606 research outputs found
Decoherence by a spin thermal bath: Role of the spin-spin interactions and initial state of the bath
We study the decoherence of two coupled spins that interact with a spin-bath
environment. It is shown that the connectivity and the coupling strength
between the spins in the environment are of crucial importance for the
decoherence of the central system. For the anisotropic spin-bath, changing the
connectivity or coupling strenghts changes the decoherence of the central
system from Gaussian to exponential decay law. The initial state of the
environment is shown to affect the decoherence process in a qualitatively
significant manner.Comment: submitted to PR
Photons uncertainty solves Einstein-Podolsky-Rosen paradox
Einstein, Podolsky and Rosen (EPR) pointed out that the quantum-mechanical
description of "physical reality" implied an unphysical, instantaneous action
between distant measurements. To avoid such an action at a distance, EPR
concluded that Quantum Mechanics had to be incomplete. However, its extensions
involving additional "hidden variables", allowing for the recovery of
determinism and locality, have been disproved experimentally (Bell's theorem).
Here, I present an opposite solution of the paradox based on the greater
indeterminism of the modern Quantum Field Theory (QFT) description of Particle
Physics, that prevents the preparation of any state having a definite number of
particles. The resulting uncertainty in photons radiation has interesting
consequences in Quantum Information Theory (e.g. cryptography and
teleportation). Moreover, since it allows for less elements of EPR physical
reality than the old non-relativistic Quantum Mechanics, QFT satisfies the EPR
condition of completeness without the need of hidden variables. The residual
physical reality does never violate locality, thus the unique objective proof
of "quantum nonlocality" is removed in an interpretation-independent way. On
the other hand, the supposed nonlocality of the EPR correlations turns out to
be a problem of the interpretation of the theory. If we do not rely on hidden
variables or new physics beyond QFT, the unique viable interpretation is a
minimal statistical one, that preserves locality and Lorentz symmetry.Comment: Published version, with updated referenc
Hardy's argument and successive spin-s measurements
We consider a hidden-variable theoretic description of successive
measurements of non commuting spin observables on a input spin-s state. In this
scenario, the hidden-variable theory leads to a Hardy-type argument that
quantum predictions violate it. We show that the maximum probability of success
of Hardy's argument in quantum theory is , which is more
than in the spatial case.Comment: 7 page
Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach
We study spin and charge currents pumped by precessing magnetization of a
single ferromagnetic layer within F|I|N or F|I|F (F-ferromagnet; I-insulator;
N-normal-metal) multilayers of nanoscale thickness attached to two normal metal
electrodes with no applied bias voltage between them. Both simple
one-dimensional model, consisting of a single precessing spin and a potential
barrier as the "sample," and realistic three-dimensional devices are
investigated. In the rotating reference frame, where the magnetization appears
to be static, these junctions are mapped onto a four-terminal dc circuit whose
effectively half-metallic ferromagnetic electrodes are biased by the frequency
of microwave radiation driving magnetization precession at the
ferromagnetic resonance (FMR) conditions. We show that pumped spin current in
F|I|F junctions, diminished behind the tunnel barrier and increased in the
opposite direction, is filtered into charge current by the second layer to
generate dc pumping voltage of the order of V (at FMR frequency
GHz) in an open circuit. In F|I|N devices, several orders of
magnitude smaller charge current and the corresponding dc voltage appear
concomitantly with the pumped spin current due to barrier induced asymmetry in
the transmission coefficients connecting the four electrodes in the rotating
frame picture of pumping.Comment: 8 pages, 5 figure
The Quantum Mechanics of Hyperion
This paper is motivated by the suggestion [W. Zurek, Physica Scripta, T76,
186 (1998)] that the chaotic tumbling of the satellite Hyperion would become
non-classical within 20 years, but for the effects of environmental
decoherence. The dynamics of quantum and classical probability distributions
are compared for a satellite rotating perpendicular to its orbital plane,
driven by the gravitational gradient. The model is studied with and without
environmental decoherence. Without decoherence, the maximum quantum-classical
(QC) differences in its average angular momentum scale as hbar^{2/3} for
chaotic states, and as hbar^2 for non-chaotic states, leading to negligible QC
differences for a macroscopic object like Hyperion. The quantum probability
distributions do not approach their classical limit smoothly, having an
extremely fine oscillatory structure superimposed on the smooth classical
background. For a macroscopic object, this oscillatory structure is too fine to
be resolved by any realistic measurement. Either a small amount of smoothing
(due to the finite resolution of the apparatus) or a very small amount of
environmental decoherence is sufficient ensure the classical limit. Under
decoherence, the QC differences in the probability distributions scale as
(hbar^2/D)^{1/6}, where D is the momentum diffusion parameter. We conclude that
decoherence is not essential to explain the classical behavior of macroscopic
bodies.Comment: 17 pages, 24 figure
Entanglement for all quantum states
It is shown that a state that is factorizable in the Hilbert space
corresponding to some choice of degrees of freedom, becomes entangled for a
different choice of degrees of freedom. Therefore, entanglement is not a
special case but is ubiquitous in quantum systems. Simple examples are
calculated and a general proof is provided. The physical relevance of the
change of tensor product structure is mentioned.Comment: 9 page
Quantum oscillations and black hole ringing
We show that strongly coupled field theories with holographic gravity duals
at finite charge density and low temperatures can undergo de Haas - van Alphen
quantum oscillations as a function of an external magnetic field. Exhibiting
this effect requires computation of the one loop contribution of charged bulk
fermions to the free energy. The one loop calculation is performed using a
formula expressing determinants in black hole backgrounds as sums over
quasinormal modes. At zero temperature, the periodic nonanalyticities in the
magnetic susceptibility as a function of the inverse magnetic field depend on
the low energy scaling behavior of fermionic operators in the field theory, and
are found to be softer than in weakly coupled theories. We also obtain
numerical and WKB results for the quasinormal modes of charged bosons in dyonic
black hole backgrounds, finding evidence for nontrivial periodic behavior as a
function of the magnetic field.Comment: 1+53 pages. 9 figures. v2: important changes to sections 3.4 - 3.6.
contribution of branch cut poles include
Entropy production and equilibration in Yang-Mills quantum mechanics
The Husimi distribution provides for a coarse grained representation of the
phase space distribution of a quantum system, which may be used to track the
growth of entropy of the system. We present a general and systematic method of
solving the Husimi equation of motion for an isolated quantum system, and we
construct a coarse grained Hamiltonian whose expectation value is exactly
conserved. As an application, we numerically solve the Husimi equation of
motion for two-dimensional Yang-Mills quantum mechanics (the x-y model) and
calculate the time evolution of the coarse grained entropy of a highly excited
state. We show that the coarse grained entropy saturates to a value that
coincides with the microcanonical entropy corresponding to the energy of the
system.Comment: 23 pages, 23 figure
Quantum mechanics and elements of reality inferred from joint measurements
The Einstein-Podolsky-Rosen argument on quantum mechanics incompleteness is
formulated in terms of elements of reality inferred from joint (as opposed to
alternative) measurements, in two examples involving entangled states of three
spin-1/2 particles. The same states allow us to obtain proofs of the
incompatibility between quantum mechanics and elements of reality.Comment: LaTeX, 12 page
Invalidity of Classes of Approximated Hall Effect Calculations
In this comment, I point out a number of approximated derivations for the
effective equation of motion, now been applied to d-wave superconductors by
Kopnin and Volovik are invalid. The major error in those approximated
derivations is the inappropriate use of the relaxation time approximation in
force-force correlation functions, or in force balance equations, or in similar
variations. This approximation is wrong and unnecessary.Comment: final version, minor changes, to appear in Phys. Rev. Let
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