5,799 research outputs found
Statistical Mechanics of finite arrays of coupled bistable elements
We discuss the equilibrium of a single collective variable characterizing a
finite set of coupled, noisy, bistable systems as the noise strength, the size
and the coupling parameter are varied. We identify distinct regions in
parameter space. The results obtained in prior works in the asymptotic infinite
size limit are significantly different from the finite size results. A
procedure to construct approximate 1-dimensional Langevin equation is adopted.
This equation provides a useful tool to understand the collective behavior even
in the presence of an external driving force
Vacuum fluctuations and the conditional homodyne detection of squeezed light
Conditional homodyne detection of quadrature squeezing is compared with
standard nonconditional detection. Whereas the latter identifies
nonclassicality in a quantitative way, as a reduction of the noise power below
the shot noise level, conditional detection makes a qualitative distinction
between vacuum state squeezing and squeezed classical noise. Implications of
this comparison for the realistic interpretation of vacuum fluctuations
(stochastic electrodynamics) are discussed.Comment: 14 pages, 7 figures, to appear in J. Opt. B: Quantum Semiclass. Op
Checking the validity of truncating the cumulant hierarchy description of a small system
We analyze the behavior of the first few cumulant in an array with a small
number of coupled identical particles. Desai and Zwanzig (J. Stat. Phys., {\bf
19}, 1 (1978), p. 1) studied noisy arrays of nonlinear units with global
coupling and derived an infinite hierarchy of differential equations for the
cumulant moments. They focused on the behavior of infinite size systems using a
strategy based on truncating the hierarchy. In this work we explore the
reliability of such an approach to describe systems with a small number of
elements. We carry out an extensive numerical analysis of the truncated
hierarchy as well as numerical simulations of the full set of Langevin
equations governing the dynamics. We find that the results provided by the
truncated hierarchy for finite systems are at variance with those of the
Langevin simulations for large regions of parameter space. The truncation of
the hierarchy leads to a dependence on initial conditions and to the
coexistence of states which are not consistent with the theoretical
expectations based on the multidimensional linear Fokker-Planck equation for
finite arrays
Dynamics of an inhomogeneous quantum phase transition
We argue that in a second order quantum phase transition driven by an
inhomogeneous quench density of quasiparticle excitations is suppressed when
velocity at which a critical point propagates across a system falls below a
threshold velocity equal to the Kibble-Zurek correlation length times the
energy gap at freeze-out divided by . This general prediction is
supported by an analytic solution in the quantum Ising chain. Our results
suggest, in particular, that adiabatic quantum computers can be made more
adiabatic when operated in an "inhomogeneous" way.Comment: 7 pages; version to appear in a special issue of New J. Phy
Quantum Zeno and anti-Zeno effects in surface diffusion of interacting adsorbates
Surface diffusion of interacting adsorbates is here analyzed within the
context of two fundamental phenomena of quantum dynamics, namely the quantum
Zeno effect and the anti-Zeno effect. The physical implications of these
effects are introduced here in a rather simple and general manner within the
framework of non-selective measurements and for two (surface) temperature
regimes: high and very low (including zero temperature). The quantum
intermediate scattering function describing the adsorbate diffusion process is
then evaluated for flat surfaces, since it is fully analytical in this case.
Finally, a generalization to corrugated surfaces is also discussed. In this
regard, it is found that, considering a Markovian framework and high surface
temperatures, the anti-Zeno effect has already been observed, though not
recognized as such.Comment: 17 pages, 1 figur
The nature of dark matter and the density profile and central behavior of relaxed halos
We show that the two basic assumptions of the model recently proposed by
Manrique and coworkers for the universal density profile of cold dark matter
(CDM) halos, namely that these objects grow inside out in periods of smooth
accretion and that their mass profile and its radial derivatives are all
continuous functions, are both well understood in terms of the very nature of
CDM. Those two assumptions allow one to derive the typical density profile of
halos of a given mass from the accretion rate characteristic of the particular
cosmology. This profile was shown by Manrique and coworkers to recover the
results of numerical simulations. In the present paper, we investigate its
behavior beyond the ranges covered by present-day N-body simulations. We find
that the central asymptotic logarithmic slope depends crucially on the shape of
the power spectrum of density perturbations: it is equal to a constant negative
value for power-law spectra and has central cores for the standard CDM power
spectrum. The predicted density profile in the CDM case is well fitted by the
3D S\'ersic profile over at least 10 decades in halo mass. The values of the
S\'ersic parameters depend on the mass of the structure considered. A practical
procedure is provided that allows one to infer the typical values of the best
NFW or S\'ersic fitting law parameters for halos of any mass and redshift in
any given standard CDM cosmology.Comment: 9 pages, 6 figures, to appear in the ApJ vol. 647, september 20,
2007. Minor changes to match the published versio
Line Shape Broadening in Surface Diffusion of Interacting Adsorbates with Quasielastic He Atom Scattering
The experimental line shape broadening observed in adsorbate diffusion on
metal surfaces with increasing coverage is usually related to the nature of the
adsorbate-adsorbate interaction. Here we show that this broadening can also be
understood in terms of a fully stochastic model just considering two noise
sources: (i) a Gaussian white noise accounting for the surface friction, and
(ii) a shot noise replacing the physical adsorbate-adsorbate interaction
potential. Furthermore, contrary to what could be expected, for relatively weak
adsorbate-substrate interactions the opposite effect is predicted: line shapes
get narrower with increasing coverage.Comment: 4 pages, 2 figures (slightly revised version
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