4,861 research outputs found
Normalization of Collisional Decoherence: Squaring the Delta Function, and an Independent Cross-Check
We show that when the Hornberger--Sipe calculation of collisional decoherence
is carried out with the squared delta function a delta of energy instead of a
delta of the absolute value of momentum, following a method introduced by
Di\'osi, the corrected formula for the decoherence rate is simply obtained. The
results of Hornberger and Sipe and of Di\'osi are shown to be in agreement. As
an independent cross-check, we calculate the mean squared coordinate diffusion
of a hard sphere implied by the corrected decoherence master equation, and show
that it agrees precisely with the same quantity as calculated by a classical
Brownian motion analysis.Comment: Tex: 14 pages 7/30/06: revisions to introduction, and references
added 9/29/06: further minor revisions and references adde
Sum-over-histories origin of the composition laws of relativistic quantum mechanics and quantum cosmology
The scope of the paper has been broadened to include a more complete
discussion of the following topics: The derivation of composition laws in
quantum cosmology. The connection between the existence of a composition law in
the sum over histories approach to relativistic quantum mechanics and quantum
cosmology, and the existence of a canonical formulation.Comment: 36 page
Approximate Decoherence of Histories and 't Hooft's Deterministic Quantum Theory
This paper explores the possibility that an exactly decoherent set of
histories may be constructed from an approximately decoherent set by small
distortions of the operators characterizing the histories. In particular, for
the case of histories of positions and momenta, this is achieved by doubling
the set of operators and then finding, amongst this enlarged set, new position
and momentum operators which commute, so decohere exactly, and which are
``close'' to the original operators. The enlarged, exactly decoherent, theory
has the same classical dynamics as the original one, and coincides with the
so-called deterministic quantum theories of the type recently studied by 't
Hooft. These results suggest that the comparison of standard and deterministic
quantum theories may provide an alternative method of characterizing emergent
classicality. A side-product is the surprising result that histories of momenta
in the quantum Brownian motion model (for the free particle in the
high-temperature limit) are exactly decoherent.Comment: 41 pages, plain Te
Effective Theories of Coupled Classical and Quantum Variables from Decoherent Histories: A New Approach to the Backreaction Problem
We use the decoherent histories approach to quantum theory to derive the form
of an effective theory describing the coupling of classical and quantum
variables. The derivation is carried out for a system consisting of a large
particle coupled to a small particle with the important additional feature that
the large particle is also coupled to a thermal environment producing the
decoherence necessary for classicality. The effective theory is obtained by
tracing out both the environment and the small particle variables. It consists
of a formula for the probabilities of a set of histories of the large particle,
and depends on the dynamics and initial quantum state of the small particle. It
has the form of an almost classical particle coupled to a stochastic variable
whose probabilities are determined by a formula very similar to that given by
quantum measurement theory for continuous measurements of the small particle's
position. The effective theory gives intuitively sensible answers when the
small particle is in a superposition of localized states.Comment: 27 pages, plain Te
Disentanglement and Decoherence by Open System Dynamics
The destruction of quantum interference, decoherence, and the destruction of
entanglement both appear to occur under the same circumstances. To address the
connection between these two phenomena, we consider the evolution of arbitrary
initial states of a two-particle system under open system dynamics described by
a class of master equations which produce decoherence of each particle. We show
that all initial states become separable after a finite time, and we produce
the explicit form of the separated state. The result extends and amplifies an
earlier result of Di\'osi. We illustrate the general result by considering the
case in which the initial state is an EPR state (in which both the positions
and momenta of a particle pair are perfectly correlated). This example clearly
illustrates how the spreading out in phase space produced by the environment
leads to certain disentanglement conditions becoming satisfied.Comment: 15 Page
Half Quantization
A general dynamical system composed by two coupled sectors is considered. The
initial time configuration of one of these sectors is described by a set of
classical data while the other is described by standard quantum data. These
dynamical systems will be named half quantum. The aim of this paper is to
derive the dynamical evolution of a general half quantum system from its full
quantum formulation. The standard approach would be to use quantum mechanics to
make predictions for the time evolution of the half quantum initial data. The
main problem is how can quantum mechanics be applied to a dynamical system
whose initial time configuration is not described by a set of fully quantum
data. A solution to this problem is presented and used, as a guideline to
obtain a general formulation of coupled classical-quantum dynamics. Finally, a
quantization prescription mapping a given classical theory to the correspondent
half quantum one is presented.Comment: 20 pages, LaTex file, Substantially revised versio
An Examination of the Challenges Experienced by Canadian Ice-Hockey Players in the National Hockey League
Semistructured interviews were used in this study to learn about the challenges experienced by four groups of National Hockey League (NHL) players (N=11): prospects (n=3), rookies (n=3), veterans (n=2), and retirees (n=3). The database is comprised of 757 meaning units grouped into 11 contextual challenges pertaining to scouting demands, training camp, increased athletic demands, team expectations, and earning team trust. The veterans spoke mostly of challenges including scouting demands, athletic demands, and team expectations. Retirees considered mostly challenges pertaining to team expectations, athletic demands, lifestyle, media demands, transactions, cross-cultural encounters, and playoffs. An expert panel ensured that the interview guide, data analysis, and the findings represented the participants’ experiences in the NHL. Recommendations for practitioners and researchers working with NHL players are proposed
- …