5,708 research outputs found
The density matrix in the de Broglie-Bohm approach
If the density matrix is treated as an objective description of individual
systems, it may become possible to attribute the same objective significance to
statistical mechanical properties, such as entropy or temperature, as to
properties such as mass or energy. It is shown that the de Broglie-Bohm
interpretation of quantum theory can be consistently applied to density
matrices as a description of individual systems. The resultant trajectories are
examined for the case of the delayed choice interferometer, for which Bell
appears to suggest that such an interpretation is not possible. Bell's argument
is shown to be based upon a different understanding of the density matrix to
that proposed here.Comment: 15 pages, 4 figure
Irreversible Quantum Mechanics in the Neutral K-System
The neutral Kaon system is used to test the quantum theory of resonance
scattering and decay phenomena. The two dimensional Lee-Oehme-Yang theory with
complex Hamiltonian is obtained by truncating the complex basis vector
expansion of the exact theory in Rigged Hilbert space. This can be done for K_1
and K_2 as well as for K_S and K_L, depending upon whether one chooses the
(self-adjoint, semi-bounded) Hamiltonian as commuting or non-commuting with CP.
As an unexpected curiosity one can show that the exact theory (without
truncation) predicts long-time 2 pion decays of the neutral Kaon system even if
the Hamiltonian conserves CP.Comment: 36 pages, 1 PostScript figure include
Chern number spins of Mn acceptor magnets in GaAs
We determine the effective total spin of local moments formed from
acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern
numbers. We find that when individual Mn atoms are close to the sample surface,
the total spin changes from to , due to quenching of the
acceptor orbital moment. For Mn pairs in bulk, the total depends on the
pair orientation in the GaAs lattice and on the separation between the Mn
atoms. We point out that Berry curvature variation as a function of local
moment orientation can profoundly influence the quantum spin dynamics of these
magnetic entities.Comment: 4 pages, 3 figure
The free energy in a class of quantum spin systems and interchange processes
We study a class of quantum spin systems in the mean-field setting of the
complete graph. For spin the model is the Heisenberg ferromagnet,
for general spin it has a probabilistic representation
as a cycle-weighted interchange process. We determine the free energy and the
critical temperature (recovering results by T\'oth and by Penrose when
). The critical temperature is shown to coincide (as a function of
) with that of the state classical Potts model, and the phase
transition is discontinuous when .Comment: 22 page
Evolution of field spiral galaxies up to redshifts z=1
We have gained VLT/FORS spectra and HST/ACS images of a sample of 220 distant
field spiral galaxies. Spatially resolved rotation curves were extracted and
fitted with synthetic velocity fields that take into account all geometric and
observational effects, like blurring due to the slit width and seeing
influence. The maximum rotation velocity Vmax could be determined for 124
galaxies that cover the redshift range 0.1<z<1.0. The luminosity-rotation
velocity distribution of this sample is offset from the Tully-Fisher relation
(TFR) of local low-mass spirals, whereas the distant high-mass spirals are
compatible with the local TFR. We show that the slope of the local and the
intermediate-z TFR would be in compliance if its scatter decreased by more than
a factor of 3 between z~0.5 and z~0. On the other hand, the distant
low-luminosity disks have much lower stellar M/L ratios than their local
counterparts, while high-luminosity disks barely evolved in M/L over the
covered redshift range. This could be the manifestation of the "downsizing"
effect, i.e. the succesive shift of the peak of star formation from high-mass
to low-mass galaxies towards lower redshifts. This trend might be canceled out
in the TF diagram due to the simultaneous evolution of multiple parameters. We
also estimate the ratios between stellar and total masses, finding that these
remained constant since z=1, as would be expected in the context of
hierarchically growing structure. (Abridged)Comment: 20 pages, 5 figures, ApJ, accepte
Entanglement and State Preparation
When a subset of particles in an entangled state is measured, the state of
the subset of unmeasured particles is determined by the outcome of the
measurement. This first measurement may be thought of as a state preparation
for the remaining particles. In this paper, we examine how the duration of the
first measurement effects the state of the unmeasured subsystem. The state of
the unmeasured subsytem will be a pure or mixed state depending on the nature
of the measurement.
In the case of quantum teleportation we show that there is an eigenvalue
equation which must be satisfied for accurate teleportation. This equation
provides a limitation to the states that can be accurately teleported.Comment: 24 pages, 3 figures, submitted to Phys. Rev.
On the mean value of the energy for resonant states
In this work we discuss possible definitions of the mean value of the energy
for a resonant (Gamow) state. The mathematical and physical aspects of the
formalism are reviewed. The concept of rigged Hilbert space is used as a
supportive tool in dealing with Gamow-resonances.Comment: 9 page
Time Asymmetric Quantum Physics
Mathematical and phenomenological arguments in favor of asymmetric time
evolution of micro-physical states are presented.Comment: Tex file with 2 figure
Relating the Lorentzian and exponential: Fermi's approximation,the Fourier transform and causality
The Fourier transform is often used to connect the Lorentzian energy
distribution for resonance scattering to the exponential time dependence for
decaying states. However, to apply the Fourier transform, one has to bend the
rules of standard quantum mechanics; the Lorentzian energy distribution must be
extended to the full real axis instead of being bounded from
below (``Fermi's approximation''). Then the Fourier transform
of the extended Lorentzian becomes the exponential, but only for times , a time asymmetry which is in conflict with the unitary group time evolution
of standard quantum mechanics. Extending the Fourier transform from
distributions to generalized vectors, we are led to Gamow kets, which possess a
Lorentzian energy distribution with and have exponential
time evolution for only. This leads to probability predictions
that do not violate causality.Comment: 23 pages, no figures, accepted by Phys. Rev.
Measuring ^{12}C(&alpha,&gamma)^{16}O from White Dwarf Asteroseismology
During helium burning in the core of a red giant, the relative rates of the
3&alpha and ^{12}C(&alpha,&gamma)^{16}O reactions largely determine the final
ratio of carbon to oxygen in the resulting white dwarf star. The uncertainty in
the 3&alpha reaction at stellar energies due to the extrapolation from
high-energy laboratory measurements is relatively small, but this is not the
case for the ^{12}C(&alpha,&gamma)^{16}O reaction. Recent advances in the
analysis of asteroseismological data on pulsating white dwarf stars now make it
possible to obtain precise measurements of the central ratio of carbon to
oxygen, providing a more direct way to measure the ^{12}C(&alpha,&gamma)^{16}O
reaction rate at stellar energies. We assess the systematic uncertainties of
this approach and quantify small shifts in the measured central oxygen
abundance originating from the observations and from model settings that are
kept fixed during the optimization. Using new calculations of white dwarf
internal chemical profiles, we find a rate for the ^{12}C(&alpha,&gamma)^{16}O
reaction that is significantly higher than most published values. The accuracy
of this method may improve as we modify some of the details of our description
of white dwarf interiors that were not accessible through previous
model-fitting methods.Comment: 8 pages, 4 figures, 3 tables, uses emulateapj5.sty, Accepted for
publication in the Astrophysical Journa
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