5,588 research outputs found
Cloning of Gaussian states by linear optics
We analyze in details a scheme for cloning of Gaussian states based on linear
optical components and homodyne detection recently demonstrated by U. L.
Andersen et al. [PRL 94 240503 (2005)]. The input-output fidelity is evaluated
for a generic (pure or mixed) Gaussian state taking into account the effect of
non-unit quantum efficiency and unbalanced mode-mixing. In addition, since in
most quantum information protocols the covariance matrix of the set of input
states is not perfectly known, we evaluate the average cloning fidelity for
classes of Gaussian states with the degree of squeezing and the number of
thermal photons being only partially known.Comment: 8 pages, 7 figure
Giant planets around two intermediate-mass evolved stars and confirmation of the planetary nature of HIP67851 c
Precision radial velocities are required to discover and characterize planets
orbiting nearby stars. Optical and near infrared spectra that exhibit many
hundreds of absorption lines can allow the m/s precision levels required for
such work. However, this means that studies have generally focused on
solar-type dwarf stars. After the main-sequence, intermediate-mass stars
(former A-F stars) expand and rotate slower than their progenitors, thus
thousands of narrow absorption lines appear in the optical region, permitting
the search for planetary Doppler signals in the data for these types of stars.
We present the discovery of two giant planets around the intermediate-mass
evolved star HIP65891 and HIP107773. The best Keplerian fit to the HIP65891 and
HIP107773 radial velocities leads to the following orbital parameters: P=1084.5
d; msin = 6.0 M; =0.13 and P=144.3 d; msin = 2.0
M; =0.09, respectively. In addition, we confirm the planetary nature
of the outer object orbiting the giant star HIP67851. The orbital parameters of
HIP67851c are: P=2131.8 d, msin = 6.0 M and =0.17. With
masses of 2.5 M and 2.4 M HIP65891 and HIP107773 are two of the
most massive stars known to host planets. Additionally, HIP67851 is one of five
giant stars that are known to host a planetary system having a close-in planet
( 0.7 AU). Based on the evolutionary states of those five stars, we
conclude that close-in planets do exist in multiple systems around subgiants
and slightly evolved giants stars, but probably they are subsequently destroyed
by the stellar envelope during the ascent of the red giant branch phase. As a
consequence, planetary systems with close-in objects are not found around
horizontal branch stars.Comment: Accepted for publication in A&
Dynamics of quasiparticle trapping in Andreev levels
We present a theory describing the trapping and untrapping of quasiparticles
in the Andreev bound level of a single-channel weak link between two
superconductors. We calculate the rates of the transitions between even and odd
occupations of the Andreev level induced by absorption and emission of both
photons and phonons. We apply the theory to a recent experiment [Phys. Rev.
Lett. 106, 257003 (2011)] in which the dynamics of the trapping of
quasiparticles in the Andreev levels of superconducting atomic contacts coupled
to a Josephson junction was measured. We show that the plasma energy
of the Josephson junction defines a rather abrupt transition between a fast
relaxation regime dominated by coupling to photons and a slow relaxation regime
dominated by coupling to phonons. With realistic parameters the theory provides
a semi-quantitative description of the experimental results.Comment: 11 pages, 9 figures. Accepted for publication in Physical Review
The Seven Sisters DANCe III: Projected spatial distribution
Methods. We compute Bayesian evidences and Bayes Factors for a set of
variations of the classical radial models by King (1962), Elson et al. (1987)
and Lauer et al. (1995). The variations incorporate different degrees of model
freedom and complexity, amongst which we include biaxial (elliptical) symmetry,
and luminosity segregation. As a by-product of the model comparison, we obtain
posterior distributions and maximum a posteriori estimates for each set of
model parameters. Results. We find that the model comparison results depend on
the spatial extent of the region used for the analysis. For a circle of 11.5
parsecs around the cluster centre (the most homogeneous and complete region),
we find no compelling reason to abandon Kings model, although the Generalised
King model, introduced in this work, has slightly better fitting properties.
Furthermore, we find strong evidence against radially symmetric models when
compared to the elliptic extensions. Finally, we find that including mass
segregation in the form of luminosity segregation in the J band, is strongly
supported in all our models. Conclusions. We have put the question of the
projected spatial distribution of the Pleiades cluster on a solid probabilistic
framework, and inferred its properties using the most exhaustive and least
contaminated list of Pleiades candidate members available to date. Our results
suggest however that this sample may still lack about 20% of the expected
number of cluster members. Therefore, this study should be revised when the
completeness and homogeneity of the data can be extended beyond the 11.5
parsecs limit. Such study will allow a more precise determination of the
Pleiades spatial distribution, its tidal radius, ellipticity, number of objects
and total mass.Comment: 39 pages, 31 figure
Optimal unambiguous comparison of two unknown squeezed vacua
We propose a scheme for unambiguous state comparison (USC) of two unknown
squeezed vacuum states of an electromagnetic field. Our setup is based on
linear optical elements and photon-number detectors, and achieves optimal USC
in an ideal case of unit quantum efficiency. In realistic conditions, i.e., for
non-unit quantum efficiency of photodetectors, we evaluate the probability of
getting an ambiguous result as well as the reliability of the scheme, thus
showing its robustness in comparison to previous proposals.Comment: 7 pages, 4 figures (revised version
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