13,471 research outputs found
Quantum measurements and Paul traps in gravitational backgrounds
In the present work we solve the motion equations of a particle in a Paul
trap embeded in the gravitational field of a spherically symmetric mass. One of
the ideas behind this work concerns the analysis of the effects that the
gravity--induced quantum noise, stemming from the bodies in the neighborhood of
the Paul trap, could have upon the enhancement of the quantum behavior of this
system. This will be done considering a series expansion for the gravitational
field of the source, and including in the Hamiltonian of the Paul trap only the
first two terms. Higher--order contributions will be introduced as part of the
environment of the system, and in consequence will not appear in the
Hamiltonian. In other words, we put forward an argument that allows us to
differentiate those gravitational degrees of freedom that will appear as an
uncontrollable influence on the Paul trap. Along the ideas of the so called
restricted path integral formalism, we take into account the continuous
monitoring of the position of our particle, and in consequence the
corresponding propagators and probabilities, associated with the different
measurements outputs, are obtained.
Afterwards, the differential equation related to a quantum nondemolition
variable is posed and solved, i.e., a family of quantum nondemolition
parameters is obtained. Finally, a qualitative analysis of the effects on the
system, of the gravity--induced environment, will be done.Comment: Accepted in International Journal of Modern Physics
Comment on "Chain Length Scaling of Protein Folding Time", PRL 77, 5433 (1996)
In a recent Letter, Gutin, Abkevich, and Shakhnovich (GAS) reported on a
series of dynamical Monte Carlo simulations on lattice models of proteins.
Based on these highly simplified models, they found that four different
potential energies lead to four different folding time scales tau_f, where
tau_f scales with chain length as N^lambda (see, also, Refs. [2-4]), with
lambda varying from 2.7 to 6.0. However, due to the lack of microscopic models
of protein folding dynamics, the interpretation and origin of the data have
remained somewhat speculative. It is the purpose of this Comment to point out
that the application of a simple "mesoscopic" model (cond-mat/9512019, PRL 77,
2324, 1996) of protein folding provides a full account of the data presented in
their paper. Moreover, we find a major qualitative disagreement with the
argumentative interpretation of GAS. Including, the origin of the dynamics, and
size of the critical folding nucleus.Comment: 1 page Revtex, 1 fig. upon request. Submitted to PR
Quantum nondemolition measurements of a particle in electric and gravitational fields
In this work we obtain a nondemolition variable for the case in which a
charged particle moves in the electric and gravitational fields of a spherical
body. Afterwards we consider the continuous monitoring of this nondemolition
parameter, and calculate along the ideas of the so called restricted path
integral formalism, the corresponding propagator. Using these results the
probabilities associated with the possible measurement outputs are evaluated.
The limit of our results, as the resolution of the measuring device goes to
zero, is analyzed, and the dependence of the corresponding propagator upon the
strength of the electric and gravitational fields are commented. The role that
mass plays in the corresponding results, and its possible connection with the
equivalence principle at quantum level, are studied.Comment: Accepted in International Journal of Modern Physics D, 14 page
Landau Effective Interaction between Quasiparticles in a Bose-Einstein Condensate
Landau's description of the excitations in a macroscopic system in terms of
quasiparticles stands out as one of the highlights in quantum physics. It
provides an accurate description of otherwise prohibitively complex many-body
systems, and has led to the development of several key technologies. In this
paper, we investigate theoretically the Landau effective interaction between
quasiparticles, so-called Bose polarons, formed by impurity particles immersed
in a Bose-Einstein condensate (BEC). In the limit of weak interactions between
the impurities and the BEC, we derive rigorous results for the effective
interaction. They show that it can be strong even for weak impurity-boson
interaction, if the transferred momentum/energy between the quasiparticles is
resonant with a sound mode in the BEC. We then develop a diagrammatic scheme to
calculate the effective interaction for arbitrary coupling strengths, which
recovers the correct weak coupling results. Using this, we show that the Landau
effective interaction in general is significantly stronger than that between
quasiparticles in a Fermi gas, mainly because a BEC is more compressible than a
Fermi gas. The interaction is particularly large near the unitarity limit of
the impurity-boson scattering, or when the quasiparticle momentum is close to
the threshold for momentum relaxation in the BEC. Finally, we show how the
Landau effective interaction leads to a sizeable shift of the quasiparticle
energy with increasing impurity concentration, which should be detectable with
present day experimental techniques.Comment: 12 page
Determining distributions of weakly bound nuclei from breakup cross sections using Continuum Discretized Coupled Channels calculations. Application to Be
A novel method to extract the strength of a weakly bound nucleus from
experimental Coulomb dissociation data is proposed. The method makes use of
continuum discretized coupled channels (CDCC) calculations, in which both
nuclear and Coulomb forces are taken into account to all orders. This is a
crucial advantage with respect to the standard procedure based on the
Equivalent Photon Method (EPM) which does not properly take into account
nuclear distortion, higher order coupling effects, or Coulomb-nuclear
interference terms. The procedure is applied to the Be nucleus using two
sets of available experimental data at different energies, for which seemingly
incompatible have been reported using the EPM. We show that the present
procedure gives consistent strengths, thus solving the aforementioned
long-standing discrepancy between the two measurements.Comment: Submitted for publicatio
OB stars at the lowest Local Group metallicity: GTC-OSIRIS observations of Sextans A
Our aim is to find and classify OB stars in Sextans A, to later determine
accurate stellar parameters of these blue massive stars in this low metallicity
region .
Using UBV photometry, the reddening-free index Q and GALEX imaging, we built
a list of blue massive star candidates in Sextans A. We obtained low resolution
(R 1000) GTC-OSIRIS spectra for a fraction of them and carried out
spectral classification. For the confirmed O-stars we derive preliminary
stellar parameters.
The target selection criteria and observations were successful and have
produced the first spectroscopic atlas of OB-type stars in Sextans A. From the
whole sample of 18 observed stars, 12 were classified as early OB-types,
including 5 O-stars. The radial velocities of all target stars are in agreement
with their Sextans A membership, although three of them show significant
deviations. We determined the stellar parameters of the O-type stars using the
stellar atmosphere code FASTWIND, and revisited the sub-SMC temperature scale.
Two of the O-stars are consistent with relatively strong winds and enhanced
helium abundances, although results are not conclusive. We discuss the position
of the OB stars in the HRD. Initial stellar masses run from slightly below 20
up to 40 solar masses.
The target selection method worked well for Sextans A, confirming the
procedure developed in Garcia \& Herrero (2013). The stellar temperatures are
consistent with findings in other galaxies. Some of the targets deserve
follow-up spectroscopy because of indications of a runaway nature, an enhanced
helium abundance or a relatively strong wind. We observe a correlation between
HI and OB associations similar to the irregular galaxy IC1613, confirming the
previous result that the most recent star formation of Sextans A is currently
on-going near the rim of the H\,{\sc I} cavity
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