327 research outputs found
Phase-Space Decoherence: a comparison between Consistent Histories and Environment Induced Superselection
We examine the decoherence properties of a quantum open system as modeled by
a quantum optical system in the Markov regime. We look for decoherence in both
the Environment Induced Superselection (EIS) and Consistent Histories (CH)
frameworks. We propose a general measure of the coherence of the reduced
density matrix and find that EIS decoherence occurs in a number of bases for
this model. The degree of ``diagonality'' achieved increases with bath
temperature. We evaluate the Decoherence Functional of Consistent Histories for
coarse grained phase space two-time projected histories. Using the measures
proposed by Dowker and Halliwell we find that the consistency of the histories
improves with increasing bath temperature, time and final grain size and
decreases with initial grain size. The peaking increases with increasing grain
size and decreases with increasing bath temperature. Adopting the above
proposed measure of ``coherence'' to the Decoherence Functional gives similar
results. The results agree in general with expectations while the anomalous
dependence of the consistency on the initial grain size is discussed.Comment: 27 pages, 5 postscript figs in uuencoded compressed tar format
Replaced: definition of special character for the complex number
Exactly soluble models of decoherence
Superselection rules induced by the interaction with the environment are a
basis to understand the emergence of classical observables within quantum
theory. The aim of this article is to investigate the decoherence effects,
which lead to superselection sectors, with the help of exactly soluble
Hamiltonian models. Starting from the examples of Araki and of Zurek more
general models with scattering are presented for which the projection operators
onto the induced superselection sectors do no longer commute with the
Hamiltonian. The example of an environment given by a free quantum field
indicates that infrared divergence plays an essential role for the emergence of
induced superselection sectors. For all models the induced superselection
sectors are uniquely determined by the Hamiltonian, whereas the time scale of
the decoherence depends crucially on the initial state of the total system.Comment: 12 pages, Late
A model-theoretic interpretation of environmentally-induced superselection
Environmentally-induced superselection or "einselection" has been proposed as
an observer-independent mechanism by which apparently classical systems
"emerge" from physical interactions between degrees of freedom described
completely quantum-mechanically. It is shown that einselection can only
generate classical systems if the "environment" is assumed \textit{a priori} to
be classical; einselection therefore does not provide an observer-independent
mechanism by which classicality can emerge from quantum dynamics. Einselection
is then reformulated in terms of positive operator-valued measures (POVMs)
acting on a global quantum state. It is shown that this re-formulation enables
a natural interpretation of apparently-classical systems as virtual machines
that requires no assumptions beyond those of classical computer science.Comment: 15 pages, 1 figure; minor correction
Non-Equilibrium Quantum Electrodynamics
We employ the influence functional technique to trace out the photonic
contribution from full quantum electrodynamics. The reduced density matrix
propagator for the spinor field is then constructed. We discuss the role of
time-dependent renormalization in the propagator and focus on the possibility
of obtaining dynamically induced superselection rules. Finally, we derive the
master equation for the case of the field being in an one-particle state in a
non-relativistic regime and discuss whether EM vacuumm fluctuations are
sufficient to produce decoherence in the position basis.Comment: 28 pages, 2 figures. Substantially revised, one important mistake
corrected; discussion on decoherence upgraded, section 4 essentially
rewritte
Coherent states and the classical-quantum limit considered from the point of view of entanglement
Three paradigms commonly used in classical, pre-quantum physics to describe
particles (that is: the material point, the test-particle and the diluted
particle (droplet model)) can be identified as limit-cases of a quantum regime
in which pairs of particles interact without getting entangled with each other.
This entanglement-free regime also provides a simplified model of what is
called in the decoherence approach "islands of classicality", that is,
preferred bases that would be selected through evolution by a Darwinist
mechanism that aims at optimising information. We show how, under very general
conditions, coherent states are natural candidates for classical pointer
states. This occurs essentially because, when a (supposedly bosonic) system
coherently exchanges only one quantum at a time with the (supposedly bosonic)
environment, coherent states of the system do not get entangled with the
environment, due to the bosonic symmetry.Comment: This is the definitive version of a paper entitled The
classical-quantum limit considered from the point of view of entanglement: a
survey (author T. Durt). The older version has been replaced by the
definitive on
Decoherence: Concepts and Examples
We give a pedagogical introduction to the process of decoherence - the
irreversible emergence of classical properties through interaction with the
environment. After discussing the general concepts, we present the following
examples: Localisation of objects, quantum Zeno effect, classicality of fields
and charges in QED, and decoherence in gravity theory. We finally emphasise the
important interpretational features of decoherence.Comment: 24 pages, LATEX, 9 figures, needs macro lamuphys.sty, to appear in
the Proceedings of the 10th Born Symposiu
Decoherence as a sequence of entanglement swaps
Standard semi-classical models of decoherence do not take explicit account of
the classical information required to specify the system - environment
boundary. I show that this information can be represented as a finite set of
reference eigenvalues that must be encoded by any observer, including any
apparatus, able to distinguish the system from its environment. When the
information required for system identification is accounted for in this way,
decoherence can be described as a sequence of entanglement swaps between
reference and pointer components of the system and their respective
environments. Doing so removes the need for the a priori assumptions of ontic
boundaries required by semi-classical models.Comment: 13 pgs, 3 figures. Accepted by Results in Physic
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