18,227 research outputs found
Non-equilibrium Goldstone phenomenon in tachyonic preheating
The dominance of the direct production of elementary Goldstone waves is
demonstrated in tachyonic preheating by numerically determining the evolution
of the dispersion relation, the equation of state and the kinetic power spectra
for the angular degree of freedom of the complex matter field. The importance
of the domain structure in the order parameter distribution for the
quantitative understanding of the excitation mechanism is emphasized. Evidence
is presented for the very early decoupling of the low-momentum Goldstone modes.Comment: 14 LaTeX pages, 5 figures, version published in Phys. Rev.
Out-of-equilibrium tricritical point in a system with long-range interactions
Systems with long-range interactions display a short-time relaxation towards
Quasi Stationary States (QSSs) whose lifetime increases with system size. With
reference to the Hamiltonian Mean Field (HMF) model, we here show that a
maximum entropy principle, based on Lynden-Bell's pioneering idea of "violent
relaxation", predicts the presence of out-of-equilibrium phase transitions
separating the relaxation towards homogeneous (zero magnetization) or
inhomogeneous (non zero magnetization) QSSs. When varying the initial condition
within a family of "water-bags" with different initial magnetization and
energy, first and second order phase transition lines are found that merge at
an out--of--equilibrium tricritical point. Metastability is theoretically
predicted and numerically checked around the first-order phase transition line.Comment: Physical Review Letters (2007
Hybrid quantum repeater using bright coherent light
We describe a quantum repeater protocol for long-distance quantum
communication. In this scheme, entanglement is created between qubits at
intermediate stations of the channel by using a weak dispersive light-matter
interaction and distributing the outgoing bright coherent light pulses among
the stations. Noisy entangled pairs of electronic spin are then prepared with
high success probability via homodyne detection and postselection. The local
gates for entanglement purification and swapping are deterministic and
measurement-free, based upon the same coherent-light resources and weak
interactions as for the initial entanglement distribution. Finally, the
entanglement is stored in a nuclear-spin-based quantum memory. With our system,
qubit-communication rates approaching 100 Hz over 1280 km with fidelities near
99% are possible for reasonable local gate errors.Comment: title changed, final published versio
Site-site memory equation approach in study of density/pressure dependence of translational diffusion coefficient and rotational relaxation time of polar molecular solutions: acetonitrile in water, methanol in water, and methanol in acetonitrile
We present results of theoretical study and numerical calculation of the
dynamics of molecular liquids based on combination of the memory equation
formalism and the reference interaction site model - RISM. Memory equations for
the site-site intermediate scattering functions are studied in the
mode-coupling approximation for the first order memory kernels, while
equilibrium properties such as site-site static structure factors are deduced
from RISM. The results include the temperature-density(pressure) dependence of
translational diffusion coefficients D and orientational relaxation times t for
acetonitrile in water, methanol in water and methanol in acetonitrile, all in
the limit of infinite dilution. Calculations are performed over the range of
temperatures and densities employing the SPC/E model for water and optimized
site-site potentials for acetonitrile and methanol. The theory is able to
reproduce qualitatively all main features of temperature and density
dependences of D and t observed in real and computer experiments. In
particular, anomalous behavior, i.e. the increase in mobility with density, is
observed for D and t of methanol in water, while acetonitrile in water and
methanol in acetonitrile do not show deviations from the ordinary behavior. The
variety exhibited by the different solute-solvent systems in the density
dependence of the mobility is interpreted in terms of the two competing origins
of friction, which interplay with each other as density increases: the
collisional and dielectric frictions which, respectively, increase and decrease
with increasing density.Comment: 13 pages, 8 eps-figures, 3 tables, RevTeX4-forma
Evolution of a global string network in a matter dominated universe
We evolve the network of global strings in the matter-dominated universe by
means of numerical simulations. The existence of the scaling solution is
confirmed as in the radiation-dominated universe but the scaling parameter
takes a slightly smaller value, , which is
defined as with the energy density of
global strings and the string tension per unit length. The change of
from the radiation to the matter-dominated universe is consistent with
that obtained by Albrecht and Turok by use of the one-scale model. We also
study the loop distribution function and find that it can be well fitted with
that predicted by the one-scale model, where the number density of
the loop with the length is given by with and . Thus, the evolution of the
global string network in the matter-dominated universe can be well described by
the one-scale model as in the radiation-dominated universe.Comment: 10 pages, 5 figure
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