104 research outputs found
On Birkhoff's theorem for doubly stochastic completely positive maps of matrix algebras
AbstractA study is made of the extreme points of the convex set of doubly stochastic completely positive maps of the matrix algebra Mn. If n = 2 the extreme points are precisely the unitary maps, but if n â©ľ 3 there are nonunitary extreme points, examples of which are exhibited. A tilde operation is defined on the linear maps of Mn and used to give an elementary derivation of a result of Kummerer and Maassen
Ambivalence of the anisotropy of the vortex lattice in an anisotropic type-II superconductor
We present a geometry-based discussion of possible vortex configurations in
the mixed state of anisotropic type-II superconductors. It is shown that, if
energy considerations assign six nearest neighbors to each vortex, two distinct
modifications of the vortex lattice are possible. It is expected that certain
conditions lead to a first order phase transition from one modification of the
vortex lattice to the other upon varying the external magnetic field.Comment: 3 pages, 2 figure
The quantum world is not built up from correlations
It is known that the global state of a composite quantum system can be
completely determined by specifying correlations between measurements performed
on subsystems only. Despite the fact that the quantum correlations thus suffice
to reconstruct the quantum state, we show, using a Bell inequality argument,
that they cannot be regarded as objective local properties of the composite
system in question. It is well known since the work of J.S. Bell, that one
cannot have locally preexistent values for all physical quantities, whether
they are deterministic or stochastic. The Bell inequality argument we present
here shows this is also impossible for correlations among subsystems of an
individual isolated composite system. Neither of them can be used to build up a
world consisting of some local realistic structure. As a corrolary to the
result we argue that entanglement cannot be considered ontologically robust.
The argument has an important advantage over others because it does not need
perfect correlations but only statistical correlations. It can therefore easily
be tested in currently feasible experiments using four particle entanglement.Comment: Published version. Title change
Unital quantum operators on the Bloch ball and Bloch region
For one qubit systems, we present a short, elementary argument characterizing
unital quantum operators in terms of their action on Bloch vectors. We then
show how our approach generalizes to multi-qubit systems, obtaining
inequalities that govern when a ``diagonal'' superoperator on the Bloch region
is a quantum operator. These inequalities are the n-qubit analogue of the
Algoet-Fujiwara conditions. Our work is facilitated by an analysis of
operator-sum decompositions in which negative summands are allowed.Comment: Revised and corrected, to appear in Physical Review
Second-quantized Landau-Zener theory for dynamical instabilities
State engineering in nonlinear quantum dynamics sometimes may demand driving
the system through a sequence of dynamically unstable intermediate states. This
very general scenario is especially relevant to dilute Bose-Einstein
condensates, for which ambitious control schemes have been based on the
powerful Gross-Pitaevskii mean field theory. Since this theory breaks down on
logarithmically short time scales in the presence of dynamical instabilities,
an interval of instabilities introduces quantum corrections, which may possibly
derail a control scheme. To provide a widely applicable theory for such quantum
corrections, this paper solves a general problem of time-dependent quantum
mechanical dynamical instability, by modelling it as a second-quantized
analogue of a Landau-Zener avoided crossing: a `twisted crossing'.Comment: 4 pages, 3 figure
Some Aspects of Rotational and Magnetic Energies for a Hierarchy of Celestial Objects
Celestial objects, from earth like planets to clusters of galaxies, possess
angular momentum and magnetic fields. Here we compare the rotational and
magnetic energies of a whole range of these celestial objects together with
their gravitational self energies and find a number of interesting
relationships. The celestial objects, due to their magnetic fields, also posses
magnetic moments. The ratio of magnetic moments of these objects with the
nuclear magnetic moments also exhibits interesting trends. We also compare
their gyromagnetic ratio which appears to fall in a very narrow range for the
entire hierarchy of objects. Here we try to understand the physical aspects
implied by these observations and the origin of these properties in such a wide
range of celestial objects, spanning some twenty orders in mass, magnetic field
and other parameters.Comment: 12 pages, 37 equation
Reconstructing Bohr's Reply to EPR in Algebraic Quantum Theory
Halvorson and Clifton have given a mathematical reconstruction of Bohr's
reply to Einstein, Podolsky and Rosen (EPR), and argued that this reply is
dictated by the two requirements of classicality and objectivity for the
description of experimental data, by proving consistency between their
objectivity requirement and a contextualized version of the EPR reality
criterion which had been introduced by Howard in his earlier analysis of Bohr's
reply. In the present paper, we generalize the above consistency theorem, with
a rather elementary proof, to a general formulation of EPR states applicable to
both non-relativistic quantum mechanics and algebraic quantum field theory; and
we clarify the elements of reality in EPR states in terms of Bohr's
requirements of classicality and objectivity, in a general formulation of
algebraic quantum theory.Comment: 13 pages, Late
Electronic states and optical properties of GaAs/AlAs and GaAs/vacuum superlattices by the linear combination of bulk bands method
The linear combination of bulk bands method recently introduced by Wang,
Franceschetti and Zunger [Phys. Rev. Lett.78, 2819 (1997)] is applied to a
calculation of energy bands and optical constants of (GaAs)/(AlAs) and
(GaAs)/(vacuum) (001) superlattices with n ranging from 4 to 20.
Empirical pseudopotentials are used for the calculation of the bulk energy
bands. Quantum-confined induced shifts of critical point energies are
calculated and are found to be larger for the GaAs/vacuum system. The
peak in the absorption spectra has a blue shift and splits into two peaks for
decreasing superlattice period; the transition instead is found to be
split for large-period GaAs/AlAs superlattices. The band contribution to linear
birefringence of GaAs/AlAs superlattices is calculated and compared with recent
experimental results of Sirenko et al. [Phys. Rev. B 60, 8253 (1999)]. The
frequency-dependent part reproduces the observed increase with decreasing
superlattice period, while the calculated zero-frequency birefringence does not
account for the experimental results and points to the importance of
local-field effects.Comment: 10 pages, 11 .eps figures, 1 tabl
Effect of quantum noise on Coulomb blockade in normal tunnel junctions at high voltages
We have investigated asymptotic behavior of normal tunnel junctions at
voltages where even the best ohmic environments start to look like RC
transmission lines. In the experiments, this is manifested by an exceedingly
slow approach to the linear behavior above the Coulomb gap. As expected on the
basis of the quantum theory taking into account interaction with the
environmental modes, better fits are obtained using 1/sqrt{V}- than 1/V-
dependence for the asymptote. These results agree with the horizon picture if
the frequency-dependent phase velocity is employed instead of the speed of
light in order to determine the extent of the surroundings seen by the
junction.Comment: 9 pages, 4 figures, submitted to Phys. Rev.
Bell's inequalities for states with positive partial transpose
We study violations of n particle Bell inequalities (as developed by Mermin
and Klyshko) under the assumption that suitable partial transposes of the
density operator are positive. If all transposes with respect to a partition of
the system into p subsystems are positive, the best upper bound on the
violation is 2^((n-p)/2). In particular, if the partial transposes with respect
to all subsystems are positive, the inequalities are satisfied. This is
supporting evidence for a recent conjecture by Peres that positivity of partial
transposes could be equivalent to existence of local classical models.Comment: 4 pages, REVTe
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