110 research outputs found
Quantum non-locality co-exists with locality
Quantum non-locality is normally defined via violations of Bell's
inequalities that exclude certain classical hidden variable theories from
explaining quantum correlations. Another definition of non-locality refers to
the wave-function collapse thereby one can prepare a quantum state from
arbitrary far away. In both cases one can debate on whether non-locality is a
real physical phenomenon, e.g. one can employ formulations of quantum mechanics
that does not use collapse, or one can simply refrain from explaining quantum
correlations via classical hidden variables. Here we point out that there is a
non-local effect within quantum mechanics, i.e. without involving hidden
variables or collapse. This effect is seen via imprecise (i.e. interval-valued)
joint probability of two observables, which replaces the ill-defined notion of
the precise joint probability for non-commuting observables. It is consistent
with all requirements for the joint probability, e.g. those for commuting
observales. The non-locality amounts to a fact that (in a two-particle system)
the joint imprecise probability of non-commuting two-particle observables (i.e.
tensor product of single-particle observables) does not factorize into
single-particle contributions, even for uncorrelated states of the two-particle
system. The factorization is recovered for a less precise (i.e. the one
involving a wider interval) joint probability. This approach to non-locality
reconciles it with locality, since the latter emerges as a less precise
description.Comment: 6 pages, no figure
Emergence of Leadership in Communication
We study a neuro-inspired model that mimics a discussion (or information
dissemination) process in a network of agents. During their interaction, agents
redistribute activity and network weights, resulting in emergence of leader(s).
The model is able to reproduce the basic scenarios of leadership known in
nature and society: laissez-faire (irregular activity, weak leadership, sizable
inter-follower interaction, autonomous sub-leaders); participative or
democratic (strong leadership, but with feedback from followers); and
autocratic (no feedback, one-way influence). Several pertinent aspects of these
scenarios are found as well---e.g., hidden leadership (a hidden clique of
agents driving the official autocratic leader), and successive leadership (two
leaders influence followers by turns). We study how these scenarios emerge from
inter-agent dynamics and how they depend on behavior rules of agents---in
particular, on their inertia against state changes.Comment: 17 pages, 11 figure
Quantum Brownian motion and its conflict with the second law
The Brownian motion of a harmonically bound quantum particle and coupled to a
harmonic quantum bath is exactly solvable. At low enough temperatures the
stationary state is non-Gibbsian due to an entanglement with the bath. This
happens when a cloud of bath modes around the particle is formed. Equilibrium
thermodynamics for particle plus bath together, does not imply standard
thermodynamics for the particle itself at low T. Various formulations of the
second law are then invalid. First, the Clausius inequality can be violated.
Second, when the width of the confining potential is suddenly changed, there
occurs a relaxation to equilibrium during which the rate of entropy production
is partly negative. Third, for non-adiabatic changes of system parameters the
rate of energy dissipation can be negative, and, out of equilibrium, cyclic
processes are possible which extract work from the bath. Conditions are put
forward under which perpetuum mobile of the second kind, having several work
extraction cycles, enter the realm of condensed matter physics.Comment: 6 pages Latex, uses aip-proceedings style files. Proceedings `Quantum
Limits to the Second Law', San Diego, July 200
Phase transitions and quantum measurements
In a quantum measurement, a coupling between the system S and the
apparatus A triggers the establishment of correlations, which provide
statistical information about S. Robust registration requires A to be
macroscopic, and a dynamical symmetry breaking of A governed by S allows the
absence of any bias. Phase transitions are thus a paradigm for quantum
measurement apparatuses, with the order parameter as pointer variable. The
coupling behaves as the source of symmetry breaking. The exact solution of
a model where S is a single spin and A a magnetic dot (consisting of
interacting spins and a phonon thermal bath) exhibits the reduction of the
state as a relaxation process of the off-diagonal elements of S+A, rapid due to
the large size of . The registration of the diagonal elements involves a
slower relaxation from the initial paramagnetic state of A to either one of its
ferromagnetic states. If is too weak, the measurement fails due to a
``Buridan's ass'' effect. The probability distribution for the magnetization
then develops not one but two narrow peaks at the ferromagnetic values. During
its evolution it goes through wide shapes extending between these values.Comment: 12 pages, 2 figure
The Quantum Measurement Process: Lessons from an Exactly Solvable Model
The measurement of a spin-\half is modeled by coupling it to an apparatus,
that consists of an Ising magnetic dot coupled to a phonon bath. Features of
quantum measurements are derived from the dynamical solution of the
measurement, regarded as a process of quantum statistical mechanics.
Schr\"odinger cat terms involving both the system and the apparatus, die out
very quickly, while the registration is a process taking the apparatus from its
initially metastable state to one of its stable final states. The occurrence of
Born probabilities can be inferred at the macroscopic level, by looking at the
pointer alone. Apparent non-unitary behavior of the measurement process is
explained by the arisal of small many particle correlations, that characterize
relaxation.Comment: 13 pages, discussion of pre-measurement added. World Scientific
style. To appear in proceedings "Beyond The Quantum", Th.M. Nieuwenhuizen et
al, eds, (World Scientific, 2007
- …