51,192 research outputs found
Noise-Free Measurement of Harmonic Oscillators with Instantaneous Interactions
We present a method of measuring the quantum state of a harmonic oscillator
through instantaneous probe-system selective interactions of the
Jaynes-Cummings type. We prove that this scheme is robust to general
decoherence mechanisms, allowing the possibility of measuring fast-decaying
systems in the weak-coupling regime. This method could be applied to different
setups: motional states of trapped ions, microwave fields in cavity/circuit
QED, and even intra-cavity optical fields.Comment: 4 pages, no figure, published in Physical Review Letter
CB damping of primordial gravitational waves and the fine-tuning of the CB temperature anisotropy
Damping of primordial gravitational waves due to the anisotropic stress
contribution owing to the cosmological neutrino background (CB) is
investigated in the context of a radiation-to-matter dominated Universe.
Besides its inherent effects on the gravitational wave propagation, the
inclusion of the CB anisotropic stress into the dynamical equations also
affects the tensor mode contribution to the anisotropy of the cosmological
microwave background (CB) temperature. Given that the fluctuations of
the CB temperature in the (ultra)relativistic regime are driven by a
multipole expansion, the mutual effects on the gravitational waves and on the
CB are obtained through a unified prescription for a
radiation-to-matter dominated scenario. The results are confronted with some
preliminary results for the radiation dominated scenario. Both scenarios are
supported by a simplified analytical framework, in terms of a scale independent
dynamical variable, , that relates cosmological scales, , and the
conformal time, . The background relativistic (hot dark) matter
essentially works as an effective dispersive medium for the gravitational waves
such that the damping effect is intensified for the Universe evolving to the
matter dominated era. Changes on the temperature variance owing to the
inclusion of neutrino collision terms into the dynamical equations result into
spectral features that ratify that the multipole expansion coefficients
's die out for .Comment: 24 pages, 8 figure
Non-equilibrium tube length fluctuations of entangled polymers
We investigate the nonequilibrium tube length fluctuations during the
relaxation of an initially stretched, entangled polymer chain. The
time-dependent variance of the tube length follows in the early-time
regime a simple universal power law originating in the
diffusive motion of the polymer segments. The amplitude is calculated
analytically both from standard reptation theory and from an exactly solvable
lattice gas model for reptation and its dependence on the initial and
equilibrium tube length respectively is discussed. The non-universality
suggests the measurement of the fluctuations (e.g. using flourescence
microscopy) as a test for reptation models.Comment: 12 pages, 2 figures. Minor typos correcte
Magnetism of Substitutional Co Impurities in Graphene: Realization of Single -Vacancies
We report {\it ab initio} calculations of the structural, electronic and
magnetic properties of a graphene monolayer substitutionally doped with Co
(Co) atoms. We focus in Co because among traditional ferromagnetic
elements (Fe, Co and Ni), only Co atoms induce spin-polarization in
graphene. Our results show the complex magnetism of Co substitutional impurites
in graphene, which is mapped into simple models such as the -vacancy and
Heisenberg model. The links established in our work can be used to bring into
contact the engineering of nanostructures with the results of -models in
defective graphene. In principle, the structures considered here can be
fabricated using electron irradiation or Ar ion bombardment to create
defects and depositing Co at the same time
Evidence for an inflationary phase transition from the LSS and CMB anisotropy data
In the light of the recent Boomerang and Maxima observations of the CMB which
show an anomalously low second acoustic peak, we reexamine the prediction by
Adams et al (1997) that this would be the consequence of a 'step' in the
primordial spectrum induced by a spontaneous symmetry breaking phase transition
during primordial inflation. We demonstrate that a deviation from
scale-invariance around ~Mpc can simultaneously explain both
the feature identified earlier in the APM galaxy power spectrum as well the
recent CMB anisotropy data, with a baryon density consistent with the BBN
value. Such a break also allows a good fit to the data on cluster abundances
even for a critical density matter-dominated universe with zero cosmological
constant.Comment: 4 pages with 3 figures, LaTeX file using espcrc2.sty to appear on the
Proceedings of "Euroconference on Frontiers in Particle Astrophysics and
Cosmology",Sant Feliu de Guixols,Spain,30th September-5th October of 200
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