1,490 research outputs found
Quantum characterization of bipartite Gaussian states
Gaussian bipartite states are basic tools for the realization of quantum
information protocols with continuous variables. Their complete
characterization is obtained by the reconstruction of the corresponding
covariance matrix. Here we describe in details and experimentally demonstrate a
robust and reliable method to fully characterize bipartite optical Gaussian
states by means of a single homodyne detector. We have successfully applied our
method to the bipartite states generated by a sub-threshold type-II optical
parametric oscillator which produces a pair of thermal cross-polarized
entangled CW frequency degenerate beams. The method provide a reliable
reconstruction of the covariance matrix and allows to retrieve all the physical
information about the state under investigation. These includes observable
quantities, as energy and squeezing, as well as non observable ones as purity,
entropy and entanglement. Our procedure also includes advanced tests for
Gaussianity of the state and, overall, represents a powerful tool to study
bipartite Gaussian state from the generation stage to the detection one
An effective method to estimate multidimensional Gaussian states
A simple and efficient method for characterization of multidimensional
Gaussian states is suggested and experimentally demonstrated. Our scheme shows
analogies with tomography of finite dimensional quantum states, with the
covariance matrix playing the role of the density matrix and homodyne detection
providing Stern-Gerlach-like projections. The major difference stems from a
different character of relevant noises: while the statistics of
Stern-Gerlach-like measurements is governed by binomial statistics, the
detection of quadrature variances correspond to chi-square statistics. For
Gaussian and near Gaussian states the suggested method provides, compared to
standard tomography techniques, more stable and reliable reconstructions. In
addition, by putting together reconstruction methods for Gaussian and arbitrary
states, we obtain a tool to detect the non-Gaussian character of optical
signals.Comment: 8 pages, 5 fis, accepted for publication on PR
Full characterization of Gaussian bipartite entangled states by a single homodyne detector
We present the full experimental reconstruction of Gaussian entangled states
generated by a type--II optical parametric oscillator (OPO) below threshold.
Our scheme provides the entire covariance matrix using a single homodyne
detector and allows for the complete characterization of bipartite Gaussian
states, including the evaluation of purity, entanglement and nonclassical
photon correlations, without a priori assumptions on the state under
investigation. Our results show that single homodyne schemes are convenient and
robust setups for the full characterization of OPO signals and represent a tool
for quantum technology based on continuous variable entanglement.Comment: 4 pages, 3 figures, slightly longer version of published PR
Quantum Decoherence of Single-Photon Counters
The interaction of a quantum system with the environment leads to the
so-called quantum decoherence. Beyond its fundamental significance, the
understanding and the possible control of this dynamics in various scenarios is
a key element for mastering quantum information processing. Here we report the
quantitative probing of what can be called the quantum decoherence of
detectors, a process reminiscent of the decoherence of quantum states in the
presence of coupling with a reservoir. We demonstrate how the quantum features
of two single-photon counters vanish under the influence of a noisy
environment. We thereby experimentally witness the transition between the
full-quantum operation of the measurement device to the "semi-classical
regime", described by a positive Wigner function. The exact border between
these two regimes is explicitely determined and measured experimentally
Positive Energy Unitary Irreducible Representations of the Superalgebras osp(1|2n,R)
We give the classification of the positive energy (lowest weight) unitary
irreducible representations of the superalgebras osp(1|2n,R).Comment: 20 pages, LATEX2e (revtex4,amsmath,amssymb), Plenary talk by VKD at X
International Conference on Symmetry Methods in Physics, Yerevan,
13-21.8.2003; added acknowledgements; corrected misprint
Integration of massive states as contractions of non linear -models
We consider the contraction of some non linear sigma models which appear in
effective supergravity theories. In particular we consider the contractions of
maximally symmetric spaces corresponding to N=1 and N=2 theories, as they
appear in certain low energy effective supergravity actions with mass
deformations.
The contraction procedure is shown to describe the integrating out of massive
modes in the presence of interactions, as it happens in many supergravity
models after spontaneous supersymmetry breaking.Comment: AMS-LaTeX, 33 page
Alleviating the non-ultralocality of coset sigma models through a generalized Faddeev-Reshetikhin procedure
The Faddeev-Reshetikhin procedure corresponds to a removal of the
non-ultralocality of the classical SU(2) principal chiral model. It is realized
by defining another field theory, which has the same Lax pair and equations of
motion but a different Poisson structure and Hamiltonian. Following earlier
work of M. Semenov-Tian-Shansky and A. Sevostyanov, we show how it is possible
to alleviate in a similar way the non-ultralocality of symmetric space sigma
models. The equivalence of the equations of motion holds only at the level of
the Pohlmeyer reduction of these models, which corresponds to symmetric space
sine-Gordon models. This work therefore shows indirectly that symmetric space
sine-Gordon models, defined by a gauged Wess-Zumino-Witten action with an
integrable potential, have a mild non-ultralocality. The first step needed to
construct an integrable discretization of these models is performed by
determining the discrete analogue of the Poisson algebra of their Lax matrices.Comment: 31 pages; v2: minor change
Fermionic Wigs for AdS-Schwarzschild Black Holes
We provide the metric, the gravitino fields and the gauge fields to all
orders in the fermionic zero modes for D=5 and D=4, N=2 gauged supergravity
solutions starting from non-extremal AdS--Schwarzschild black holes. We compute
the Brown-York stress--energy tensor on the boundary of AdS_5 / AdS_4 spaces
and we discuss some implications of the fermionic corrections to perfect fluid
interpretation of the boundary theory. The complete non-linear solution, which
we denote as fermionic wig, is achieved by acting with supersymmetry
transformations upon the supergravity fields and that expansion naturally
truncates at some order in the fermionic zero modes.Comment: 27 pages, Latex2e, no figures, 3 ancillary file
Supersymmetric branes with (almost) arbitrary tensions
We present a supersymmetric version of the two-brane Randall-Sundrum
scenario, with arbitrary brane tensions T_1 and T_2, subject to the bound
|T_{1,2}| \leq \sqrt{-6\Lambda_5}, where \Lambda_5 < 0 is the bulk cosmological
constant. Dimensional reduction gives N=1, D=4 supergravity, with cosmological
constant \Lambda_4 in the range \half\Lambda_5 \leq \Lambda_4 \leq 0. The case
with \Lambda_4 = 0 requires T_1 = -T_2 = \sqrt{-6\Lambda_5}. This work unifies
and generalizes previous approaches to the supersymmetric Randall-Sundrum
scenario. It also shows that the Randall-Sundrum fine-tuning is not a
consequence of supersymmetry.Comment: 19pp; Published versio
Investigation of Coolant Mixing in Reactor VVER-1000
We present an experimental investigation of coolant mixing in downcomer and lower plenum of VVER-1000 here. The arrangement of the problem, the methodology and results are discussed. Three groups of experiments simulating coolant mixing were executed: in conditions of RCP start-up, during natural circulation recovery in the course of SB LOCA and in conditions of stable operation of different amount of RCPs. Results of experiments are used for validation of numerical codes
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