575 research outputs found
All-optical phase-regenerative multicasting of 40 Gbit/s DPSK signal in a degenerate phase sensitive amplifier
We demonstrate all-optical 1-to-5 differential phase-shift keyed (DPSK) wavelength multicasting at 40 Gbit/s using a degenerate four-wave mixing (FWM) based phase sensitive amplifier (PSA). Phase regenerative properties are reported with a sensitivity improvement of more that 10 dB
Saturation effects in degenerate phase sensitive fiber optic parametric amplifiers
We experimentally study saturation effects in degenerate phase sensitive amplifiers, revealing and explaining a gain regime suitable for all-optical signal processing functions such as phase regeneration
Comment on superluminality in general relativity
General relativity provides an appropriate framework for addressing the issue
of sub- or superluminality as an apparent effect. Even though a massless
particle travels on the light cone, its average velocity over a finite path
measured by different observers is not necessarily equal to the velocity of
light, as a consequence of the time dilation or contraction in gravitational
fields. This phenomenon occurs in either direction (increase or depletion)
irrespectively of the details and strength of the gravitational interaction.
Hence, it does not intrinsically guarantee superluminality, even when the
gravitational field is reinforced.Comment: 6 page
G3-homogeneous gravitational instantons
We provide an exhaustive classification of self-dual four-dimensional
gravitational instantons foliated with three-dimensional homogeneous spaces,
i.e. homogeneous self-dual metrics on four-dimensional Euclidean spaces
admitting a Bianchi simply transitive isometry group. The classification
pattern is based on the algebra homomorphisms relating the Bianchi group and
the duality group SO(3). New and general solutions are found for Bianchi III.Comment: 24 pages, few correction
All-optical phase and amplitude regeneration properties of a 40 Gbit/s DPSK black-box phase sensitive amplifier
We experimentally study the pure amplitude and phase regeneration capabilities of a blackbox degenerate four wave mixing (FWM) based bit-rate-flexible phase sensitive amplifier (PSA) for a 40 Gbit/s differential phase-shift keyed (DPSK) signal
Fluxes, Gaugings and Gaugino Condensates
Based on the correspondence between the N = 1 superstring compactifications
with fluxes and the N = 4 gauged supergravities, we study effective N = 1
four-dimensional supergravity potentials arising from fluxes and gaugino
condensates in the framework of orbifold limits of (generalized) Calabi-Yau
compactifications. We give examples in heterotic and type II orientifolds in
which combined fluxes and condensates lead to vacua with small supersymmetry
breaking scale. We clarify the respective roles of fluxes and condensates in
supersymmetry breaking, and analyze the scaling properties of the gravitino
mass.Comment: 17 pages, C
Boundary States for D-branes in
We construct boundary states representing D-strings in . These wrap
twisted conjugacy classes of SL(2,R), and the boundary states are therefore
based on continuous representations only. We check Cardy's condition and find a
consistent open string spectrum. The open string spectrum on all the D-branes
is the same.Comment: 23 pages, late
From the WZWN Model to the Liouville Equation: Exact String Dynamics in Conformally Invariant AdS Background
It has been known for some time that the SL(2,R) WZWN model reduces to
Liouville theory. Here we give a direct and physical derivation of this result
based on the classical string equations of motion and the proper string size.
This allows us to extract precisely the physical effects of the metric and
antisymmetric tensor, respectively, on the {\it exact} string dynamics in the
SL(2,R) background. The general solution to the proper string size is also
found. We show that the antisymmetric tensor (corresponding to conformal
invariance) generally gives rise to repulsion, and it precisely cancels the
dominant attractive term arising from the metric.
Both the sinh-Gordon and the cosh-Gordon sectors of the string dynamics in
non-conformally invariant AdS spacetime reduce here to the Liouville equation
(with different signs of the potential), while the original Liouville sector
reduces to the free wave equation. Only the very large classical string size is
affected by the torsion. Medium and small size string behaviours are unchanged.
We also find illustrative classes of string solutions in the SL(2,R)
background: dynamical closed as well as stationary open spiralling strings, for
which the effect of torsion is somewhat like the effect of rotation in the
metric. Similarly, the string solutions in the 2+1 BH-AdS background with
torsion and angular momentum are fully analyzed.Comment: 24 pages including 4 postscript figures. Enlarged version including a
section on string solutions in 2+1 black hole background. To be published in
Phys. Rev. D., December 199
Quantum Coherent String States in AdS_3 and SL(2,R) WZWN Model
In this paper we make the connection between semi-classical string
quantization and exact conformal field theory quantization of strings in 2+1
Anti de Sitter spacetime. More precisely, considering the WZWN model
corresponding to SL(2,R) and its covering group, we construct quantum {\it
coherent} string states, which generalize the ordinary coherent states of
quantum mechanics, and show that in the classical limit they correspond to
oscillating circular strings. After quantization, the spectrum is found to
consist of two parts: A continuous spectrum of low mass states (partly
tachyonic) fulfilling the standard spin-level condition necessary for unitarity
|j|< k/2, and a discrete spectrum of high mass states with asymptotic behaviour
m^2\alpha'\propto N^2 (N positive integer). The quantization condition for the
high mass states arises from the condition of finite positive norm of the
coherent string states, and the result agrees with our previous results
obtained using semi-classical quantization. In the k\to\infty limit, all the
usual properties of coherent or {\it quasi-classical} states are recovered. It
should be stressed that we consider the circular strings only for simplicity
and clarity, and that our construction can easily be used for other string
configurations too. We also compare our results with those obtained in the
recent preprint hep-th/0001053 by Maldacena and Ooguri.Comment: Misprints corrected. Final version to appear in Phys. Rev.
Isolation and primary cultures of human intrahepatic bile ductular epithelium
A technique for the isolation of human intrahepatic bile ductular epithelium, and the establishment of primary cultures using a serum- and growth-factor-supplemented medium combined with a connective tissue substrata is described. Initial cell isolates and monolayer cultures display phenotypic characteristics of biliary epithelial cells (low molecular weight prekeratin positive; albumin, alphafetoprotein, and Factor VIII-related antigen negative). Ultrastructural features of the cultured cells show cell polarization with surface microvilli, numerous interepithelial junctional complexes and cytoplasmic intermediate prekeratin filaments. © 1988 Tissue Culture Association, Inc
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