measurement apparatuses; a generalisation of coherent states to hybrid harmonic
oscillators; and an equation for describing the interaction of quantum matter
fields with classical gravity, that implies the radius of a Robertson-Walker
universe with a quantum massive scalar field can be sharply defined only for
particular `quantized' values.Comment: 31 pages, minor clarifications and one Ref. added, to appear in PR
On the Decoherence of Primordial Fluctuations During Inflation
We study the process whereby quantum cosmological perturbations become
classical within inflationary cosmology. By setting up a master-equation
formulation we show how quantum coherence for super-Hubble modes can be
destroyed by their coupling to the environment provided by sub-Hubble modes. We
identify what features the sub-Hubble environment must have in order to
decohere the longer wavelengths, and identify how the onset of decoherence (and
how long it takes) depends on the properties of the sub-Hubble physics which
forms the environment. Our results show that the decoherence process is largely
insensitive to the details of the coupling between the sub- and super-Hubble
scales. They also show how locality implies, quite generally, that the
decohered density matrix at late times is diagonal in the field representation
(as is implicitly assumed by extant calculations of inflationary density
perturbations). Our calculations also imply that decoherence can arise even for
couplings which are as weak as gravitational in strength.Comment: 31 pages, 1 figur
This elusive objective existence
Zurek's existential interpretation of quantum mechanics suffers from three
classical prejudices, including the belief that space and time are
intrinsically and infinitely differentiated. They compel him to relativize the
concept of objective existence in two ways. The elimination of these prejudices
makes it possible to recognize the quantum formalism's ontological implications
- the relative and contingent reality of spatiotemporal distinctions and the
extrinsic and finite spatiotemporal differentiation of the physical world -
which in turn makes it possible to arrive at an unqualified objective
existence. Contrary to a widespread misconception, viewing the quantum
formalism as being fundamentally a probability algorithm does not imply that
quantum mechanics is concerned with states of knowledge rather than states of
Nature. On the contrary, it makes possible a complete and strongly objective
description of the physical world that requires no reference to observers. What
objectively exists, in a sense that requires no qualification, is the
trajectories of macroscopic objects, whose fuzziness is empirically irrelevant,
the properties and values of whose possession these trajectories provide
indelible records, and the fuzzy and temporally undifferentiated states of
affairs that obtain between measurements and are described by counterfactual
probability assignments.Comment: To appear in IJQI; 21 pages, LaTe
Are There Quantum Effects Coming from Outside Space-time? Nonlocality, free will and "no many-worlds"
Observing the violation of Bell's inequality tells us something about all
possible future theories: they must all predict nonlocal correlations. Hence
Nature is nonlocal. After an elementary introduction to nonlocality and a brief
review of some recent experiments, I argue that Nature's nonlocality together
with the existence of free will is incompatible with the many-worlds view of
quantum physics.Comment: Talk presented at the meeting "Is Science Compatible with Our Desire
for Freedom?" organised by the Social Trends Institute at the IESE Business
School in Barcelona, Octobre 201
Robustness and diffusion of pointer states
Classical properties of an open quantum system emerge through its interaction
with other degrees of freedom (decoherence). We treat the case where this
interaction produces a Markovian master equation for the system. We derive the
corresponding distinguished local basis (pointer basis) by three methods. The
first demands that the pointer states mimic as close as possible the local
non-unitary evolution. The second demands that the local entropy production be
minimal. The third imposes robustness on the inherent quantum and emerging
classical uncertainties. All three methods lead to localized Gaussian pointer
states, their formation and diffusion being governed by well-defined quantum
Langevin equations.Comment: 5 pages, final versio
Signatures of Extragalactic Dust in pre-Swift GRB Afterglows
We present the results of a systematic analysis of gamma-ray burst afterglow
spectral energy distributions (SEDs) in the optical/near-infrared bands. Our
input list includes the entire world sample of afterglows observed in the
pre-Swift era by the end of 2004 that have sufficient publicly available data.
We apply various dust extinction models to fit the observed SEDs (Milky Way,
Large Magellanic Cloud and Small Magellanic Cloud) and derive the corresponding
intrinsic extinction in the GRB host galaxies and the intrinsic spectral slopes
of the afterglows. We then use these results to explore the parameter space of
the power-law index of the electron distribution function and to derive the
absolute magnitudes of the unextinguished afterglows.Comment: Submitted to ApJ 25 May 2005, accepted for publication 16 December
2005, updated 22 December 2005. 50 Pages, 12 Figures, 5 Tables. Figures 1a to
1af (30 subfigures) merged into three subfigures with downgraded resolution.
Figures 7 and 8 severly downsampled in resolutio
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