854 research outputs found
Roles of resonance and dark irradiance for infrared photorefractive self-focusing and solitons in bi-polar InP:Fe
This paper shows experimental evidence of photorefractive steady state
self-focusing in InP:Fe for a wide range of intensities, at both 1.06 and
1.55m. To explain those results, it is shown that despite the bi-polar
nature of InP:Fe where one photocarrier and one thermal carrier are to be
considered, the long standing one photocarrier model for photorefractive
solitons can be usefully applied. The relationship between the dark irradiance
stemming out of this model and the known resonance intensity is then discussed
Locking Local Oscillator Phase to the Atomic Phase via Weak Measurement
We propose a new method to reduce the frequency noise of a Local Oscillator
(LO) to the level of white phase noise by maintaining (not destroying by
projective measurement) the coherence of the ensemble pseudo-spin of atoms over
many measurement cycles. This scheme uses weak measurement to monitor the phase
in Ramsey method and repeat the cycle without initialization of phase and we
call, "atomic phase lock (APL)" in this paper. APL will achieve white phase
noise as long as the noise accumulated during dead time and the decoherence are
smaller than the measurement noise. A numerical simulation confirms that with
APL, Allan deviation is averaged down at a maximum rate that is proportional to
the inverse of total measurement time, tau^-1. In contrast, the current atomic
clocks that use projection measurement suppress the noise only down to the
level of white frequency, in which case Allan deviation scales as tau^-1/2.
Faraday rotation is one of the possible ways to realize weak measurement for
APL. We evaluate the strength of Faraday rotation with 171Yb+ ions trapped in a
linear rf-trap and discuss the performance of APL. The main source of the
decoherence is a spontaneous emission induced by the probe beam for Faraday
rotation measurement. One can repeat the Faraday rotation measurement until the
decoherence become comparable to the SNR of measurement. We estimate this
number of cycles to be ~100 cycles for a realistic experimental parameter.Comment: 18 pages, 7 figures, submitted to New Journal of Physic
Large self-deflection of soliton beams in LiNbO3
We report the observation of large self-deflection of 2-D bright photorefractive solitons in LiNbO(3) crystal under a dc applied field. Beam deflection as large as 300 mu m after a 7 mm. propagation distance is reported, leading to formation of curved 2-D waveguides. We attribute this large deflection to the low level of impurity acceptors present in the samples, as confirmed by numerical results from a time-dependent photorefractive model
Quantum frequency estimation with trapped ions and atoms
We discuss strategies for quantum enhanced estimation of atomic transition
frequencies with ions stored in Paul traps or neutral atoms trapped in optical
lattices. We show that only marginal quantum improvements can be achieved using
standard Ramsey interferometry in the presence of collective dephasing, which
is the major source of noise in relevant experimental setups. We therefore
analyze methods based on decoherence free subspaces and prove that quantum
enhancement can readily be achieved even in the case of significantly imperfect
state preparation and faulty detections.Comment: 5 pages + 6 pages appendices; published versio
Isotropization of Bianchi type models and a new FRW solution in Brans-Dicke theory
Using scaled variables we are able to integrate an equation valid for
isotropic and anisotropic Bianchi type I, V, IX models in Brans-Dicke (BD)
theory. We analyze known and new solutions for these models in relation with
the possibility that anisotropic models asymptotically isotropize, and/or
possess inflationary properties. In particular, a new solution of curve
() Friedmann-Robertson-Walker (FRW) cosmologies in Brans-Dicke theory
is analyzed.Comment: 15 pages, 4 postscript figures, to appear in Gen. Rel. Grav., special
issue dedicated in honour of Prof. H. Dehne
Can induced gravity isotropize Bianchi I, V, or IX Universes?
We analyze if Bianchi I, V, and IX models in the Induced Gravity (IG) theory
can evolve to a Friedmann--Roberson--Walker (FRW) expansion due to the
non--minimal coupling of gravity and the scalar field. The analytical results
that we found for the Brans-Dicke (BD) theory are now applied to the IG theory
which has ( being the square ratio of the Higgs to
Planck mass) in a cosmological era in which the IG--potential is not
significant. We find that the isotropization mechanism crucially depends on the
value of . Its smallness also permits inflationary solutions. For the
Bianch V model inflation due to the Higgs potential takes place afterwads, and
subsequently the spontaneous symmetry breaking (SSB) ends with an effective FRW
evolution. The ordinary tests of successful cosmology are well satisfied.Comment: 24 pages, 5 figures, to be published in Phys. Rev. D1
Low frequency Raman studies of multi-wall carbon nanotubes: experiments and theory
In this paper, we investigate the low frequency Raman spectra of multi-wall
carbon nanotubes (MWNT) prepared by the electric arc method. Low frequency
Raman modes are unambiguously identified on purified samples thanks to the
small internal diameter of the MWNT. We propose a model to describe these
modes. They originate from the radial breathing vibrations of the individual
walls coupled through the Van der Waals interaction between adjacent concentric
walls. The intensity of the modes is described in the framework of bond
polarization theory. Using this model and the structural characteristics of the
nanotubes obtained from transmission electron microscopy allows to simulate the
experimental low frequency Raman spectra with an excellent agreement. It
suggests that Raman spectroscopy can be as useful regarding the
characterization of MWNT as it is in the case of single-wall nanotubes.Comment: 4 pages, 2 eps fig., 2 jpeg fig., RevTex, submitted to Phys. Rev.
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Decision making through use of interoperable simulation software
Many building simulation computer programs, originally developed on mainframe computers for research purposes, can now run on the powerful workstation and personal computers that are available to most architectural and engineering firms. Major efforts have been underway during the last decade to compile these programs on personal computers and make them available to a wider range of building professionals. However, even with the addition of user-friendly front- and back-ends, their use is still limited to a small number of specialized consultants. Considering the tremendous benefits of informed decisions that these programs can support, it is critical to address and resolve the issues that are associated with their limited acceptance. In this paper, the authors report on their research and development efforts to better understand decision-making and develop computer tools that will facilitate the use of simulation software during the building design process. They present a brief analysis of decision-making and then describe how they try to address it in building design through the development of the Building Design Advisor (BDA). Moreover, the authors elaborate on the major issues that they have encountered, discuss lessons learned, and offer recommendations for short- and long-term developments in this area
Localization, Coulomb interactions and electrical heating in single-wall carbon nanotubes/polymer composites
Low field and high field transport properties of carbon nanotubes/polymer
composites are investigated for different tube fractions. Above the percolation
threshold f_c=0.33%, transport is due to hopping of localized charge carriers
with a localization length xi=10-30 nm. Coulomb interactions associated with a
soft gap Delta_CG=2.5 meV are present at low temperature close to f_c. We argue
that it originates from the Coulomb charging energy effect which is partly
screened by adjacent bundles. The high field conductivity is described within
an electrical heating scheme. All the results suggest that using composites
close to the percolation threshold may be a way to access intrinsic properties
of the nanotubes by experiments at a macroscopic scale.Comment: 4 pages, 5 figures, Submitted to Phys. Rev.
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